Millimeter-Wave Broadband Anti-Reflection Coatings Using Laser Ablation   of Sub-Wavelength Structures
  We report on the first use of laser ablation to make sub-millimeter, broad-band, anti-reflection coatings (ARC) based on sub-wavelength structures (SWS) on alumina and sapphire. We used a 515 nm laser to produce pyramid-shaped structures with pitch of about 320 $\mu$m and total height of near 800 $\mu$m. Transmission measurements between 70 and 140 GHz are in agreement with simulations using electromagnetic propagation software. The simulations indicate that SWS ARC with the fabricated shape should have a fractional bandwidth response of $\Delta \nu / \nu_{center}\ = 0.55$ centered on 235 GHz for which reflections are below 3%. Extension of the bandwidth to both lower and higher frequencies, between few tens of GHz and few THz, should be straightforward with appropriate adjustment of laser ablation parameters. 
Tomotake Matsumura, Karl Young, Qi Wen, Shaul Hanany, Hirokazu Ishino,   Yuki Inoue, Masashi Hazumi, J\"urgen Koch, Oliver S...
astro-ph.IM physics.optics