  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.
