Joseph Talghader obtained his B.S. in electrical engineering from Rice University. He was awarded an NSF Graduate Fellowship and attended the University of California at Berkeley where he received his M.S. in 1993 and Ph.D. in 1995. He worked at Texas Instruments as a Process Development Engineer, where he investigated EEPROM memory design and reliability issues. After graduating from Berkeley in 1995, he joined Waferscale Integration where he developed microfabrication processes for high-density nonvolatile memory devices. In 1997 Dr. Talghader joined the faculty at the University of Minnesota as an Assistant Professor and was later promoted to Associate and then Full Professor. He has been extensively involved in thermal infrared and radiation heat transfer devices, optical coatings materials science, and the miniaturization of micro-opto-mechanical systems. His group has demonstrated the highest sensitivity uncooled thermal detectors and the first tunable multispectral thermal detectors. He has several patents and patent applications in these areas. His group works in a number of different fields including recent research on the measurement of the optical and materials properties of glacial ice. Dr. Talghader has received 3M Faculty Awards on three occasions. His technology has been a Finalist for the Minnesota Cup for entrepreneurs. He has served on various program committees and reviews, including service on the triennial strategic planning panel for the Army Research Office Electronics Division, twice as Program chair of the IEEE/LEOS Optical Microelectromechanical Systems Conference, and as Guest Editor of the IEEE Journal of Selected Topics in Quantum Electronics Special Issue on Optical Microsystems. He was Conference chair of the IEEE/LEOS Optical MEMS and Nanophotonics Conference and served as Lead Editor of the JSTQE Special Issue on Optical Micro and Nanosystems. He currently an Editor of the NPG journal Light: Science and Applications.
Abstract The optical absorption of contaminants on high reflectivity mirrors was measured using photo thermal common-path interferometry before and after exposure to high power continuous-wave laser light. The contaminants were micron-sized graphite flakes on hafnia-silica distributed Bragg reflectors illuminated by a ytterbium-doped fiber laser. After one-second periods of exposure, the mirrors demonstrated reduced absorption for irradiances as low as 11 kW cm−2 and had an obvious threshold near 20 kW cm−2. Final absorption values were reduced by up to 90% of their initial value for irradiances of 92 kW cm−2. For shorter pulses at 34 kW cm−2, a minimum exposure time required to begin absorption reduction was found between 100 μs and 200 μs, with particles reaching their final minimum absorption value within 300 ms. Microscope images of the surface showed agglomerated particles fragmenting with some being removed completely, probably by evaporation for exposures between 200 μs to 10 ms. Exposures of 100 ms and longer left behind a thin semi-transparent residue, covering much of the conditioned area. An order of magnitude estimate of the time necessary to begin altering the surface contaminants (also known as ”conditioning”) indicates about 200 μs seconds at 34 kW cm−2, based on heating an average carbon particle to its sublimation temperature including energy loss to thermal contact and radiation. This estimation is close to the observed exposure time required to begin absorption reduction.
Solar selective coatings that absorb solar incident light but that inhibit emission of thermal light can have a dramatic impact on the performance of Direct Steam Generation (DSG) systems. In prior art, perfect coatings with instantaneous transitions between high absorption and low absorption have been modeled. In this paper non-ideal spectral, environmental, and material properties of solar selective coatings are shown to have significant effects on the efficiencies and optimum transition wavelengths of real systems. By introducing more real world parameters into the coating optimization it is desired that a more cost effective and efficient system can be built. It is shown that using ideal conditions the optimum transition wavelength for a DSG system is 1.4μm with an efficiency of 55.7%. Whereas for a realistic non-ideal DSG system the optimum transition is at 3.4μm resulting in an efficiency around 30% depending on the concentration factor used. It is then shown that an optimized selective coating will be outperformed by a simple non-selective black absorber with 95% absorption at concentration factors above 80 and above 130 when the AM1.5 spectrum is used.
Many current quantum optical systems, such as microcavities, interact with thermal light through a small number of widely separated modes. Previous theories for photon number fluctuations of thermal light have been primarily limited to special cases that are appropriate for large volumes or distances, such as single modes, many modes, or modes of uniform spectral distribution. Herein, a theory for the general case of spectrally dependent photon number fluctuations is developed for thermal light. The error in variance of prior art is quantitatively derived for an example cavity in the case where photon counting noise dominates. A method to reduce the spectral impact of this variance is described.
A subsampling technique for real-time phase retrieval of high-speed thermal signals is demonstrated with heated metal lines such as those found in microelectronic interconnects. The thermal signals were produced by applying a current through aluminum resistors deposited on soda–lime–silica glass, and the resulting refractive index changes were measured using a Mach–Zehnder interferometer with a microscope objective and high-speed camera. The temperatures of the resistors were measured both by the phase-retrieval method and by monitoring the resistance of the aluminum lines. The method used to analyze the phase is at least 60× faster than the state of the art but it maintains a small spatial phase noise of 16 nm, remaining comparable to the state of the art. For slowly varying signals, the system is able to perform absolute phase measurements over time, distinguishing temperature changes as small as 2 K. With angular scanning or structured illumination improvements, the system could also perform fast thermal tomography.
Spatial modulation spectroscopy (SMS) is a powerful method for interrogating single nanoparticles. In these experiments optical extinction is measured by moving the particle in and out of a tightly focused laser beam. SMS is typically used for particles that are much smaller than the laser spot size. In this paper, we extend the analysis of the SMS signal to particles with sizes comparable to or larger than the laser spot, where the shape of the particle matters. These results are important for the analysis of polydisperse samples that have a wide range of sizes. The presented example images and analysis of a carbon microparticle sample show the utility of the derived expressions. In particular, we show that SMS can be used to generate extinction cross-section information about micrometer-sized particles with complex shapes.
High temperature microheaters have been designed and constructed to reduce the background thermal emission radiation produced by the heater. Such heaters allow one to probe luminescence with very low numbers of photons where the background emission would overwhelm the desired signal. Two methods to reduce background emission are described: one with low emission materials and the other with interference coating design. The first uses platforms composed of material that is transparent to mid-infrared light and therefore of low emissivity. Heating elements are embedded in the periphery of the heater. The transparent platform is composed of aluminum oxide, which is largely transparent for wavelengths less than about 8 μ m. In the luminescent microscopy used to test the heater, an optical aperture blocks emission from the heating coils while passing light from the heated objects on the transparent center of the microheater. The amount of infrared light transmitted through the aperture was reduced by 90% as the aperture was moved from the highly emissive heater coils at 450 \,^∘C to the largely transparent center at the same temperature. The second method uses microheaters with integrated multilayer interference structures designed to limit background emission in the spectral range of the low-light luminescence object being measured. These heaters were composed of aluminum oxide, titanium dioxide, and platinum and were operated over a large range of temperatures, from 50 \,^∘C to 600 \,^∘C. At 600 \,^∘C, they showed a background photon emission only 1/800 that of a comparison heater without the multilayer interference structure. In this structure, the radiation background was sufficiently reduced to easily monitor weak thermoluminescent emission from CaSO 4 :Ce,Tb microparticles.
While there are innumerable devices that measure temperature, the nonvolatile measurement of thermal history is far more difficult, particularly for sensors embedded in extreme environments such as fires and explosions. In this review, an extensive analysis is given of one such technology: thermoluminescent microparticles. These are transparent dielectrics with a large distribution of trap states that can store charge carriers over very long periods of time. In their simplest form, the population of these traps is dictated by an Arrhenius expression, which is highly dependent on temperature. A particle with filled traps that is exposed to high temperatures over a short period of time will preferentially lose carriers in shallow traps. This depopulation leaves a signature on the particle luminescence, which can be used to determine the temperature and time of the thermal event. Particles are prepared—many months in advance of a test, if desired—by exposure to deep ultraviolet, X-ray, beta, or gamma radiation, which fills the traps with charge carriers. Luminescence can be extracted from one or more particles regardless of whether or not they are embedded in debris or other inert materials. Testing and analysis of the method is demonstrated using laboratory experiments with microheaters and high energy explosives in the field. It is shown that the thermoluminescent materials LiF:Mg,Ti, MgB4O7:Dy,Li, and CaSO4:Ce,Tb, among others, provide accurate measurements of temperature in the 200 to 500 \,^∘C range in a variety of high-explosive environments.
A new instrument for high-resolution optical logging has been built and tested in Antarctica. Its purpose is to obtain records of volcanic products and other scattering features, such as bubbles and impurities, preserved in polar ice sheets, and it achieves this by using long wavelength near-infrared light that is absorbed by the ice before many scattering events occur. Longer wavelengths ensure that the return signal is composed primarily of a single or few backscattering event(s) that limit its spatial spread. The compact optical logger features no components on its body that draw power, which minimizes its size and weight. A prototype of the logger was built and tested at Siple Dome A borehole, and the results were correlated with prior optical logging profiles and records of volcanic products from collected ice core samples.
The present work describes the procedures and results from the first temperature measurements in closed chamber detonations obtained using the thermoluminescence (TL) of particles specifically developed for temperature sensing. Li2B4O7:Ag,Cu (LBO), MgB4O7:Dy,Li (MBO) and CaSO4:Ce,Tb (CSO) were tested separately in a total of 12 independent detonations using a closed detonation chamber at the Naval Surface Warfare Center, Indian Head Explosive Ordnance Disposal Technology Division (NSWC IHEODTD). Detonations were carried out using two different explosives: a high temperature plastic bonded explosive (HPBX) and a low temperature plastic bonded explosive (LPBX). The LPBX and HPBX charges produced temperatures experienced by the TL particles to be between ~550–670 K and ~700–780 K, respectively, depending on the shot. The measured temperatures were reproducible and typically higher than the thermocouple temperatures. These tests demonstrate the survivability of the TL materials and the ability to obtain temperature estimates in realistic conditions, indicating that TL may represent a reliable way of estimating the temperature experienced by free-flowing particles inside an opaque post-detonation fireball.
Thermal effects in optical substrates are vitally important in determining laser damage resistance in long-pulse and continuous-wave laser systems. Thermal deformation waves in a soda-lime-silica glass substrate have been measured using high-speed interferometry during a series of laser pulses incident on the surface. Two-dimensional images of the thermal waves were captured at a rate of up to six frames per thermal event using a quantitative phase measurement method. The system comprised a Mach–Zehnder interferometer, along with a high-speed camera capable of up to 20,000 frames-per-second. The sample was placed in the interferometer and irradiated with 100 ns, 2 kHz 𝑄-switched pulses from a high-power Nd:YAG laser operating at 1064 nm. Phase measurements were converted to temperature using known values of thermal expansion and temperature-dependent refractive index for glass. The thermal decay at the center of the thermal wave was fit to a function derived from first principles with excellent agreement. Additionally, the spread of the thermal distribution over time was fit to the same function. Both the temporal decay fit and the spatial fit produced a thermal diffusivity of 5×10E-7 m^2/s.
Incorporation of bismuth telluride/antimony telluride co-sputtered thermoelectric junctions into MEMS devices requires process developments for patterning and encapsulation as well as characterization of properties such as film stress and contact resistance. Test structures are presented for measuring important thermoelectric properties, resistivity, thermal conductivity, carrier concentration, and Seebeck coefficient. A fabrication process is presented that allows the junctions to be deposited, patterned, encapsulated, and etch released. Measurement of the thermoelectric junctions reveals a room temperature figure of merit, Z_T, of 0.43 with a total Seebeck coefficient difference of 150 μV/K, resistivities of 17.4 and 7.6 μΩ-m, and thermal conductivity of 0.34 and 0.30 W/mK for antimony telluride and bismuth telluride, respectively. The junctions have been incorporated into state of the art uncooled thermopile infrared detectors with a peak detectivity of 3 × 10^9 cm*Hz^(1/2)/W.
