Low loss THz window

This paper presents a method to manufacture a low loss window applicable for THz frequencies. The window is made out of high resistivity silicon (3 k Omega cm, n = 3.42, and alpha = 0.1/cm). Reflective loss due to the impedance mismatch between the substrate and free space is overcome by etching (Bosch-process) rectangular grooves of depth lambda/4 into the substrate as an antireflection (AR) layer. The refractive index of the AR-layer depends on the incident wave mode (TE/TM), i.e., the AR-layer is a birefringent medium. A short theoretical description is given. Simulation of the AR-layer was done by using a transmission line analogue and the Scatter-program written by Padman. FTS measurements yield a transmittance greater than 96% at 2.1 THz and a band width of 400 GHz (1.9-2.3 THz) with >= 90% transmission. (c) 2006 Elsevier B.V. All rights reserved.