Browsing by Subject "Optical Design"

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    Development of Novel Anti-reflection Coatings for Millimeter/Submillimeter-wave Telescopes and Optimization of Cosmic Microwave Background Instruments
    (2021-07) Wen, Qi
    I report the development of the anti-reflection coatings (ARC) using laser ablated sub-wavelength structures (SWS) in millimeter and sub-millimeter (MSM) wavelengths. This technology provides a promising solution for broadband, cryogenically robust ARC on high-index materials (HIM) - alumina, sapphire and silicon - for a broad range of MSM telescopes. The effective behaviors of SWS are studied using numerical simulations by finite element method. We observe complex behaviors of SWS when the pitch of SWS is not negligibly small compared to the electromagnetic wavelength. Based on the study, a practical guide to design optimal SWS ARC is provided. Ultrashort pulsed lasers are used to fabricate SWS on HIM, majority of which are difficult to be modified by other traditional fabrication methods such as dicing and chemical etching. We have successfully ablated structures with height from a few hundred  m to around 2.5 mm. Excellent anti-reflection performances have been demonstrated by experimental measurements of transmittance/reflectance as well as by numerical simulations based on the measured structure shapes. Higher than 20 mm^3/min average ablation rates have been experimentally verified on alumina and sapphire through an optimization effort using a high-power picosecond laser. The demonstrated high rates strongly support the feasibility of laser ablated SWS ARC on large (>=30 cm) optical elements. A novel ablation model that relates the structure height and laser cumulative fluence is presented. Using a best-fit procedure with experimental data, for both alumina and sapphire, we find threshold fluence \theta_{th} \approx 2 J/cm^3 and average absorption length \bar{\delta} \approx 650 nm for peak fluence values between 30 and 70 J/cm^2. With the best-fit values, the model and data values for cumulative fluence agree to within 10%. The model is used to predict average ablation rate as a function of SWS height and average laser power. I also report the results of several projects that aim to optimize cosmic microwave background instruments. These projects include (1) an optical design study of cross Dragone system for PICO, the Probe of Inflation and Cosmic Origins, a next-generation space telescope; (2) a mechanical design of the focal plane for PICO; (3) a trade study on the aperture size for Tau Surveyor, a balloon-borne instrument aiming to measure the optical depth to reionization \tau; (4) development of low-conductance, lenslet coupled, multichroic bolometers for balloon-borne platforms.
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    Light Management and Optical Loss Mitigation for Photovoltaics: Downshifting, Downconversion, and Tandem Solar Cell Designs
    (2023-07) Keil, John
    The efficiency of single-junction Si photovoltaic cells has continually increased over the past several decades, but is approaching fundamental thermodynamic limits. Holding over 95% of the solar module market share, Si modules will continue to be an integral part of the rapidly expanding photovoltaic industry, so different device technologies that increaseSi cell efficiencies beyond thermodynamic limits, or that expand the available installation sites for solar cells, are needed. In this thesis, three types of technologies are discussed that use optical design to more efficiently use the high energy solar spectrum for Si PV: downshifting, downconversion, and tandem solar cells. We first discuss the design of downshifting and concentrating devices called luminescent solar concentrators (LSCs). Tandem LSC architectures, which combine multiple luminophores to broaden the absorption spectrum, are one potential route to increase the efficiency of these devices. We first describe an analytical model to develop luminophore selection criteria for tandem LSCs. We find that luminophores with high photoluminescent quantum yield, minimal overlap between the absorption and photoluminescence spectrum, and an absorption onset closely matched to the band gap of the chosen photovoltaic cell yield the best LSC performance. We then create bilayer LSCs, which combine CdSe/CdS and Si nanocrystals in a monolithic waveguide. Through a combination of transmission measurements, position-dependent photoluminescence measurements, and ray-tracing simulations, the bilayer LSC was found to sensitize Si nanocrystal absorption and enhance the optical efficiency by 30% relative to a single layer LSC. We discuss the use of the bilayer device in agrivoltaic applications, and then explore this use further using a thin-film stack optimization methods to direct emission out one LSC side toward the plant species. The LSC extraction efficiency is increased from 13.9% to 15.1%. We next consider optical designs for downconversion, a process by which one high energy photon is converted into two lower energy photons. We consider the coupling efficiency from the downconverter to a realistic Si solar module in several different configurations, finding an optical coupling efficiency of 95.25% by placing the downconverting film directly on the Si cell. This enhances the power conversion efficiency by 2% absolute. Lastly, CdTe/Si four-terminal tandem solar cells are studied to improve the sub-band gap transparency of CdTe solar cells. We find that the surface texture of the CdTe significantly impacts light transmission into the Si bottom cell, and that the losses are dominated by the transparent conductive oxide absorption. An optical design solution is proposed that mitigates transparency loss and enhances the short circuit current density of the Si cell by 2.5 mA/cm2 , which enhances the tandem efficiency by a relative increase of 5.6%.