A model is presented and confirmed experimentally that explains the anomalous behavior observed in continuous wave (CW) excitation of thermally isolated optics. Distributed Bragg Reflector (DBR) high reflective optical thin film coatings of HfO2 and SiO2 were prepared with a very low absorption, about 7 ppm, measured by photothermal common-path interferometry. When illuminated with a 17 kW CW laser for 30 s, the coatings survived peak irradiances of 13 MW/cm2, on 500 μm diameter spot cross sections. The temperature profile of the optical surfaces was measured using a calibrated thermal imaging camera for illuminated spot sizes ranging from 500 μm to 5 mm; about the same peak temperatures were recorded regardless of spot size. This phenomenon is explained by solving the heat equation for an optic of finite dimensions and taking into account the non-idealities of the experiment. An analytical result is also derived showing the relationship between millisecond pulse to CW laser operation where (1) the heating is proportional to the laser irradiance (W/m2) for millisecond pulses, (2) the heating is proportional to the beam radius (W/m) for CW, and (3) the heating is proportional to W/m⋅tan−1(t√/m) in the transition region between the two.
Most thermoluminescent materials are created using crystal growth techniques; however, it would be of great utility to identify those few thermoluminescent materials that can be deposited using simpler methods, for example to be compatible with the early portions of a silicon integrated circuit or microelectromechanical fabrication process. In this work, thin films of yttrium oxide with a terbium impurity (Y2O3:Tb) were deposited on silicon wafers by electron beam evaporation. The source for the Y2O3:Tb was made by combining Y2O3 and Tb4O7 powders. The approximate thicknesses of the deposited films were 350 nm. After deposition, the films were annealed at 1100 \,^∘C for 30 s to improve crystallinity. There is a strong correlation between the x-ray diffraction (XRD) peak intensity and the thermoluminescent glow curve intensity. The glow curve displays at least two peaks at 140 \,^∘C and 230 \,^∘C. The emission spectra was measured using successive runs with a monochromator set to a different wavelength for each run. There are two main emission peaks at 490 nm and 540 nm. The terbium impurity concentration of approximately 1 mol% was measured using Rutherford backscattering spectrometry (RBS). The Y2O3:Tb is sensitive to UV, x-ray, and gamma radiation. The luminescent intensity per unit mass of UV irradiated Y2O3:Tb was about 2 times that of x-ray irradiated TLD-100.
On extremely small scales, traditional microcooler performance estimates must be corrected to include losses due to radiation. We present a method for analysis of microcoolers having a significant radiative contribution to their thermal conductance. We have fabricated ultrasmall microcoolers from sputtered Bi2Te3/Sb2Te3 thermoelectric junctions with cooling volumes of 200 μm × 200 μm × 65 nm, which we believe to be the smallest microcoolers ever made. The devices are highly thermally isolated with total thermal conductance under 5 × 10−7 W/K in vacuum at room temperature. By fitting the temperature response to input power of the devices in vacuum, we have quantified the nonlinearity of the response to calculate the radiative and film contributions to the total thermal conductance of the device. Three device geometries are presented, with radiative contributions to thermal conductance of 15%, 26%, and 100% depending on their emissive area and support structure. The cooling capabilities of these devices are also measured with maximum cooling of 3.1 K for the 15% radiation-limited device and 2.6 K for the 26% radiation-limited device, with power consumptions below 5 μW.
The laser-damage thresholds of single material and nanolaminate thin films were compared under continuous-wave (CW) illumination conditions. Nanolaminate films consist of uniform material interrupted by the periodic insertion of one or more atomic layers of an alternative material. Hafnia and titania were used as the base materials, and the films were deposited using atomic-layer deposition. The nanolaminates were less polycrystalline than the uniform films, as quantified using x-ray diffraction. It was found that the nanolaminate films had reduced laser-damage thresholds on smooth and patterned substrates as compared to uniform single-material films. This behavior is unusual as prior art indicates that amorphous (less polycrystalline) materials have higher laser-damage thresholds under short-pulse excitation. It is speculated that this may indicate that local thermal conduction affects breakdown more strongly under CW excitation than the dielectric properties that are important for short-pulse excitation.
A model is presented and confirmed experimentally that explains the anomalous behavior observed in continuous wave (CW) excitation of thermally isolated optics. Distributed Bragg Reflector (DBR) high reflective optical thin film coatings of HfO2 and SiO2 were prepared with a very low absorption, about 7 ppm, measured by photothermal common-path interferometry. When illuminated with a 17 kW CW laser for 30 s, the coatings survived peak irradiances of 13 MW/cm2, on 500 μm diameter spot cross sections. The temperature profile of the optical surfaces was measured using a calibrated thermal imaging camera for illuminated spot sizes ranging from 500 μm to 5 mm; about the same peak temperatures were recorded regardless of spot size. This phenomenon is explained by solving the heat equation for an optic of finite dimensions and taking into account the non-idealities of the experiment. An analytical result is also derived showing the relationship between millisecond pulse to CW laser operation where (1) the heating is proportional to the laser irradiance (W/m2) for millisecond pulses, (2) the heating is proportional to the beam radius (W/m) for CW, and (3) the heating is proportional to W/m⋅tan−1(t√/m) in the transition region between the two.
Strong correlations have been found in the polarization of light transmitted through a polycrystalline material and the grain sizes and orientations of that material. Experiments and supporting simulations with irregularly shaped single quartz crystals show that linear polarization is lost more rapidly as grain sizes decrease and the angular spread of the crystal orientations increase. A quantitative method using Stokes matrices to predict such changes is described and experimentally verified using an apparatus to vary the orientation of irregular quartz crystals. Grain sizes are varied between 1mm and 4mm, and the angular spreads in the crystal orientation are varied between 9\,^∘ and 27\,^∘. This technique has applications to identify changes in crystal structure of transparent uniaxial polycrystalline materials, especially in the nondestructive characterization of glacial ice.
Micromachined structures that repeatedly contact their substrate must be actuated in such a way to minimize stiction. A study of electrostatic actuation for adaptive thermal detectors has been performed that shows only a few tenths of a volt can separate normal actuation, snap-down without stiction, and snap-down with stiction. The applied voltage needed to achieve snap-down without stiction is largely constant if the pulse width exceeds the mechanical response time, but increases dramatically if the pulse is applied for a shorter time.
A continuously tunable vertical actuator with subnanometer resolution is presented. It consists of a heterostructure cantilever which has collapsed over a 125 nm thick nanogap. Its operating principle relies on the temperature dependence of the adhesion energy between two InGaAs surface quantum wells. Deflections from −17 to 5 nm with a precision better than three atomic layers have been measured.
The adhesion energy is measured between InGaAsquantum wells that have collapsed across a 125 nm air gap in an InP/InGaAs heterostructure. The method relies on measuring the unadhered length and shape of collapsed microcantilevers with optical interferometry. The adhesion energy is found to be 72 \pm 16 mJ m^-2 . Since the air gap is much smaller than has been measured previously, the influence of van der Waals forces across the gap was included in theoretical modeling. It was found that the forces should not cause significant deviation from the standard adhesion models unless the adhesion energy drops below 25 mJ m^-2 .
The thermal contact conductance (TCC) of microactuated mechanical interfaces has been characterized using an electronic technique, where micromachined test structures were heated with a current and the TCC was inferred from the change in resistance. For every device tested, the TCC was higher in vacuum than in air. This is in stark contrast to the behavior of bulk interfaces, and several experiments suggest that it may be the result of a decreased solid–solid contact area in air caused by the pressure of the interstitial gas. The average effective TCC of a polysilicon/nitride interface brought together by electrostatic actuation varies about values of 6.0×10 4 W /( K m 2 ) in air and 9.5×10 4 W /( K m 2 ) under vacuum for applied pressures of 1 MPa. These values are significantly higher than commonly reported for nonmetallic materials and probably reflect the very smooth surfaces of depositedthin films.
With the recent interest in adaptive IR imaging,focal plane arrays are desired that can operate linearly over an enormous dynamic range. Unfortunately, large signals can cause thermal detectors to operate at temperatures significantly above their ambient resulting in intensity dependent performance or even device damage. In this letter, the responsivity of microbolometer devices is controlled using the detector and substrate as a simple electrostatic actuator. Microbolometers are demonstrated to switch between states that are over a factor of 50 apart in responsivity. The limits of the switching are theoretically separated by four to five orders of magnitude. In addition, intermediate values of responsivity can be obtained by designing devices in which the support beams snap down at lower voltage than the detector plate. Combining this idea with the pressure dependence of the thermal contact conductance, continuous thermal conductance tuning over a factor of 3 is demonstrated.
Free carrier generation due to the presence of absorbing contaminants is considered as a primary mechanism for initiating continuous-wave laser damage in low absorption, high bandgap optical coatings. Thermal, optical, and ion-generation are examined as means of exciting electrons above the bandgap of the material. Once electrons exist above the bandgap, they absorb incident light, causing a runway thermal-absorption process leading to material breakdown. Testing this theory, high reflectivity distributed Bragg reflectors and half-wave coatings were tested with a 17kW 1070nm continuous wave laser in the presence of carbon contamination. Damage thresholds of titania, tantala, hafnia, alumina, and silica were compared to theory and found to follow similar trends. In an effort to prevent laser damage, samples were conditioned using lower irradiance levels, prior to higher exposure. This was found to reduce absorption by up to 90% as well as increase damage thresholds by over an order of magnitude for some samples.
The minimum and maximum laser irradiances that are effective in cleaning etch-released membranes and microstructure was measured for carbon microparticles on LPCVD silicon nitride thin films and suspended platforms. A 1064nm Nd:Yag laser was scanned across the samples to ablate the contaminants. Microscope images of the membranes showed mass removal for an irradiance as low as 600W cm-2. More thorough cleaning was achieved by increasing the irradiance. For 2.5×2.5mm, 205nm thin silicon nitride membranes, 79% of the contaminated area was removed with an exposure of 3.7kW cm-2. Catastrophic damage was seen at a power level of 8.4kW cm-2. Ablation effects were also measured as a change in optical absorption using a photo thermal common-path interferometer. Peak absorption values were decreased from over 100,000ppm to less than 20,000ppm. Silicon nitride platforms were also tested. Despite significant substrate heating, the platforms survived intact up to power levels of 8.4kW cm-2 with near perfect cleaning of carbon particles from their surfaces.
Infrared-transparent microheaters have been constructed to reduce the background blackbody radiation produced by the heater. Among other applications, such heaters allow one to probe the high temperature peaks of thermoluminescent(TL) materials. The microheater consists of peripheral platinum heating elements on a mid-infrared transparent alumina platform. Alumina has a relatively low blackbody signal at high temperature for wavelengths less than 8μm. To test the reduced blackbody emission, an aperture was placed over the heating coils and then the transparent center of the microheater. The amount of infrared light transmitted through the aperture was reduced by 90% as the aperture moved from the highly emissive heater coils at 450\,^∘C to the largely transparent center at the same temperature.
