Development of Novel Anti-reflection Coatings for Millimeter/Submillimeter-wave Telescopes and Optimization of Cosmic Microwave Background Instruments

Abstract

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.