1.6:1 bandwidth two-layer antireflection structure for silicon matched to the 190–310  GHz atmospheric window

Abstract

Although high-resistivity, low-loss silicon is an excellent material for terahertz transmission optics, its high refractive index necessitates an 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<−20 dB (<1%<1%) reflectance over 187–317 GHz at a 15° angle of incidence in TE polarization. We also demonstrated that bonding wafers introduce no reflection features above the −20 dB−20 dB level (also in TE at 15°), 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.

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