Andy Brown received his bachelors summa cum laude in Electrical Engineering from the University of Minnesota in 2011 and continued there for his masters and doctoral degrees, researching continuous-wave laser induced damage of optical materials. Andrew has published in Applied Optics and the Journal of Optics and Laser Technology as well as the Optical Interference Coatings, the Directed Energy Professional Society, IEEE Photonics, OSA Advanced Solid State Lasers, and IEEE Optical MEMS conferences. Outside of work, Andy likes cross-country skiing. A lot. Maybe a little too much.
Doc Brown defended his thesis in June 2018, and can now be found–when the trails aren’t snowy–at Honeywell.
Likes: Fast skis, light bikes, muddy trail runs, and consistent data
Dislikes: Hoppy beer, press fit bottom brackets, and physical inactivity
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.
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.
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.
Continuous-Wave laser-induced optical breakdown affects anyone whose work requires tightly focused light, high power sources, or delicate materials. It often occurs unexpectedly and seemingly randomly at optical intensities far lower than those predicted by ultra-short pulse laser experiments. Further complicating the issue is that the majority of laser damage experiments use carefully controlled laboratory conditions with short-pulsed lasers focused to small spots on clean, pristine materials. Continuous-Wave laser damage is usually attributed to contamination, and occurs under radically different conditions. To determine the origin of contamination-induced breakdown, microparticle contaminated optics were stressed using a 17 kW continuous-wave laser. Contamination-induced breakdown occurred at intensity levels many orders of magnitude lower than expected in clean, pristine materials. For both half-wave and high reflectivity coatings, damage thresholds were found to strongly follow the bandgap energy of the film. It is theorized that surface contamination heated by the laser thermally generates free carriers in the films. If the free carrier concentration exceeds a certain threshold, runaway absorption and breakdown will occur. A thermal model incorporating the particle absorption, interfacial heat transfer, and free carrier absorption was developed, and it explains the observed data. The bandgap of the film, the absorption and thermal con- tact of the contaminant, and the evaporation time of the particle, all determine whether a material can survive. 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. These findings strongly suggest that low bandgap materials are a liability in optics exposed to environmental contamination. Laser conditioning was examined as a means of preventing damage by removing contamination without initiating damage. Absorption measurements taken using photo thermal common-path interferometry show up to a 90% absorption reduction with con- ditioned samples. Regular laser conditioning at low irradiances can prolong the life of optics that must operate in difficult environmental conditions.
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.
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.
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.
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.
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.
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.
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.