Phyo graduated with his bachelors in physics from Cornell College in 2016, and started his graduate studies in Electrical Engineering at the University of Minnesota the same year. His research demonstrated new methods of making thin films highly resistant to laser damage, such as all-silica nanocolumnar structures achieved by evaporation and sol-gel spin-coating.
Phyo defended his thesis in winter 2024. Dr. Lin can now be found at Intel in Hillsboro, OR.
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.
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.
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.
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.