An anti-reflection coating for silicon optics at terahertz frequencies

A method for reducing the reflections from silicon optics at terahertz freq uencies has been investigated. In this study, we used thin films of parylen e as an anti-reflection (AR) layer for silicon optics and show low-loss beh avior well above 1 THz. Transmittance spectra are acquired on double-sided- parylene-coated, high-resistivity, single-crystal silicon etalons between 0 .45 THz and 2.8 THz. Modeling the optical behavior of the three-layer syste m allowed fur the determination of the refractive index and absorption coef ficient of parylene at these frequencies. Our data indicate a refractive in dex, n, of 1.62 for parylene C and parylene D, and a reasonably modest abso rption coefficient make these materials a suitable AR coating for silicon a t terahertz frequencies. Coatings sufficiently thick for AR performance red uced the average transmittance of the three-layer system by <10% compared t o a lossless AR coating with an ideal refractive index.