  We present the design, fabrication, and characterization of a 100 mm
diameter, flat, gradient-index (GRIN) lens fabricated with high-resistivity
silicon, combined with a three-layer anti-reflection (AR) structure optimized
for 160-355 GHz. Multi-depth, deep reactive-ion etching (DRIE) enables
patterning of silicon wafers with sub-wavelength structures (posts or holes) to
locally change the effective refractive index and thus create anti-reflection
layers and a radial index gradient. The structures are non-resonant and, for
sufficiently long wavelengths, achromatic. Hexagonal holes varying in size with
distance from the optical axis create a parabolic index profile decreasing from
3.15 at the center of the lens to 1.87 at the edge. The AR structure consists
of square holes and cross-shaped posts. We have fabricated a lens consisting of
a stack of five 525 $\mu$m thick GRIN wafers and one AR wafer on each face. We
have characterized the lens over the frequency range 220-330 GHz, obtaining
behavior consistent with Gaussian optics down to -14 dB and transmittance
between 75% and 100%.
