Kyle likes to bike fast. During his PhD studies, he started the Android app maker HiQ, co-founded bike spoke company Berd, published six peer-reviewed papers, and still found time to bike fast.
Dr. Olson defended his dissertation in fall 2015 and now researches energy harvesting at Starkey Laboratories. And yes, he’s still biking fast.
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.
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.
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 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.
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.
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.
A model is presented and confirmed experimentally that explains the anomalous behavior observed in the continuous wave (CW) excitation of thermally-isolated optics. Very low absorption, high reflective optical thin film coatings of HfO2 and SiO2 were prepared. When illuminated with a laser for 30s the coatings survived peak irradiances of 13MW/cm 2. The temperature profile of the optical surfaces was measured using a calibrated thermal imaging camera; about the same peak temperatures were recorded regardless of spot size, which ranged between 500μm and 5mm. This phenomenon is explained by solving the heat diffusion equation for an optic of finite dimensions, including the non-idealities of the measurement. An analytical result is also derived showing the transition from millisecond pulses to CW, where the heating is proportional to the laser irradiance (W/m 2) for millisecond pulses, and proportional to the beam radius (W/m) for CW. Contamination-induced laser breakdown is often viewed as random and simple physical models are difficult to apply. Under continuous wave illumination conditions, failure appears to be induced by a runaway free-carrier absorption process. High power laser illumination is absorbed by the contaminant particles or regions, which heat rapidly. Some of this heat transfers to the substrate, raising its temperature towards that of the vaporizing particle. This generates free carriers, causing more absorption and more heating. If a certain threshold concentration is created, the process becomes unstable, thermally heating the material to catastrophic breakdown. Contamination-induced breakdown is exponentially bandgap dependent, and this prediction is borne out in experimental data from TiO2, Ta2O5, HfO2, Al 2O3, and SiO2. The spectral dependence of blackbody radiation and thermal photon noise is derived analytically for the first time as a function of spectra and mode density. An algorithm by which the analytical expression for the variance can be found for any spectral distribution is also presented. The analytical results of some simple distributions are found and shown to be inaccurately approximated with a uniform spectral distribution highlighting the importance of the finding. Two microcavities are then presented to exemplify enhanced or inhibited photon statistics effects on the cavity.
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.
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.
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.