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

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 (<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 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.