Mid-infrared Optics Using Dielectrics with Refractive Indices Below Unity

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 well-studied dielectric with strong optical-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 functionalities to optical devices.