Pushing the Limits of Broadband and High Frequency Metamaterial Silicon
  Antireflection Coatings
  Broadband refractive optics realized from high index materials provide
compelling design solutions for the next generation of observatories for the
Cosmic Microwave Background (CMB), and for sub-millimeter astronomy. In this
paper, work is presented which extends the state of the art in silicon lenses
with metamaterial antireflection (AR) coatings towards larger bandwidth and
higher frequency operation. Examples presented include octave bandwidth
coatings with less than $0.5\%$ reflection, a prototype 4:1 bandwidth coating,
and a coating optimized for 1.4 THz. For these coatings the detailed design,
fabrication and testing processes are described as well as the inherent
performance trade offs.
