Merlin received B.S. degrees in Electrical Engineering and Computer Engineering (cum laude) from the University of Minnesota. Working with the Optical MEMS group since his undergrad days, he defended in July 2017 his Ph.D thesis on the dual topics of thermoluminescent microparticle thermometry for high-explosive detonations and the development of optical methods and instruments for characterizing glacial ice. He is currently a postdoc attached to the latter project.
Outside of the lab, Merlin enjoys (further) ventures into programming, photography, graphics and web design (including this site), a few musical instruments, and swimming.
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.
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.
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.
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.
A system includes a detector and a computing device communicatively coupled to the detector. The detector detects spatial or temporal spectral features of a light beam after transmission of the light beam through a turbulent or aberrated medium and generate a measurement signal indicative of the spectral feature. The computing device receives the measurement signal and a comparative signal indicative of a spectral feature of the light beam prior to or after transmission of the light beam through the medium. The computing device compares the measurement signal and the comparative signal and determines, based on the comparison of the measurement signal and the comparative signal, one or more values related to variations in refractive indices of the medium.
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.
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.
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.
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.
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 environment around a detonating high explosive is incredibly energetic and dynamic, generating shock waves, turbulent mixing, chemical reactions, and temperature excursions of thousands of Kelvin. Probing this violent but short-lived phenomena requires durable sensors with fast response times. By contrast, the glacier ice sheets of Antarctica and Greenland change on geologic time scales; the accumulation and compression of snow into ice preserves samples of atmospheric gas, dust, and volcanic ash, while the crystal orientations of the ice reflect its conditions and movement over hundreds of thousands of years. Here, difficulty of characterization stems primarily from the location, scale, and depth of the ice sheet. This work describes new sensing technologies for both of these environments. Microparticles of thermoluminescent materials are proposed as high-survivability, bulk-deployable temperature sensors for applications such as assessing bioagent inactivation. A technique to reconstruct thermal history from subsequent thermoluminescence observations is described. MEMS devices were designed and fabricated to assist in non-detonation testing: large-area electrostatic membrane actuators were used to apply mechanical stress to thermoluminescent Y2O3:Tb thin film, and microheaters impose rapid temperature excursions upon particles of Mg2SiO4:Tb,Co to demonstrate predictable thermoluminescent response. Closed- and open-chamber explosive detonation tests using dosimetric LiF:Mg,Ti and two experimental thermometry materials were performed to test survivability and attempt thermal event reconstruction. Two borehole logging devices are described for optical characterization of glacier ice. For detecting and recording layers of volcanic ash in glacier ice, we developed a lightweight, compact probe which uses optical fibers and purely passive downhole components to detect single-scattered long-wavelength light. To characterize ice fabric orientation, we propose a technique which uses reflection measurements from a small, fixed set of geometries. The design and construction of a borehole logger implementing these techniques is described, and its testing discussed.
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.
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.
To the uninitiated, the phrase “optical microelectromechanical systems”or optical MEMS must appear to refer to a field of incredible specialization. Ironically, the number of disciplines involved, optics, mechanics, and electronics, make the field most accessible to scientists of great technical breadth. This is especially true when optical MEMS is used in chemical and biological applications - the theme of this text. Underlying all of them is the technology of microfabrication. One chapter could not possibly cover all of the techniques developed over the decades for very-large-scale integration (VLSI) and general MEMS systems. Indeed there are entire textbooks devoted specifically to both types. In this chapter then, we present the characteristics of fabrication and design that are specific to bring optics into the system. In particular, there are a number of materials and fabrication techniques that are specific to optical MEMS systems. When dealing with light, one may have to handle visible, ultraviolet, or infrared portions of the spectrum, each of which has its own special set of optimal substances. Since one often has to emit light or detect it in special wavelength regions, semiconductors other than silicon often must be incorporated, each with their own set of wet and dry chemical etching techniques and their own set of mechanical properties. Standard mechanical characteristics that play no role in “normal”MEMS systems may prove problematic in optical MEMS. For example small size may lead to diffraction, typical surface roughness may limit optical cavity resolution, and mechanical or motion may deform mirrors to limit the number of resolvable spots. Even thermal noise may place limits on optical design. Each of these topics is covered in the pages that follow. For the reader who is interested in further exploring many of these areas, we recommend the text by Solgaard.
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.
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.
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.
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.
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.
Sensor technologies are a rapidly growing area of interest in science and product design, embracing developments in electronics, photonics, mechanics, chemistry, and biology. Their presence is widespread in everyday life, where they are used to sense sound, movement, and optical or magnetic signals. The demand for portable and lightweight sensors is relentless in several industries, from consumer electronics to biomedical engineering to the military. Smart Sensors for Industrial Applications brings together the latest research in smart sensors technology and exposes the reader to myriad applications that this technology has enabled. Organized into five parts, the book explores: Photonics and optoelectronics sensors, including developments in optical fibers, Brillouin detection, and Doppler effect analysis. Chapters also look at key applications such as oxygen detection, directional discrimination, and optical sensing. Infrared and thermal sensors, such as Bragg gratings, thin films, and microbolometers. Contributors also cover temperature measurements in industrial conditions, including sensing inside explosions. Magnetic and inductive sensors, including magnetometers, inductive coupling, and ferro-fluidics. The book also discusses magnetic field and inductive current measurements in various industrial conditions, such as on airplanes. Sound and ultrasound sensors, including underwater acoustic modem, vibrational spectroscopy, and photoacoustics. Piezoresistive, wireless, and electrical sensors, with applications in health monitoring, agrofood, and other industries. Featuring contributions by experts from around the world, this book offers a comprehensive review of the groundbreaking technologies and the latest applications and trends in the field of smart sensors.
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.
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%.