Browsing by Subject "optics"

Now showing 1 - 4 of 4
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    Contamination Induced Continuous-Wave Laser Damage of Optical Elements
    (2018-06) Brown, Andrew
    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 contact 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 conditioned samples. Regular laser conditioning at low irradiances can prolong the life of optics that must operate in difficult environmental conditions.
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    High Power Continuous Wave Laser Heating and Damage with Contamination, and Non-Uniform Spectrally Dependent Thermal Photon Statistics
    (2015-12) Olson, Kyle
    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/cm2. 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/m2) 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, Al2O3, 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.
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    Low-Signal Passive Non-Line-of-Sight Imaging
    (2023-12) Hashemi, Connor
    In recent years, significant progress has been made in passive non-line-of-sight imaging, which looks around corners at hidden objects using just scattered light off a rough surface. Since passive non-line-of-sight imaging can obtain information about the surrounding environment that was previously deemed irrecoverable, it has many far-reaching applications, such as improving autonomous vehicles, aiding search-and-rescue operations, and performing military surveillance. While the bulk of progress has focused on improving the resolution and capabilities of existing non-line-of-sight imaging algorithms, most methods assume a high signal-to-noise ratio and low amounts of background signals. These conditions are impossible to find outside of highly-controlled laboratories and do not correspond to real-world applications. This thesis considers "low-signal'' passive non-line-of-sight imaging, where the desired imaging signal is either swamped by stochastic noise or structured background signals that interfere with conventional non-line-of-sight reconstructions. These scenarios mimic what is found in real-world environments. To image in these difficult low-signal scenarios, this thesis delves into two main works: unmixing spectral content of the scattered light and denoising thermal scattered imagery. As a result of these methods, non-line-of-sight imaging can be performed in many scenarios previously deemed too difficult, and future low-signal imaging can build off the principles laid in this thesis.
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    Silicon quantum dot luminescent solar concentrators
    (2019-08) Hill, Samantha
    Silicon quantum dots (Si QDs) have previously been established as a unique class of quantum-confined materials with potential for a wide variety of optoelectronic applications. In this work, we examine their application to luminescent solar concentrators, or LSCs, for the first time by developing high-quality Si QD / polymer nanocomposites. By encasing Si QDs with in a polymer slab, most of their photoluminescence becomes trapped via total internal reflection and escapes only at the slab edges where solar cells can be placed to harvest the concentrated light. We find that Si QDs are suitable for such LSC devices due to their unique combination of indirect band gap absorption with efficient photoluminescence. The resulting low overlap between the absorption and photoluminescence spectra yields low reabsorption losses in large-area LSCs without the use of rare or toxic elements in the luminophore. We demonstrate effective Si QD LSC prototypes consisting of flexible and rigid bulk nanocomposites as well as films on glass using methacrylate-based polymers. We find the Si QDs maintain their optical properties throughout radically-initiated polymerization processes but are prone to forming light scattering agglomerates in the solid phase. These agglomerates drastically reduce the LSC waveguiding efficiency due to their light scattering properties. We find that light scattering from these nanocomposites increases with Si QD concentration. One approach for improving the dispersion of the Si QDs within solid polymers is to choose surface ligands which mimic the structure of the encasing polymer. We demonstrate this with ester-capped Si QDs compared to alkane-capped Si QDs in poly(methyl methacrylate), or PMMA. Furthermore, we find that fast polymer solidification rates also reduce the formation of light scattering agglomerates. We show ester-Si QD / PMMA films cast from prepolymer solutions have an order of magnitude higher concentration limit before the onset of light scattering compared to their bulk-polymerized counterparts. Overall, this work establishes Si QDs as a promising luminophore for visibly transparent LSCs which may be used in the future for solar harvesting windows and architectural elements or in concert with other LSCs to form more efficient tandem structures.