The blackbody radiation limit has traditionally been set forth as the ultimate performance limit of thermal detectors. However, this fundamental limit assumes that the detector absorbs uniformly throughout the thermal spectrum. In much the same way as photon detectors can achieve very high D* because they do not absorb photon energies below their bandgap, so too can thermal detectors except that thermal detectors are not limited to cryogenic operation. In both cases, the enhanced theoretical D* is achieved because the radiation noise is reduced in a device that does not absorb at a uniform high level throughout the thermal emission band. There are multiple ways to achieve high D* in thermal detectors. One is to use materials that absorb only in a certain spectral range, just as in photon detectors. For example a detector made from PbSe, with proper optical coupling, absorbs only photons with wavelengths shorter than 4.9μm. The radiation limited detectivity of such a device can theoretically exceed 9 x 1010cmHz1/2/W in the MWIR. Even with Johnson and 1/f noise estimates included, it can still exceed 2.5x1010cmHz1/2/W in the MWIR. Another technique, applicable for narrowband thermal detectors, is probably even more powerful. Consider a thermal detector that is almost completely transparent. Here, the radiation noise has been reduced but the signal has been reduced even more. However, if the device is now placed inside an optical cavity, then at one wavelength and in one direction, the nearly transparent detector couples to the cavity resonance to absorb at 100%. Radiation from all other wavelengths and directions are rejected by the cavity or are absorbed only weakly by the detector. It is shown that theoretically, the D* of these devices are roughly proportional to the inverse square root of the spectral resonant width under certain conditions. It is also shown that even including Johnson noise and 1/f noise, the practically achievable D* approaches or exceeds 1011 cmHz^1/2/W.
Microbolometers and other thermal detectors have traditionally been limited to seeing objects in a broad wavelength band at a single sensitivity. Recent advances in interface heat transfer and optical cavity design promise to change that. In this paper, we present recent work on thermal infrared detectors with tunable responsivity and wavelength. First, we demonstrate that extended dynamic range in thermal detectors can be achieved by electrostatically bringing a portion of the detector support structure in contact with the substrate. The exact amount of heat transfer can be controlled by adjusting the contact area and pressure. The thermal conductance and responsivity can be switched more than an order of magnitude using this technique. Next, we demonstrate that a wavelength tunable device in the LWIR can be achieved by modifying the structure of a microbolometer to incorporate a modified Gires Tournois optical cavity. The cavity couples light at a single wavelength into the microbolometer while other wavelengths are rejected. We demonstrate that resonance can be tuned from 8.7 to 11.1 μm with applied voltages from 0 to 42 V. The FWHM of the resonance can be switched between around 1.5 μm in a narrow-band mode and 2.83 μm in a broad-band mode.
An electrostatic actuation scheme for creating a soft contact is described and applied to cantilevers and doubly clamped beams. The actuation voltage form is composed of a high voltage pulse with a low DC voltage. The voltage pulse height and duration is mapped for achieving minimum impact force, and the results are well predicted by the simulation. Furthermore, a new phenomenon is observed for clamped beams that pulse longer than the soft-landing requirement bounces the beam back rather than stick.
This paper covers the design, fabrication, and testing of a step-wise tunable long-wavelength infrared filter. The design uses a modified type of microbolometer to filter infrared light in the 7-10 micrometer wavelength range. Fabrication of the device is accomplished using standard microelectronic and microelectromechanical systems (MEMS) fabrication techniques. Germanium and zinc sulfide are used as optical layers for good reflectivity in the desired infrared range. Testing of the device shows that it filters light in three absorption bands in the specified wavelength range. The tuning of the device is found to be step-wise in response to electrostatic actuation using voltages up to 100 volts.
Multiwavelength adaptive optic systems can experience phase errors when corrections measured at one wavelength are applied to others. Such errors originate from several sources, but poor optical coating design can play a major role since the phase dependence of any coating varies greatly with wavelength, angle, and thickness. To address these errors, a multilayer coating design is proposed where the effective electric field penetration depth into the coating is similar at all design wavelengths. Using this method, a two-wavelength coating is designed for 1070 and 1550 nm and compared to a standard two-wavelength high-reflectivity coating consisting of stacked Bragg reflectors. The standard stacked Bragg reflector design induces up to 173.0 nm of phase error across the wavelength band that reflects off the more deeply buried part of the multilayer. Alternatively, the two-wavelength equal field penetration design maintains a high reflectivity of 99.999%, while only inducing phase variations of 28.6 nm for both target wavelengths.
The use of a laser to cut or drill ice has been proposed and demonstrated multiple times in previous decades as a novel, but never adopted, machining tool in glaciology and paleoclimate studies. However, with the rapid development of high power fiber-laser technology over the past few decades, it is timely to perform further studies using this new tool. An investigation is made herein on the cutting of ice using a Yb-doped fiber laser emitting at a wavelength of 1070 nm, the most extensively developed and highest power fiber laser technology, in pulsed and continuous-wave operation. Visible-light observations of clear tap water ice samples, moving at a constant velocity relative to a pulsed laser beam, demonstrate a linear relationship between the duration of a millisecond-range laser pulse and the depth of the meltwater-free cut formed in response. Thermal imaging of the irradiated face shows that peripheral heating trends linearly for pulse lengths greater than 5 ms. A comparison of pulse trains with a constant time-averaged power suggests that shorter pulses are advantageous in slot-cutting efficiency and in minimizing visible alterations in the surrounding ice. These results demonstrate the viability of powerful fiber-compatible lasers as a tool for ice sample retrieval and processing.
An uncooled detector has reached the thermodynamic temperature fluctuation limit, such that 98% of its total noise consisted of phonon and photon fluctuations of the detector body. The device has performed with a detectivity of 3.8\texttimes109 c m H z/W, which is the highest reported for any room temperature device operating in the long-wave infrared (λ∼8-12\textmum). The device has shown a noise-equivalent temperature difference of 4.5 mK and a time constant of 7.4 ms. The detector contains a subwavelength perforated absorber with an absorption-per-unit-thermal mass-per-area of 1.54\texttimes1022 k g -1 m -2, which is approximately 1.6–32.1 times greater than the state-of-the-art absorbers reported for any infrared application. The perforated absorber membrane is mostly open space, and the solid portion consists of Ti, S i N x, and Ni layers with an overall fill factor of ∼28%, where subwavelength interference, cavity coupling, and evanescent field absorption among units induce the high absorption-per-unit-thermal mass-per-area. Readout of the detector occurs via infrared-absorption-induced deformation using a Mach–Zehnder interferometry technique (at λ=633n m), chosen for its long-term compatibility with array reads using a single integrated transceiver.
Silica aerogels synthesized by different techniques have been studied for their electrical and thermal insulating properties, mostly in their bulk structures; however, the optical properties of a silica aerogel thin film have remained largely unexplored. Due to their high porosities, silica aerogel thin films may be useful as very low-index optical materials, especially in multilayer coatings for high intensity and high-power lasers, where the large bandgap of silica is particularly valuable. In this paper, silica aerogel thin films were fabricated by spin-coating a silica sol, derived from a two-step acid/base catalyzed technique at an ambient pressure, on fused silica substrates. The films showed very low refractive indices (n) around 1.1 (approximately 72% porosity) and low absorptions between about 6 and 16ppm, lower than plasma-enhanced chemical vapor deposition (PECVD) comparison films. Optical scatterings of the silica aerogel films were measured and found to be comparable to a PECVD silica film and a bare fused silica substrate, with most films showing slightly higher scattering but one film showing lower. The laser-induced damage thresholds (LIDT) of all films were measured using carbon particle contamination, which allows testing over statistically large areas using continuous wave (CW) laser illumination. The LIDT of silica aerogels was similar to that of pure high-density silica and much higher than that of other common high-LIDT films such as hafnia and alumina. Most damage spots on silica aerogel samples only showed slight discoloration at irradiance levels of 150 kW/cm2 (1.5 \texttimes 109 W/m2), while similar tantala high reflectivity coatings failed catastrophically at 75 or 86 kW/cm2 (7.5 \texttimes 108 or 8.6 \texttimes 108 W/m2). Moreover, aerogels performed somewhat better than PECVD silica, where most damage occurred at a lower irradiance level of 100 kW/cm2 (1 \texttimes 109 W/m2) with a larger discolored spot.
The absorption-to-mass ratio of the infrared arrays is enhanced to ∼1.33-7.33 times larger than the previously reported structures by incorporating two design characteristics: first, the coupling of evanescent fields in the air gaps around pixels to create effectively larger pixel sizes and, second, the use of guided-mode resonance (GMR) within the subwavelength metal–dielectric gratings. The bilayer Ti-Si3N4 gratings achieve broadband long-wave infrared (LWIR, λ∼8-12\textmum) absorption by the combined effects of free carrier absorption by the thin Ti films and vibrational phonon absorption by the thick Si3N4 films. In the presence of GMR, this broadband absorption can be enormously enhanced even with low fill factor subwavelength grating cells. Further, the spacing and design of the cells can be modified to form a pixel array structure that couples the light falling in the air gaps via evanescent field coupling. Calculations are performed using the finite difference time domain technique. Excellent broadband absorption is observed for the optimized arrays, yielding maximum absorption of 90% across the LWIR and an average absorption per unit mass (absorption/mass) per pixel of 3.45\texttimes1013kg-1.
Optical multilayers created from a single material have been demonstrated to have lower stress and lower thermal expansion mismatch than standard two-material coatings; however, questions of high power operation and vulnerability to environmental contamination remain. This study examines the particle-induced laser damage properties of all-silica high reflectivity multilayers, specifically those for high average power illumination. All-silica infrared reflectors were deposited by oblique angle deposition (OAD). All-silica mirrors showed low overall stress and had smaller stress changes with temperature. Their laser-induced damage thresholds (LIDTs) under high-power continuous wave (CW) laser illumination with carbon particulate contamination are significantly higher than corresponding high reflectivity multilayers composed of two materials.
The scattered light distribution of surfaces in the long-wave infrared (λ∼8-12\textmum) is measured using a small set of thermal camera images. This method can extract scatter patterns considerably faster than standard laboratory bidirectional reflectance distribution function measurements and is appropriate for passive homogeneous surfaces. Specifically, six images are used in this study, each taken with respect to a thermal light source at an angle ranging from 10^∘ to 60^∘ to the normal of the surface. This data is deconvolved with the shape of the light source to estimate the scattering pattern. Both highly specular (black Masonite) and diffuse (painted drywall) surfaces are tested. Errors between the estimated scattering distribution and a directly measured one using a goniometer stage and quantum-cascade laser (QCL) are less than or equal to 3% except for extremely specular surfaces where viable QCL measurements cannot be made due to the increased relative contribution of speckle noise.
An aberrated wavefront incident upon an optical resonator will excite higher order spatial modes in the cavity, and the spectral width and distribution of these modes are indicative of the type and magnitude of the aberration. We apply this concept to atmospheric turbulence modeled by the Kolmogorov distribution. The spectral widths of the cavity transmission spectra are demonstrated via simulations to correspond to the structure constant that characterizes the variation in the optical index of refraction and thus the turbulence strength. Such a relationship can be harnessed to build a sensor for simply and quickly assessing optical turbulence strength.
High quality factor (Q) photonic devices in the room temperature thermal infrared region, corresponding to deeper long-wave infrared with wavelengths beyond 9 microns, have been demonstrated for the first time. Whispering gallery mode diamond microresonators were fabricated using single crystal diamond substrates and oxygen-based inductively coupled plasma (ICP) reactive ion etching (RIE) at high angles. The spectral characteristics of the devices were probed at room temperature using a tunable quantum cascade laser that was free space-coupled into the resonators. Light was extracted via an arsenic selenide (As2Se3) chalcogenide infrared fiber and directed to a cryogenically cooled mercury cadmium telluride (HgCdTe) detector. The quality factors were tested in multiple microresonators across a wide spectral range from 9 to 9.7 microns with similar performance. One example resonance (of many comparables) was found to reach 3648 at 9.601 µm. Fourier analysis of the many resonances of each device showed free spectral ranges slightly greater than 40 GHz, matching theoretical expectations for the microresonator diameter and the overlap of the whispering gallery mode with the diamond.
Germanium is one of the most commonly used materials in the longwave infrared (λ∼8-12\textmum), but ironically, its absorption coefficient is poorly known in this range. An infrared photothermal common-path interferometry system with a tunable quantum cascade pump laser is used to measure the absorption coefficient of >99.999% pure undoped germanium as a function of wavelengths between 9 and 11 \textmum, varying between about 0.15 and 0.45 cm^-1 over this range.
