  Conventional transparent materials at optical frequencies have refractive
index values (n) greater than unity -- most commonly between about 1 and 4.
This paper explores optical phenomena made possible by using materials with
refractive indices less than unity. We focus primarily on fused silica (SiO2),
a polar dielectric with strong phonon resonances in the mid infrared that
result in a spectral region in which n < 1 with modest optical loss. Using this
ubiquitous easy-to-deposit material, we demonstrate infrared-frequency external
reflection, frustrated external reflection, and direct coupling to surface
plasmon polaritons from free space. Our work suggests that materials with
refractive indices below unity can bring significant new functionality to
optical devices.

Optics