Mid-infrared optics using dielectrics with refractive indices below unity

TL;DR

This paper investigates the use of dielectric materials with refractive indices below one, like fused silica, to enable novel mid-infrared optical phenomena and device functionalities.

Contribution

It demonstrates the practical use of low-index dielectrics in mid-infrared optics, including reflection and surface plasmon coupling, expanding the scope of optical device design.

Findings

Fused silica exhibits n<1 in mid-infrared with low loss

Achieved external reflection and surface plasmon coupling from free space

Materials with n<1 enable new optical functionalities

Abstract

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.