Laser-Induced optical breakdown often occurs unexpectedly at optical intensities far lower than those predicted by ultra-short pulse laser experiments, and is usually attributed to contamination. To determine the physical mechanism, optical coatings were contaminated with carbon and steel microparticles and stressed using a 17 kW continuous-wave laser. Breakdown occurred at intensity levels many orders of magnitude lower than expected in clean, pristine materials. Damage thresholds were found to strongly follow the bandgap energy of the film. A thermal model incorporating the particle absorption, interface heat transfer, and free carrier absorption was developed, and it explains the observed data, indicating that surface contamination heated by the laser thermally generates free carriers in the films. The observed bandgap dependence is in direct contrast to the behavior observed for clean samples under continuous wave and long-pulse illumination, and, unexpectedly, has similarities to ultra-short pulse breakdown for clean samples, albeit with a substantially different physical mechanism.
This paper presents a theoretical and experimental investigation of photon diffusion in highly absorbing microscale graphite. A Nd:YAG continuous wave laser is used to heat the graphite samples with thicknesses of 40 μm and 100 μm. Optical intensities of 10 kW cm-2 and 20 kW cm-2 are used in the laser heating. The graphite samples are heated to temperatures of thousands of kelvins within milliseconds, which are recorded by a 2-color, high speed pyrometer. To compare the observed temperatures, differential equation of heat conduction is solved across the samples with proper initial and boundary conditions. In addition to lattice vibrations, photon diffusion is incorporated in the analytical model of thermal conductivity for solving the heat equation. The numerical simulations showed close matching between experiment and theory only when including the photon diffusion equations and existing material properties data found in the previously published works with no fitting constants. The results indicate that the commonly-overlooked mechanism of photon diffusion dominates the heat transfer of many microscale structures near their evaporation temperatures. In addition, the treatment explains the discrepancies between thermal conductivity measurements and theory that were previously described in the scientific literature.
It is shown that an aberrated wavefront incident upon a Fabry-Perot optical cavity excites higher order spatial modes in the cavity and that the spectral width and distribution of these modes is indicative of the type and magnitude of the aberration. The cavities are purely passive, and therefore frequency content is limited to that provided by the original light source. To illustrate this concept, spatial mode decomposition and transmission spectrum calculation are simulated on an example cavity; the effects of various phase delays, in the form of two basic Seidel aberrations and a composite of Zernike polynomial terms, are shown using both Laguerre-Gaussian and plane wave incident beams. The aggregate spectral width of the cavity modes excited by the aberrations is seen to widen as the magnitude of the aberrations/ phase delay increases.
It is shown that an aberrated wavefront incident upon a Fabry-Perot optical cavity excites higher order spatial modes in the cavity, and that the spectral width and distribution of these modes is indicative of the type and magnitude of the aberration. The cavities are purely passive and therefore frequency content is limited to that provided by the original light source, unless time-varying content is introduced. To illustrate this concept, spatial mode decomposition and transmission spectrum calculation are simulated on an example cavity; the effects of various phase delays, in the form of two basic Seidel aberrations and a composite of Zernike polynomial terms, are shown using both Laguerre-Gaussian and plane wave incident beams. The aggregate spectral width of the excited cavity modes is seen to widen as the magnitude of the phase delay increases.
Typically when microsensor particles are used to monitor temperatures in harsh environments, a statistical number of particles are collected and measured in aggregate. While this method is undoubtedly the most practical method for the rapid acquisition of overall temperature data, the limitations of collective measurements of numbers of particles versus individual ones has never been explored. In this paper, the thermoluminescent (TL) magnesium borate microparticles are used to measure temperatures inside the periphery of explosions with collective and individual behavior contrasted. It is found that individual measurement indicate that some particles undergo extreme temperature while others seem to have had no exposure to high temperature at all. The microparticles were irradiated with 200Gy of gamma radiation to fill the traps in the band gap. Several grams of the irradiated microparticles were placed at various distances from the source of the detonation. After the microparticles were collected the TL curve was measured for microparticles that were in the detonation and a control group of microparticles not in the detonation. The TL curve of an individual microparticle was measured by placing the microparticle ranging in size from 25 μm to 75 μm on a microheater with an area of 300 μm \texttimes 300 μm. The microheater was then used to heat the microparticle at a linear rate while the thermoluminescence of the microparticle was measured. A summation of first-order kinetics curves were used to do a fit to the thermoluminescence curves of the microparticles that were in the detonation and the control group. By comparing the ratio of first-order kinetic curve peaks of the particles that were in the detonation to the control group the temperature that the particles in the explosion were calculated. This process was carried out for many different particles that were in the same detonation and collected from the same location in the explosive test chamber. Individual extracted temperatures from the microparticles show a large distribution ranging from room temperature to 516^∘C, but in aggregate, the microparticles show a clustering of temperatures around 290^∘C.
A Yb-doped fiber laser is used to accelerate and evaporate absorbing particles in air. Optical intensities of 1MW/cm2 and 2MW/cm2 illuminate stainless steel particles. These particles are accelerated to velocities of tens of meters per second before evaporating within a few tenths of a millisecond. Position measurements are made using direct imaging with a high-speed camera. A fundamental system of coupled differential equations to track particle momentum, velocity, mass, radius, temperature, vapor opacity, and temperature distribution is developed and shown to accurately model the trajectories and lifetimes of laser heated particles. Atoms evaporating from the particle impart momentum to the larger particle, which accelerates until it is slowed by drag forces. Heat transfer within the evaporating particles is dominated by radiation diffusion, a process that usually only dominates in astrophysical objects, for example in the photospheres of stars.
The optical absorption of contaminants on high reflectivity mirrors was measured using photo thermal common-path interferometry before and after exposure to high power continuous-wave laser light. The contaminants were micron-sized graphite flakes on hafnia-silica distributed Bragg reflectors illuminated by a ytterbium-doped fiber laser. After one-second periods of exposure, the mirrors demonstrated reduced absorption for irradiances as low as 11kWcm-2 and had an obvious threshold near 20kWcm-2. Final absorption values were reduced by up to 90% of their initial value for irradiances of 92kWcm-2. For shorter pulses at 34kWcm-2, a minimum exposure time required to begin absorption reduction was found between 100μs and 200μs, with particles reaching their final minimum absorption value within 300ms. Microscope images of the surface showed agglomerated particles fragmenting with some being removed completely, probably by evaporation for exposures between 200μs to 10ms. Exposures of 100ms and longer left behind a thin semi-transparent residue, covering much of the conditioned area. An order of magnitude estimate of the time necessary to begin altering the surface contaminants (also known as ”conditioning”) indicates about 200μs seconds at 34kWcm-2, based on heating an average carbon particle to its sublimation temperature including energy loss to thermal contact and radiation. This estimation is close to the observed exposure time required to begin absorption reduction.
A new instrument for high-resolution optical logging has been built and tested in Antarctica. Its purpose is to obtain records of volcanic products and other scattering features, such as bubbles and impurities, preserved in polar ice sheets, and it achieves this by using long wavelength near-infrared light that is absorbed by the ice before many scattering events occur. Longer wavelengths ensure that the return signal is composed primarily of a single or few backscattering event(s) that limit its spatial spread. The compact optical logger features no components on its body that draw power, which minimizes its size and weight. A prototype of the logger was built and tested at Siple Dome A borehole, and the results were correlated with prior optical logging profiles and records of volcanic products from collected ice core samples.
High temperature microheaters have been designed and constructed to reduce the background thermal emission radiation produced by the heater. Such heaters allow one to probe luminescence with very low numbers of photons where the background emission would overwhelm the desired signal. Two methods to reduce background emission are described: one with low emission materials and the other with interference coating design. The first uses platforms composed of material that is transparent to mid-infrared light and therefore of low emissivity. Heating elements are embedded in the periphery of the heater. The transparent platform is composed of aluminum oxide, which is largely transparent for wavelengths less than about 8 μ m. In the luminescent microscopy used to test the heater, an optical aperture blocks emission from the heating coils while passing light from the heated objects on the transparent center of the microheater. The amount of infrared light transmitted through the aperture was reduced by 90% as the aperture was moved from the highly emissive heater coils at 450 ^∘C to the largely transparent center at the same temperature. The second method uses microheaters with integrated multilayer interference structures designed to limit background emission in the spectral range of the low-light luminescence object being measured. These heaters were composed of aluminum oxide, titanium dioxide, and platinum and were operated over a large range of temperatures, from 50 ^∘C to 600 ^∘C. At 600 ^∘C, they showed a background photon emission only 1/800 that of a comparison heater without the multilayer interference structure. In this structure, the radiation background was sufficiently reduced to easily monitor weak thermoluminescent emission from CaSO 4 :Ce,Tb microparticles.
Spatial modulation spectroscopy (SMS) is a powerful method for interrogating single nanoparticles. In these experiments optical extinction is measured by moving the particle in and out of a tightly focused laser beam. SMS is typically used for particles that are much smaller than the laser spot size. In this paper, we extend the analysis of the SMS signal to particles with sizes comparable to or larger than the laser spot, where the shape of the particle matters. These results are important for the analysis of polydisperse samples that have a wide range of sizes. The presented example images and analysis of a carbon microparticle sample show the utility of the derived expressions. In particular, we show that SMS can be used to generate extinction cross-section information about micrometer-sized particles with complex shapes.
Solar selective coatings that absorb solar incident light but that inhibit emission of thermal light can have a dramatic impact on the performance of Direct Steam Generation (DSG) systems. In prior art, perfect coatings with instantaneous transitions between high absorption and low absorption have been modeled. In this paper non-ideal spectral, environmental, and material properties of solar selective coatings are shown to have significant effects on the efficiencies and optimum transition wavelengths of real systems. By introducing more real world parameters into the coating optimization it is desired that a more cost effective and efficient system can be built. It is shown that using ideal conditions the optimum transition wavelength for a DSG system is 1.4μm with an efficiency of 55.7%. Whereas for a realistic non-ideal DSG system the optimum transition is at 3.4μm resulting in an efficiency around 30% depending on the concentration factor used. It is then shown that an optimized selective coating will be outperformed by a simple non-selective black absorber with 95% absorption at concentration factors above 80 and above 130 when the AM1.5 spectrum is used.
Many current quantum optical systems, such as microcavities, interact with thermal light through a small number of widely separated modes. Previous theories for photon number fluctuations of thermal light have been primarily limited to special cases that are appropriate for large volumes or distances, such as single modes, many modes, or modes of uniform spectral distribution. Herein, a theory for the general case of spectrally dependent photon number fluctuations is developed for thermal light. The error in variance of prior art is quantitatively derived for an example cavity in the case where photon counting noise dominates. A method to reduce the spectral impact of this variance is described.
While there are innumerable devices that measure temperature, the nonvolatile measurement of thermal history is far more difficult, particularly for sensors embedded in extreme environments such as fires and explosions. In this review, an extensive analysis is given of one such technology: thermoluminescent microparticles. These are transparent dielectrics with a large distribution of trap states that can store charge carriers over very long periods of time. In their simplest form, the population of these traps is dictated by an Arrhenius expression, which is highly dependent on temperature. A particle with filled traps that is exposed to high temperatures over a short period of time will preferentially lose carriers in shallow traps. This depopulation leaves a signature on the particle luminescence, which can be used to determine the temperature and time of the thermal event. Particles are prepared—many months in advance of a test, if desired—by exposure to deep ultraviolet, X-ray, beta, or gamma radiation, which fills the traps with charge carriers. Luminescence can be extracted from one or more particles regardless of whether or not they are embedded in debris or other inert materials. Testing and analysis of the method is demonstrated using laboratory experiments with microheaters and high energy explosives in the field. It is shown that the thermoluminescent materials LiF:Mg,Ti, MgB4O7:Dy,Li, and CaSO4:Ce,Tb, among others, provide accurate measurements of temperature in the 200 to 500\,^∘C range in a variety of high-explosive environments.
