We report on the first use of laser ablation to make submillimeter, broadband, antireflection coatings (ARCs) based on subwavelength structures (SWSs) on alumina and sapphire. We used a 515 nm laser to produce pyramid-shaped structures with a pitch of about 320 μm and a total height of near 800 μm. Transmission measurements between 70 and 140 GHz are in agreement with simulations using electromagnetic propagation software. The simulations indicate that SWS-ARCs with the fabricated shape should have a fractional bandwidth response of Δν/ν_center=0.55 centered on 235 GHz for which reflections are below 3%. Extension of the bandwidth to both lower and higher frequencies, between a few tens of gigahertz and a few terahertz, should be straightforward with appropriate adjustment of laser ablation parameters.

In this study, we worked on the formation of micro through holes in multi-component glass substrates using laser-assisted etching, in which mixed aqueous solutions of hydrofluoric acid (HF) and nitric acid (HNO 3 ) were used for the etching. Glass samples modified by femtosecond laser irradiation were immersed for 10 minutes in aqueous solutions containing 2% HF and between 0 and 32% HNO 3 . The subsequent optical microscope observations and measurements showed that the highest etch selectivity, which was 21, was obtained with an etch solution containing 6% HNO 3 . Furthermore, through holes with diameters approaching 20 μm were successfully formed in a 100 μm thick glass substrate without any cracks using the mixed solution.

We used two novel approaches to produce sub-wavelength structure (SWS) anti-reflection coatings (ARC) on silicon for the millimeter and sub-millimeter (MSM) wave band: picosecond laser ablation and dicing with beveled saws. We produced pyramidal structures with both techniques. The diced sample, machined on only one side, had pitch and height of 350 µm and 972 µm. The two laser ablated samples had pitch of 180 µm and heights of 720 µm and 580 µm; only one of these samples was ablated on both sides. We present measurements of shape and optical performance as well as comparisons to the optical performance predicted using finite element analysis and rigorous coupled wave analysis. By extending the measured performance of the one-sided diced sample to the two-sided case, we demonstrate 25 % band averaged reflectance of less than 5 % over a bandwidth of 97 % centered on 170 GHz. Using the two-sided laser ablation sample, we demonstrate reflectance less than 5 % over 83 % bandwidth centered on 346 GHz.

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