Laser-ablated sub-wavelength structure anti-reflection coating on an alumina lens

We used laser ablation to fabricate a sub-wavelength structure anti-reflection coating (SWS-ARC) on a 5-cm-diameter alumina lens. With a design aspect ratio height/pitch of 2.5, the SWS-ARC gives a calculated average reflectance of 1.3% between 110 and 290 GHz. Sub-wavelength structure shape measurements conducted on both sides of the lens give 303μm pitch and total height between 750 and 790μm, matching or exceeding the aspect ratio design values. Millimeter-wave transmittance measurements in a band between 140 and 260 GHz show the increase in transmittance expected with the anti-reflection coating when compared with finite element analysis electromagnetic simulations. To our knowledge, this is the first demonstration of SWS-ARC on an alumina lens, opening the path for implementing the technique for larger diameter lenses.