A subsampling technique for real-time phase retrieval of high-speed thermal signals is demonstrated with heated metal lines such as those found in microelectronic interconnects. The thermal signals were produced by applying a current through aluminum resistors deposited on soda–lime–silica glass, and the resulting refractive index changes were measured using a Mach–Zehnder interferometer with a microscope objective and high-speed camera. The temperatures of the resistors were measured both by the phase-retrieval method and by monitoring the resistance of the aluminum lines. The method used to analyze the phase is at least 60\texttimes faster than the state of the art but it maintains a small spatial phase noise of 16 nm, remaining comparable to the state of the art. For slowly varying signals, the system is able to perform absolute phase measurements over time, distinguishing temperature changes as small as 2 K. With angular scanning or structured illumination improvements, the system could also perform fast thermal tomography.
This paper describes the physical processes that occur when high-power continuous-wave laser light interacts with absorbing particles on a low-absorption optical surface. When a particulate-contaminated surface is illuminated by high-power continuous-wave laser light, a short burst of light is emitted from the surface, and the particles rapidly heat over a period of milliseconds to thousands of degrees Celsius, migrating over and evaporating from the surface. The surviving particles tend to coalesce into larger ones and leave a relatively flat residue on the surface. The total volume of the material on the surface has decreased dramatically. The optical surface itself heats substantially during illumination, but the surface temperature can decrease as the material is evaporated. Optical surfaces that survive this process without catastrophic damage are found to be more resistant to laser damage than surfaces that have not undergone the process. The surface temperature of the conditioned surfaces under illumination is lower than that of unconditioned surfaces. These conditioning effects on particles occurred within the first 30 s of laser exposure, with subsequent laser shots not affecting particle distributions. High-speed photography showed the actual removal and agglomeration of individual particles to occur within about 0.7 ms. Elemental changes were measured using time-of-flight secondary ion mass spectroscopy, with conditioned residuals being higher in hydrocarbon content than pristine particles. The tests in this study were conducted on high-reflectivity distributed Bragg reflector coated optics with carbon microparticles in the size range of 20–50 μm, gold particles of size 250 nm, and silica 1 μm in size.
Thermal effects in optical substrates are vitally important in determining laser damage resistance in long-pulse and continuous-wave laser systems. Thermal deformation waves in a soda-lime-silica glass substrate have been measured using high-speed interferometry during a series of laser pulses incident on the surface. Two-dimensional images of the thermal waves were captured at a rate of up to six frames per thermal event using a quantitative phase measurement method. The system comprised a Mach–Zehnder interferometer, along with a high-speed camera capable of up to 20,000 frames-per-second. The sample was placed in the interferometer and irradiated with 100 ns, 2 kHz ⬚⬚-switched pulses from a high-power Nd:YAG laser operating at 1064 nm. Phase measurements were converted to temperature using known values of thermal expansion and temperature-dependent refractive index for glass. The thermal decay at the center of the thermal wave was fit to a function derived from first principles with excellent agreement. Additionally, the spread of the thermal distribution over time was fit to the same function. Both the temporal decay fit and the spatial fit produced a thermal diffusivity of 5\texttimes10E-7\, m^2/s.
Most thermoluminescent materials are created using crystal growth techniques; however, it would be of great utility to identify those few thermoluminescent materials that can be deposited using simpler methods, for example to be compatible with the early portions of a silicon integrated circuit or microelectromechanical fabrication process. In this work, thin films of yttrium oxide with a terbium impurity (Y2O3:Tb) were deposited on silicon wafers by electron beam evaporation. The source for the Y2O3:Tb was made by combining Y2O3 and Tb4O7 powders. The approximate thicknesses of the deposited films were 350 nm. After deposition, the films were annealed at 1100 ^∘C for 30 s to improve crystallinity. There is a strong correlation between the x-ray diffraction (XRD) peak intensity and the thermoluminescent glow curve intensity. The glow curve displays at least two peaks at 140 ^∘C and 230 ^∘C. The emission spectra was measured using successive runs with a monochromator set to a different wavelength for each run. There are two main emission peaks at 490 nm and 540 nm. The terbium impurity concentration of approximately 1 mol% was measured using Rutherford backscattering spectrometry (RBS). The Y2O3:Tb is sensitive to UV, x-ray, and gamma radiation. The luminescent intensity per unit mass of UV irradiated Y2O3:Tb was about 2 times that of x-ray irradiated TLD-100.
A method has been devised and tested for measuring the c-axis orientation of crystal grains in thin sections of glacier ice. The crystal orientation and grain size of ice are of great interest to glaciologists since these parameters contain information on the prior thermal and flow history of the ice. The traditional method of determining c-axis orientation involves a transmission measurement through an ice sample, a process that is time-consuming and therefore impractical for obtaining a continuous record. A reflection- or backscatter-based method could potentially be used inside boreholes, with bubbles as reflectors to avoid such drawbacks. The concept demonstration of this paper is performed on ice slices, enabling a direct comparison of accuracy with traditional methods. Measurements of the crystal orientations (ɸ, \texttheta) in 11 grains showed an average error of \textpm0.8^∘ in ɸ, with no grain error >1.4^∘. Measurements of \texttheta showed an average error of \textpm8.2^∘ on ten grains, with unexplained disagreement on the remaining grain. Although the technique is applied specifically to glacier ice, it should be generally applicable to any transparent birefringent polycrystalline material.
We have derived an expression for the effective absorbing area for thermal infrared detectors having non-zero absorption in the support legs, which is different from the geometric areas of the constituent detector elements. This technique is particularly applicable to devices where sensitivity is more important than fill-factor, as opposed to standard imaging arrays. The effective area can simply be substituted in standard equations to obtain a good estimate of the detector performance under uniform flood illumination conditions. The formalism can also be used for estimating the contributions of the individual signal generating elements to the total measured signal. This approximation has been tested for MEMS infrared detectors with thermoelectric readout operating under vacuum. The responsivity of the same device calculated using the effective area approximation and measured using a tightly constrained absorbing area are found to match very closely, within 5% over the most wavelengths and within 15% at the shortest thermal infrared wavelengths.
A model is presented and confirmed experimentally that explains the anomalous behavior observed in continuous wave (CW) excitation of thermally isolated optics. Distributed Bragg Reflector (DBR) high reflective optical thin film coatings of HfO2 and SiO2 were prepared with a very low absorption, about 7 ppm, measured by photothermal common-path interferometry. When illuminated with a 17 kW CW laser for 30 s, the coatings survived peak irradiances of 13 MW/cm2, on 500\,μm diameter spot cross sections. The temperature profile of the optical surfaces was measured using a calibrated thermal imaging camera for illuminated spot sizes ranging from 500\,μm to 5 mm; about the same peak temperatures were recorded regardless of spot size. This phenomenon is explained by solving the heat equation for an optic of finite dimensions and taking into account the non-idealities of the experiment. An analytical result is also derived showing the relationship between millisecond pulse to CW laser operation where (1) the heating is proportional to the laser irradiance (W/m2) for millisecond pulses, (2) the heating is proportional to the beam radius (W/m) for CW, and (3) the heating is proportional to W/m⋅tan-1(t\textsurd/m) in the transition region between the two.
Incorporation of bismuth telluride/antimony telluride co-sputtered thermoelectric junctions into MEMS devices requires process developments for patterning and encapsulation as well as characterization of properties such as film stress and contact resistance. Test structures are presented for measuring important thermoelectric properties, resistivity, thermal conductivity, carrier concentration, and Seebeck coefficient. A fabrication process is presented that allows the junctions to be deposited, patterned, encapsulated, and etch released. Measurement of the thermoelectric junctions reveals a room temperature figure of merit, Z_T, of 0.43 with a total Seebeck coefficient difference of 150 μV/K, resistivities of 17.4 and 7.6 μΩ-m, and thermal conductivity of 0.34 and 0.30 W/mK for antimony telluride and bismuth telluride, respectively. The junctions have been incorporated into state of the art uncooled thermopile infrared detectors with a peak detectivity of 3 \texttimes 10^9 cm*Hz^(1/2)/W.
Vertical electrostatic wedge actuators are described that control nanometer-scale gaps between surfaces. Standard parallel-plate electrostatic actuators become difficult to stabilize across extremely small gaps because the nature of the forces and the force laws that describe them often deviate from a Coulomb’s law dependence. In this work, a nanometer-scale air gap between a collapsed cantilever structure formed by two facing In0.53Ga0.47As surfaces, with areas of tens of microns, was controlled by a wedge electrostatic actuator. Upon actuation, the gap spacing between the surfaces was tuned over a maximum range of 55 nm with an applied voltage of 60 V.
We describe uncooled thermal detectors with a peak detectivity of at least 3 \texttimes109 cm \textsurdHz/W with spectrally selective absorption in the long-wave infrared. The spectral selectivity in absorption is achieved through resonant cavity coupling of a thin metal film with a low-order air-gap optical cavity. The electrical readout uses thermoelectric thin films with a Johnson noise limited performance. The detectors are of multiple sizes but those with 100- μm2 area have time constants of 58 ms and thermal conductances of 2.3 \texttimes10-7 W/K.
On extremely small scales, traditional microcooler performance estimates must be corrected to include losses due to radiation. We present a method for analysis of microcoolers having a significant radiative contribution to their thermal conductance. We have fabricated ultrasmall microcoolers from sputtered Bi2Te3/Sb2Te3 thermoelectric junctions with cooling volumes of 200 μm \texttimes 200 μm \texttimes 65 nm, which we believe to be the smallest microcoolers ever made. The devices are highly thermally isolated with total thermal conductance under 5 \texttimes 10-7 W/K in vacuum at room temperature. By fitting the temperature response to input power of the devices in vacuum, we have quantified the nonlinearity of the response to calculate the radiative and film contributions to the total thermal conductance of the device. Three device geometries are presented, with radiative contributions to thermal conductance of 15%, 26%, and 100% depending on their emissive area and support structure. The cooling capabilities of these devices are also measured with maximum cooling of 3.1 K for the 15% radiation-limited device and 2.6 K for the 26% radiation-limited device, with power consumptions below 5 μW.
The laser-damage thresholds of single material and nanolaminate thin films were compared under continuous-wave (CW) illumination conditions. Nanolaminate films consist of uniform material interrupted by the periodic insertion of one or more atomic layers of an alternative material. Hafnia and titania were used as the base materials, and the films were deposited using atomic-layer deposition. The nanolaminates were less polycrystalline than the uniform films, as quantified using x-ray diffraction. It was found that the nanolaminate films had reduced laser-damage thresholds on smooth and patterned substrates as compared to uniform single-material films. This behavior is unusual as prior art indicates that amorphous (less polycrystalline) materials have higher laser-damage thresholds under short-pulse excitation. It is speculated that this may indicate that local thermal conduction affects breakdown more strongly under CW excitation than the dielectric properties that are important for short-pulse excitation.
The laser-damage thresholds of single material and nanolaminate thin films were compared under continuous-wave (CW) illumination conditions. Nanolaminate films consist of uniform material interrupted by the periodic insertion of one or more atomic layers of an alternative material. Hafnia and titania were used as the base materials, and the films were deposited using atomic-layer deposition. The nanolaminates were less polycrystalline than the uniform films, as quantified using x-ray diffraction. It was found that the nanolaminate films had reduced laser-damage thresholds on smooth and patterned substrates as compared to uniform single-material films. This behavior is unusual as prior art indicates that amorphous (less polycrystalline) materials have higher laser-damage thresholds under short-pulse excitation. It is speculated that this may indicate that local thermal conduction affects breakdown more strongly under CW excitation than the dielectric properties that are important for short-pulse excitation.
