A 1.6:1 Bandwidth Two-Layer Antireflection Structure for Silicon Matched to the 190-310 GHz Atmospheric Window

TL;DR

This paper presents a two-layer subwavelength structure on silicon that achieves over 20 dB reduction in reflection across 190-310 GHz, enabling advanced THz optical components.

Contribution

The authors fabricated a wide-bandwidth, two-layer antireflection structure on silicon using multi-depth DRIE, achieving low reflectance over a broad frequency range.

Findings

Reflectance below -20 dB over 190-310 GHz

Bonded wafers show no significant reflection features

Enables construction of wide-bandwidth silicon vacuum windows

Abstract

Although high-resistivity, low-loss silicon is an excellent material for THz transmission optics, its high refractive index necessitates antireflection treatment. We fabricated a wide-bandwidth, two-layer antireflection treatment by cutting subwavelength structures into the silicon surface using multi-depth deep reactive ion etching (DRIE). A wafer with this treatment on both sides has <-20 dB (<1%) reflectance over 190-310 GHz. We also demonstrated that bonding wafers introduces no reflection features above the -20 dB level, reproducing previous work. Together these developments immediately enable construction of wide-bandwidth silicon vacuum windows and represent two important steps toward gradient-index silicon optics with integral broadband antireflection treatment.