The far field radiation efficiency achievable in narrowband thermal emitters is investigated, taking into account the full spatial and spectral variation of the emissivity. A coupled Fabry-Perot cavity model is used to develop an insight into the efficiency variation with cavity coherence and device temperature. It is found that the spatial variation of emissivity has to be explicitly included in the radiation power calculations to accurately estimate the achievable power efficiencies. The calculated radiation efficiencies of an ideal coherent cavity coupled emitter were found to vary from 0.1% to 9%, with a corresponding increase in the emission linewidth from 6.3 nm to 930 nm, and were much lower than that estimated without accounting for effects of spatial coherence. The analysis presented here can be used to determine the optimal operating temperature of a coherent thermal emitter once its emission characteristics and conduction losses are known and it is demonstrated that this optimum temperature is different from the temperature of peak blackbody emission at the resonant absorption wavelength.
We correct an equation, calculating the radiating power from a selective solar absorber, which is missing an extra factor of \pi. We also correct the results of the affected figures.
A model is presented and confirmed experimentally that explains the anomalous behavior observed in continuous wave (CW) excitation of thermally isolated optics. Distributed Bragg Reflector (DBR) high reflective optical thin film coatings of HfO2 and SiO2 were prepared with a very low absorption, about 7 ppm, measured by photothermal common-path interferometry. When illuminated with a 17 kW CW laser for 30 s, the coatings survived peak irradiances of 13 MW/cm2, on 500\,μm diameter spot cross sections. The temperature profile of the optical surfaces was measured using a calibrated thermal imaging camera for illuminated spot sizes ranging from 500\,μm to 5 mm; about the same peak temperatures were recorded regardless of spot size. This phenomenon is explained by solving the heat equation for an optic of finite dimensions and taking into account the non-idealities of the experiment. An analytical result is also derived showing the relationship between millisecond pulse to CW laser operation where (1) the heating is proportional to the laser irradiance (W/m2) for millisecond pulses, (2) the heating is proportional to the beam radius (W/m) for CW, and (3) the heating is proportional to W/m⋅tan-1(t\textsurd/m) in the transition region between the two.
A review is made of the physics and technology of spectrally selective thermal detectors, especially those operating at non-cryogenic temperatures. The background radiation noise fluctuations are rederived for arbitrary spectral characteristics. Infrared absorption due to phonons and free carriers is discussed followed by a review of published works on artificial infrared absorption materials such as patterned grids, nanoparticles, plasmonic structures, metamaterials and others. Subsequently, the literature of the spectral characteristics of broadband thermal detectors and spectrally selective thermal detectors is reviewed. Finally, the authors speculate on the directions that future research and development in the area will take regarding architectures, sensitivity and spectral characteristics.
Hafnia-alumina nanolaminates show improved smoothness and reduced crystallinity relative to pure hafnia in films formed by atomic layer deposition (ALD). However, typical nanolaminates also show reduced cross-plane thermal conductivity due to the much larger interface density relative to continuous films. We find that the interface thermal resistance in hafnia-alumina nanolaminates is very low and does not dominate the film thermal conductivity, which is 1.0 to 1.2 W/(m K) at room temperature in 100 nm thin films regardless of the interface density. Measured films had a number of interfaces ranging from 2 to 40, equivalent to interface spacing varying from about 40 to 2 nm. The degree of crystallinity of these films appears to have a much larger effect on thermal conductivity than that of interface density. Cryogenic measurements show good agreement with both the minimum thermal conductivity model for disordered solids and the diffuse mismatch model of interface resistance down to about 80 K before diverging. We find that the films are quite smooth through a 400:5 ratio of hafnia to alumina in terms of ALD cycles, and the refractive index scales as expected with increasing alumina concentration.
Strong correlations have been found in the polarization of light transmitted through a polycrystalline material and the grain sizes and orientations of that material. Experiments and supporting simulations with irregularly shaped single quartz crystals show that linear polarization is lost more rapidly as grain sizes decrease and the angular spread of the crystal orientations increase. A quantitative method using Stokes matrices to predict such changes is described and experimentally verified using an apparatus to vary the orientation of irregular quartz crystals. Grain sizes are varied between 1mm and 4mm, and the angular spreads in the crystal orientation are varied between 9^∘ and 27^∘. This technique has applications to identify changes in crystal structure of transparent uniaxial polycrystalline materials, especially in the nondestructive characterization of glacial ice.
Micromachined structures that repeatedly contact their substrate must be actuated in such a way to minimize stiction. A study of electrostatic actuation for adaptive thermal detectors has been performed that shows only a few tenths of a volt can separate normal actuation, snap-down without stiction, and snap-down with stiction. The applied voltage needed to achieve snap-down without stiction is largely constant if the pulse width exceeds the mechanical response time, but increases dramatically if the pulse is applied for a shorter time.
Mechanical position is used to control the wavelength of light emission of semiconductor heterostructures. The heterostructures are coupled across a gap that varies with position to tune electron states in much the same manner that optical cavities can be coupled across a tunable reflectivity mirror to control photon states. In the experiments, a SixN/InP cantilever containing an InGaAs surface well collapses over another InGaAs quantum well. The spacing between the wells varies along the cantilever, such that the heterostructure band gap is determined by the mechanical bending of the cantilever. Photoluminescence measurements of the coupled 200 Å surface wells show a wavelength shift of up to 22 nm. Associated theory shows that mechanical quantum coupling enables interband or intersubband devices with unprecedented spectral tuning ranges for gain or absorption.
A measurement technique, pulsed thermoluminescence, is described which uses short thermal pulses to excite trapped carriers leading to radiative recombination. The pulses are obtained using microstructures with ∼500 \textmus thermal time constants. The technique has many of the advantages of pulsed optically stimulated luminescence without the need for optical sources and filters to isolate the luminescent signal. Charge carrier traps in α-Al2O3:C particles on microheaters were filled using 205 nm light. Temperature pulses of 10 and 50 ms were applied to the heaters and compared with a standard thermoluminescence curve taken at a ramp rate of 5 K s-1. This produced curves of intensity verses temperature similar to standard thermoluminescence except shifted to higher temperatures. The luminescence of single particles was read multiple times with negligible loss of population. The lower limit of the duration of useful pulses appears to be limited by particle size and thermal contact between the particle and heater.
Ice fabric and microstructure–the crystal axis orientation and size, shape, and density of grains, bubbles, and other features–are vital to studies of ice core physical properties. Clues to ice sheet flow and temperatures, glacier stresses and responses, discontinuities and more can be deduced from the distribution and evolution of c-axes and grain boundaries. The traditional method to collect this data is by observing a glass-mounted sub-millimeter thin section through crossed optical polarizers while manipulating it on a four-axis motion stage. While this venerable technique, formalized by Rigsby, achieves unparalleled crystal-level accuracy and detail, it is also highly time- and labor-intensive: manually recording the c-axes within a single thin section can take a full day of researcher effort. Automated systems have followed in the decades since, but are generally large, expensive, lab-bound, and rare. We introduce a next-generation refinement of these stationary automated c-axis analyzers. Re-analysis of the fixed-angle measurement sequence described by Wilen allows full c-axis discrimination from three motion axes, rather than the previous four.Combined with extensive use of computer-aided design, rapid prototyping technologies, and modern sensing and processing, this yields a fully-motorized low-cost system capable of analyzing 4" thin sections, robustly packaged in and operable from a single 15" x 12" x 7" transport case. The hardware design is demonstrated, and software features currently under development are discussed.
This paper presents the design, fabrication, and analysis of a subwavelength perforated long-wave infrared (LWIR) broadband absorber, which realizes an average absorption of 92% with a thermal mass 1/3 – 1/24 times smaller than the state-of-the-art infrared absorbers.
Laser-driven light sails need to withstand very high intensities of incident light, and therefore must comprise low-loss materials that remain low loss with increasing temperatures. We will describe our measurements of temperature-dependent optical properties of materials (oxides, nitrides, semiconductors) for the development of metasurfaces for laser-driven light sails. We use oscillator-based models to fit ellipsometry data at different temperatures in the wavelength region where a precise measurement can be made, and revise these models with datapoints in the low-absorption region measured using photo-thermal common-path interferometry. We also demonstrate how metasurface performance is affected by the temperature-dependent properties of constituent materials.
Very low index silica aerogel thin films were deposited by spin-coating silica sols synthesized by a two-step acid/base catalyzed method. The film shows an index as low as 1.117 and an optical absorption of 10 ppm.
Scientific questions from continuous ice core records often require additional ice samples from targeted depths: large-volume sampling of microbial communities identified by fluorescence spectrometry probes, trapped gases or insoluble atmospheric compounds of a specific age, or simply to patch breaks in a larger core record. Near-infrared laser light, efficiently carried down an existing borehole by optical fiber, offers a uniquely compact and zero-force tool for excising samples from a sidewall. This promises an unprecedented ability to retrieve new samples of ice in a lightweight, maneuverable, and fast-deploying remote field system. To optimize laser pulse parameters–a technique long used in industrial laser machining to remove material without excess heating–and better understand how ice behaves under machining, an effective method is required of monitoring ice-laser interactions in situ. Observations of cavity formation over time have been made using ~5 cm, visually clear tap-water ice samples combined with a dark field lighting method. Static-position drilling tests with a 500 W 1070 nm fiber laser, semi-collimated to a 1 mm spot and modulated at a fixed frequency, show a conical hole front which advances at least 0.35 mm per 3 ms pulse, and farther with increasing pulse durations; however, small samples show an increased risk of thermal cracking when pulses exceed 10 ms. Slot cutting tests indicate that cut depth can be adjusted with pulse duration, frequency, and scan speed. Ongoing investigations, as well as potential next steps, will be discussed.
Cryospheric science continues to expand the variety, precision, and quantity of information that can be developed from ice cores. However, these advances all share one fundamental requirement: the availability of large, high-quality ice samples from the depths of interest. The logistical costs of drilling for new cores, and ironically the burgeoning variety of ice core experiments, severely constrain the volume of ice available for scientific work on any given depth. We introduce a prototype device which promises to quickly and economically retrieve new ice samples from boreholes of any provenance and geometry, whether drilled using electromechanical coring drills or rapid-access systems. The Robotics-Enhanced Laser Ice Collection system uses high-power 1070 nm laser light, generated at the surface and transmitted downhole by optical fiber, to cleanly incise wedges of ice from the sidewalls. The use of modern laser technology provides lower vibration, less contamination, and reduced mechanical complexity compared to mechanical saws, and holds advantages for sensitive chemical or biological sampling and the handling of brittle ice. To demonstrate this concept, we show examples of ongoing laboratory experiments on cutting and sampling using a 1 kilowatt fiber laser.
Brittle ice, highly pressurized material where gas becomes pressurized into clathrates, is extremely susceptible to cracking and breakage during even gentle handling. Lasers may cut glacier ice with lower external stresses than mechanical saws, thus allowing this usually-lost material to be sampled intact. In this presentation, we discuss our initial work in the RELIC program, particularly our findings on the optimal laser wavelength for ice sampling and testing acoustic sensors to measure the fracturing of brittle ice. In order to effectively cut glacier ice at useful scales, light must have an absorption length of at most a few centimeters and a powerful yet efficient source. We have successfully tested lasers at 1.07 and 10.6 microns, the two wavelengths with the most cost-effective technologies, for cutting ice. Both laser types demonstrated the ability to quickly cut thin, fragile pieces without cracking or shattering, raising the potential that the lack of mechanical vibration may make lasers inherently better than mechanical saws for retrieving and processing brittle ice. Lasers will also eliminate contamination from cutting blades and likely improve cut surface quality. In a compact replicate coring sonde or other field application, 1.07 micron lasers hold an overwhelming advantage over longer wavelengths because of the wide availability of robust, high-power, long-lifetime, portable sources and compatibility with low-loss optical fiber. To test the reactions of brittle ice, we have examined the acoustics of ice fracture by embedding piezoelectric sensors into artificial ice stressed by extreme temperature gradients, such as that provided by liquid nitrogen cooling. The sensors were found to generate easily-detectable voltage spikes with broad high-frequency spectra consistent with destructive cracking. Use of this effect will allow us to precisely monitor ice core conditions during laser cutting experiments to compare against traditional mechanical sampling, and opens up possibilities for constant in situ monitoring of fracturing in freshly retrieved ice cores.
Traditional design involves the spectrum of an optic, but mechanical and materials properties must be included for high CW power. High bandgaps and low stress maximize damage thresholds in the presence of dirt and particles.
Radiation diffusion was investigated as a heat transfer mechanism in highly absorbing microscale graphite. A 50 μm thick graphite sheet was heated by a 1064 nm Nd:YAG continuous wave (CW) laser with optical intensities of 10 kW/cm2 and 20 kW/cm2. Extremely high temperatures (i.e., ≥2000 K) were achieved on the graphite sheet within miliseconds, which were measured by a two-color pyrometer. The recorded temperatures were later compared with numerical solutions of differential heat conduction equation. A close match was found between numerical and experimental results only when radiation diffusion was incorporated in the thermal conductivity along with the lattice vibration.
SiO2 nano-columnar films were fabricated using oblique angle deposition and characterized for their optical and mechanical properties. The films showed high damage thresholds, low scattering, and an intriguing transition to low stress at lower densities.
Materials with high bandgaps are resistant to particle-induced breakdown under CW illumination because evaporating contaminants then generate fewer free carriers near the surface. Laser-accelerated atmospheric particles induce failure even more strongly than fixed surface particles.
Modern high-power lasers have been harnessed to assist in drilling through rock, but its potential advantages in cryospheric applications—low-fracture boreholes efficiently produced in a single season with no drilling fluid—have not been explored. To test the effectiveness of laser drilling in ice, cylindrical ice samples, approximately 10 cm in diameter and ~10–30 cm in length, were frozen from tap water and exposed to the 1064 nm output of a diode-pumped fiber laser (IPG Photonics) at the Penn State Electro-Optics Center. The beam was collimated to a 30 mm spot size and various levels of output power. The ice core was situated several meters away from the laser collimator, resting on a v-shaped block lined with paper towels, and tilted upwards at ~15o to attempt to trap meltwater. Testing was conducted at room temperature and observed through thermal and visible video cameras. During laser exposure, a cylindrical hole was observed to quickly form in the irradiated face of the cylinder, and meltwater to continually flow out of the deepening cavity. Drilling was achieved at rates increasing with laser power, from 0.25 cm/sec for 1 kW to 2.1 cm/sec for 10 kW. The inner walls of the drilled cavity were lined with noticeably soft ice chunks and “slush”. Ice of visibly shorter scattering length was measured to drill at a higher rate than less-clear ice, but also quickly lost its transparency, possibly due to laser light scattering and causing localized melting within the room-temperature ice. No fractured cores, apparatus damage due to scattered light, or other adverse events were encountered in these shots. Varying spot sizes, layered and highly-transparent ice cores, and vertically-directed drilling to gauge the effects of accumulated meltwater will be tests of particular interest for drilling and coring applications.
Drills using modern high-power lasers have been demonstrated in geological petroleum exploration, but uses for cryospheric science have been given little attention. Current ice drilling technology often requires many field seasons and a large logistical footprint. A faster and more efficient method of drilling through ice sheets could also obviate the need for drilling fluid, which prohibits biological and certain trace-elemental studies, and produce a vertical borehole with a reduced number of fractures. To gauge the feasibility of laser drilling in ice, cylindrical ice samples, approximately 10 cm in diameter and 10-30 cm in length, were frozen from tap water and exposed to the 1064 nm output of a diode-pumped fiber laser (IPG Photonics) at the Penn State Electro-Optics Center. The laser was collimated to a 30 mm spot size and operated at various levels of output power. In each test an ersatz ice core was placed on a small v-shaped block lined with paper towels, tilting it upwards at 15º in an attempt to trap meltwater, and situated coaxially with and several meters away from the laser collimator. Tests were performed at room temperature. Laser exposure was observed to produce a cylindrical hole in the core, beginning immediately with the start of exposure and progressing along the beam path. Drilling rates (Fig. 1) increased with laser power, from 0.25 cm/s for 1 kW to 2.1 cm/s for 10 kW, delivering in the latter case an estimated 1100 cal/s. Meltwater was observed to continually flow out of the irradiated cylinder face during drilling, as visible in the thermal camera image of Fig. 2. No significant widening or tapering was noted of the drilled cavities, as pictured in Fig. 3, but the inner walls were lined with noticeably soft ice chunks and "slush". Visibly clear ice was measured to drill at a higher rate than opaque ice, but its transparency was also quickly reduced during exposure, possibly due to scattered laser light causing localized melting within the room-warmed ice. No fractured cores, environmental damage due to scattered light exiting the ice, or other adverse events were encountered. Varying spot sizes, vertically-directed drilling to gauge the effects of accumulated meltwater, and more realistic ice cores from layer-by-layer freezing will be future tests of particular interest for drilling and coring applications.
Aberrated optics degrade the cavity finesse and point spread function of any optical system. It is frequently overlooked that the phase errors of these aberrations cause characteristic spectral features that can be observed and measured by an optical cavity. A general theory of the spectral decomposition of aberrations is developed and applied to an electrically deformable membrane feeding a high-finesse resonator. The micromirror, which consists of a 1 cm metallized SiNxelectrostatically-actuated membrane with phase offsets of about 1.5μm, leads to optical power being distributed to over twenty higher-order Laguerre-Gaussian cavity modes.
It is well-known that continuous-wave lasers can accelerate small particles to high velocities, and, more recently, it has been observed that laser damage of optical materials occurs at laser intensities that are orders of magnitude lower in the presence of accelerated absorbing particles. At these high powers the primary interactions are ablative, and a complete set of force balance equations in this regime have only recently been described. In our experiments, small groups of 35-41m diameter stainless steel particles are accelerated using a high-power continuous wave (CW) Yb-doped fiber laser illuminating at intensities between 1MW/cm2 and 2MW/cm2. The trajectories of the particles are tracked using a high-speed camera until they leave the camera field of view or evaporate, a process typically occurring on timescales of a few milliseconds. The lifetimes and trajectories of the superheated particles are calculated using a system of coupled equations to track particle velocity, mass, momentum, radius, vapor opacity, temperature, and temperature distribution. Upon illumination, the particles heat to their vaporization temperatures, with evaporating atoms transferring momentum to the remaining particle. Significant laser attenuation occurs due to an opaque glow region around the particle consisting of dense evaporated atoms and ions. Since the particle is not uniform in temperature, the number of evaporating atoms varies with position and imparts a net acceleration to the particle until a terminal velocity on the order of a few tens of meters per second is reached where drag forces offset further acceleration. Based on the calculated interplay between temperature gradient and acceleration, heat transfer within the evaporating particles must be dominated by radiation diffusion, a process that usually only dominates in astrophysical objects, for example in the photospheres of stars. It is further observed that absorbing particles accelerated by continuous wave (CW) lasers initiate catastrophic failure in the form of micromachined drill holes. This process was tested using stainless steel, PMMA, and silica particles with fused silica, sapphire, and spinel substrates. Hole drilling occurred at laser power densities as low as 250kW/cm2. A potential dependence of accelerated particle breakdown on substrate bandgap may suggest that the underlying physical process is similar to that seen for CW laser breakdown of contaminated optical coatings.
The laser induced damage threshold of optical coatings with differing band gaps was measured using a high power 1070 nm continuous-wave laser. High reflectivity distributed Bragg reflectors of niobia-silica, tantala-silica, and hafnia-silica were tested in addition to half-wave coatings of titania, tantala, hafnia, and alumina. Absorbing contamination in the form of 20–50 \textmum carbon particles was added to the surface of the optics prior to exposure to test for particle induced damage. For both half-wave and high reflectivity coatings, the minimum damage thresholds were found to increase for larger band gap materials. Low band gap niobia reflectors and titania half wave coatings damaged at 20 kWcm-2 and 155 kWcm-2 respectively while larger bandgap hafnia reflectors failed at 1 MWcm-2 and alumina half-wave coatings failed at 9.7 MWcm-2. Most laser induced damage was catastrophic with the film and underlying substrate being damaged and the optic uncontrollably hearting. Damage effects differed for high band gap half-wave coatings that did not damage the substrate or thermally runaway with additional laser exposure.
The minimum and maximum laser irradiances that are effective in cleaning etch-released membranes and microstructure was measured for carbon microparticles on LPCVD silicon nitride thin films and suspended platforms. A 1064nm Nd:Yag laser was scanned across the samples to ablate the contaminants. Microscope images of the membranes showed mass removal for an irradiance as low as 600W cm-2. More thorough cleaning was achieved by increasing the irradiance. For 2.5\texttimes2.5mm, 205nm thin silicon nitride membranes, 79% of the contaminated area was removed with an exposure of 3.7kW cm-2. Catastrophic damage was seen at a power level of 8.4kW cm-2. Ablation effects were also measured as a change in optical absorption using a photo thermal common-path interferometer. Peak absorption values were decreased from over 100,000ppm to less than 20,000ppm. Silicon nitride platforms were also tested. Despite significant substrate heating, the platforms survived intact up to power levels of 8.4kW cm-2 with near perfect cleaning of carbon particles from their surfaces.
Under low light conditions, high temperature measurements of luminescence are limited by the overlap of the thermal emission spectra and the luminescent emission spectra being measured. A solution to this is to have a heat source that can be designed not to emit in a certain wavelength range(s) by coating it with an interference multilayer. The multilayer effectively changes the emissivity of the heat source. Microheaters made from aluminum oxide platforms with platinum heating elements were coated with aluminum oxide and titanium oxide multilayers. This multilayer structure was used to measure the thermoluminescence of CaSO4:Ce,Tb up to 420^∘C. They also showed a thermal emission background 800 times lower at 600^∘C than the same microheater with no multilayer structure.
We present a technique for high-speed imaging of the dynamic thermal deformation of transparent substrates under high-power laser irradiation. Traditional thermal sensor arrays are not fast enough to capture thermal decay events. Our system adapts a Mach-Zender interferometer, along with a high-speed camera to capture phase images on sub-millisecond time-scales. These phase images are related to temperature by thermal expansion effects and by the change of refractive index with temperature. High power continuous-wave and long-pulse laser damage often hinges on thermal phenomena rather than the field-induced effects of ultra-short pulse lasers. Our system was able to measure such phenomena. We were able to record 2D videos of 1 ms thermal deformation waves, with 6 frames per wave, from a 100 ns, 10 mJ Q-switched Nd:YAG laser incident on a yttria-coated glass slide. We recorded thermal deformation waves with peak temperatures on the order of 100 degrees Celsius during non-destructive testing.
Infrared-transparent microheaters have been constructed to reduce the background blackbody radiation produced by the heater. Among other applications, such heaters allow one to probe the high temperature peaks of thermoluminescent(TL) materials. The microheater consists of peripheral platinum heating elements on a mid-infrared transparent alumina platform. Alumina has a relatively low blackbody signal at high temperature for wavelengths less than 8μm. To test the reduced blackbody emission, an aperture was placed over the heating coils and then the transparent center of the microheater. The amount of infrared light transmitted through the aperture was reduced by 90% as the aperture moved from the highly emissive heater coils at 450^∘C to the largely transparent center at the same temperature.
The thermoluminescence characteristics of a thin film of terbium-doped yttrium oxide change upon repeated stress application through electrostatic actuation. A maximum 42% decrease in the intensity of two thermoluminescent peaks is seen when voltage is applied in 5V increments to 25V, translating to 0.15 μm of center deflection. While the overall intensity decreases, the higher temperature peak - corresponding to deeper traps - is affected more than the lower temperature one. Two possible physical explanations for the behavior are mechanical stress and dielectric charging.
We compared the laser damage performance of uniform versus nanolaminate thin films for hafnia and titania-based coatings. Our experiments showed that the films with higher crystallinity appeared to have better performance for CW systems.
Thermoluminescent (TL) particles show promise as robust direct-contact thermal history sensors for explosive events. Research with microheaters has shown that TL microparticles can measure temperature excursions of hundreds of degrees; however, microheaters do not generate the severe pressure and shock stimuli present in post- detonation environments. To address this, TL particles were tested under conditions produced by the detonation of an aluminized explosive formulation. TLD-100 (LiF:Mg,Ti) powder was irradiated with 220 Gy of gamma radiation from a ^167Cs source before being exposed to the free field detonation of a 20 gram charge. Particles were recovered post-detonation from two separate tests and their TL glow curves measured. At least two TL emission peaks 50 ^oC apart are clearly distinguishable in both samples, with peak intensity ratios decreasing 33.7% and 60.0% from an original 8.88:1, indicative of distinct carrier traps emptying at rates depending on the trap energy. These ratios agree well with thermocouple measurements from within the post-detonation fireball.
Thermal expansion mismatch between the layers of an optical coating and its substrate alters the shape of an optical element. We demonstrate predictable coating behavior using atomic layer deposition and apply to high-reflectivity mirror design.
The blackbody radiation limit has traditionally been set forth as the ultimate performance limit of thermal detectors. However, this fundamental limit assumes that the detector absorbs uniformly throughout the thermal spectrum. In much the same way as photon detectors can achieve very high D* because they do not absorb photon energies below their bandgap, so too can thermal detectors except that thermal detectors are not limited to cryogenic operation. In both cases, the enhanced theoretical D* is achieved because the radiation noise is reduced in a device that does not absorb at a uniform high level throughout the thermal emission band. There are multiple ways to achieve high D* in thermal detectors. One is to use materials that absorb only in a certain spectral range, just as in photon detectors. For example a detector made from PbSe, with proper optical coupling, absorbs only photons with wavelengths shorter than 4.9μm. The radiation limited detectivity of such a device can theoretically exceed 9 x 1010cmHz1/2/W in the MWIR. Even with Johnson and 1/f noise estimates included, it can still exceed 2.5x1010cmHz1/2/W in the MWIR. Another technique, applicable for narrowband thermal detectors, is probably even more powerful. Consider a thermal detector that is almost completely transparent. Here, the radiation noise has been reduced but the signal has been reduced even more. However, if the device is now placed inside an optical cavity, then at one wavelength and in one direction, the nearly transparent detector couples to the cavity resonance to absorb at 100%. Radiation from all other wavelengths and directions are rejected by the cavity or are absorbed only weakly by the detector. It is shown that theoretically, the D* of these devices are roughly proportional to the inverse square root of the spectral resonant width under certain conditions. It is also shown that even including Johnson noise and 1/f noise, the practically achievable D* approaches or exceeds 1011 cmHz^1/2/W.
The thermal history of a material with initially filled trap states has been probed using the thermoluminescence of microparticle sensors. Mg2SiO4:Tb,Co particles with two thermoluminescence peaks have been heated using microheaters over a 230 ºC to 310 ºC range for durations of less than 200 ms. The effect of maximum temperature during excitation on the intensity ratio of the peaks is compared with first-order kinetics theory and shown to match within an average error of 4.4%.
Microbolometers and other thermal detectors have traditionally been limited to seeing objects in a broad wavelength band at a single sensitivity. Recent advances in interface heat transfer and optical cavity design promise to change that. In this paper, we present recent work on thermal infrared detectors with tunable responsivity and wavelength. First, we demonstrate that extended dynamic range in thermal detectors can be achieved by electrostatically bringing a portion of the detector support structure in contact with the substrate. The exact amount of heat transfer can be controlled by adjusting the contact area and pressure. The thermal conductance and responsivity can be switched more than an order of magnitude using this technique. Next, we demonstrate that a wavelength tunable device in the LWIR can be achieved by modifying the structure of a microbolometer to incorporate a modified Gires Tournois optical cavity. The cavity couples light at a single wavelength into the microbolometer while other wavelengths are rejected. We demonstrate that resonance can be tuned from 8.7 to 11.1 μm with applied voltages from 0 to 42 V. The FWHM of the resonance can be switched between around 1.5 μm in a narrow-band mode and 2.83 μm in a broad-band mode.
An electrostatic actuation scheme for creating a soft contact is described and applied to cantilevers and doubly clamped beams. The actuation voltage form is composed of a high voltage pulse with a low DC voltage. The voltage pulse height and duration is mapped for achieving minimum impact force, and the results are well predicted by the simulation. Furthermore, a new phenomenon is observed for clamped beams that pulse longer than the soft-landing requirement bounces the beam back rather than stick.
The stress-induced deformation of micromirrors is examined in terms of common Seidel aberrations, and its impact on microcavity spectra is quantified. Tilt, curvature, and astigmatism cause characteristic changes in the spectra, but coma and spherical aberration are often negligible.
The adhesion energy is measured between InGaAs quantum wells that have collapsed across a 120 nm airgap in a InP/InGaAs heterostructure. The method relies on measuring the unadhered length and shape of collapsed microcantilevers with optical interferometry. The adhesion energy is found to be 72 plusmn 16 mJ m^-2. Since the airgap is much smaller than has been measured previously, the influence of van-der-Waals forces across the gap was included in theoretical modeling. It was found that the forces should not cause significant deviation from the standard adhesion models unless the adhesion energy drops below 25 mJ m^-2.
This paper covers the design, fabrication, and testing of a step-wise tunable long-wavelength infrared filter. The design uses a modified type of microbolometer to filter infrared light in the 7-10 micrometer wavelength range. Fabrication of the device is accomplished using standard microelectronic and microelectromechanical systems (MEMS) fabrication techniques. Germanium and zinc sulfide are used as optical layers for good reflectivity in the desired infrared range. Testing of the device shows that it filters light in three absorption bands in the specified wavelength range. The tuning of the device is found to be step-wise in response to electrostatic actuation using voltages up to 100 volts.
In this paper, the fabrication of microbolometers with electronically controllable responsivity is presented. The first generation devices are built in a standard polysilicon-based micromachining process with HF etch-release and demonstrated with a responsivity that can be tuned over a factor of 50. The responsivity is controlled by applying a voltage between the microbolometer and the substrate. The resulting electrostatic force causes a small portion of the support beam to contact the substrate, which thermally shorts the device at that point. The thermal contact points are defined using curved support beams with residual-stress from a Cr/Au metallization. The lowest portion of the beams contacts the substrate, and the curvature protects the device from full "snap-down," which might induce stiction. The fabrication of the second generation microbolometers based on VOx and silicon nitride materials with a polyimide etch-release is also described. The thermal contact points for these devices are defined by beam mechanics rather than by beam curvature induced by stress, and they actuate at 17 volts. The test array has a fill-factor of 91% for a pixel period of 140μm limited by our photolithography equipment.
We report the design, fabrication, and testing of a novel imaging micro-spectrometer system for the 500-1000 nm wavelength range. The space-variant design incorporates a linear array of MEMS mirrors in order to vary the dispersion spatially on a pixel-by-pixel basis. A planar-optics geometry is used so that the spectrometer optics are contained on a single piece of bulk fused silica. The object to be investigated is imaged onto a linear array of tilting MEMS mirrors that define the spectrometer slit. Each individual MEMS mirror tilts to send the light to one of three blazed gratings of differing dispersive powers. Depending on the wavelength and selected grating the spectral resolution is between 5 and 20 nm. The collimating and focusing mirrors of the spectrometer are fabricated in standard photoresist via grayscale photolithography with a custom high-energy beam-sensitive (HEBS) photomask. A reflow at 70 degrees Celsius for 40 hours is necessary to achieve diffraction-limited performance. The blazed gratings are fabricated in SU-8 via direct-write electron beam lithography. Spectrometer results with a variety of lasers and gaseous discharge tubes are presented and indicate that system performs as expected.
Materials with low coefficients of thermal expansion (CTE) are critically important in thin film design to create efficient bimorph actuators and to thermally stabilize structures of low stiffness. We report the first negative thermal expansion (NTE) material (zirconium tungstate) evaporated as a thin film and probe its CTE using a tunable curvature micromirror. The measured CTE of different films are a function of stoichiometry and annealing conditions and CTEs as low as -10/spl times/10/sup -6/ K/sup -1/ were measured. The measurements show no hysteresis after several annealing cycles. Additionally, data on optical constants, elastic modulus, film stoichiometry, and micromirror deflection have been obtained.
Micromirrors with current-controlled curvature have been fabricated and tested. The principle behind the devices is that resistive heating changes the temperature of the micromirrors, causing a change in optical coating stress. For highly symmetric mirrors, the variable stress induces a uniform and reversible change in curvature. The mirrors used in this study were surface-micromachined from polysilicon and had gold coatings. The radius of curvature of a typical mirror could be tuned from 2.5 mm to 8.2 mm over a current-induced temperature range from 22/spl deg/C to 145/spl deg/C (estimated). The corresponding change in focus over this range, as measured by the Seidel parameters of the wavefront, was 0.38/spl lambda/ while the largest aberration (astigmatism) varied from its zero current value by less than 0.02/spl lambda/. These results should be readily extendable to tuning or correcting flat mirrors by changing the design stress of the reflective coating.
When thermal infrared detectors are exposed to large signals, they are susceptible to a host of unwanted effects including intensity dependent noise and detectivity, nonlinearities in materials characteristics, and even temporary blindness or device damage. To combat these problems and to extend dynamic range, the responsivity and time constant of microbolometers have been controlled using electrostatic actuation. The responsivity has been demonstrated to switch over a factor of 60, with theoretical limits encompassing 4 to 5 orders of magnitude. High responsivity states correspond to free-standing bolometers, while discrete lower responsivity states are created by partially or completely actuating the device supports into contact with the substrate. Continuous tuning over a part of the range is demonstrated by utilizing electrostatic pressure to increase the thermal contact between discrete switching states.
All-dielectric micromirrors are reported that maintain their shape to within λ/60 at 633 nm over a range of 30 C, limited by the measurement apparatus. In tandem with the experimental results, thermally invariant coating design is discussed along with thermal expansion results from some common dielectric coating materials.
Micromachined devices are often connected to their underlying substrate by only a few small beams and are susceptible to a host of unwanted thermal effects. For many non-optical structures, thermal degradation begins with buckling or catastrophic thermal breakdown. In optical devices, however, the wavelength of light places an upper limit on the acceptable mechanical deformation, and large aberrations can be induced by relatively small changes in shape. Calculating quantitative limits on the incident optical power, applied current, and operating temperature of a device will require a strong understanding of its heat transfer and elastic response. This paper discusses the heat transfer and thermal deformation of optical MEMS components. First, the thermal conductance of microfilaments and micromirrors is discussed followed by a treatment of the thermal expansion deformation of micromirrors with temperature. Finally, mirror shape stabilization using stress compensation and thermal conductance control using electrostatic actuation are summarized.