Options for Reststrahlen Materials in Optical Surfaces and Filters

A reststrahlen material is a compound with at least partly ionic bonding. This entails that a net of alternating positively and negatively charged ion cores are arranged in a periodic pattern that constitutes the lattice of the compound. This lattice can be excited by transverse electromagnetic (EM) waves that accelerate the positive and negative ions in opposite directions. The excitation can be modeled as a damped harmonic oscillator. The oscillator has a resonance that corresponds to energy absorption. Simple considerations of ionic bonding of atomic masses in real compounds show that this resonance occurs in the infrared (IR) frequency range. In analogy with a mechanical oscillator, the resonance frequency increases with the bonding strength and decreases with the effective molecular mass [1] . An IR wave traveling through a lattice has a wavelength that is much longer than the lattice constant and the typical distances between neighboring ions. The location of this excitation in the phonon spectrum of the compound is usually associated with the transverse optical (TO) branch. Considering the long wavelength, this should be close to the origin in k-space (i.e., k�0). The frequency of the TO-branch intersecting with the frequency axis is conventionally labeled as !T [2] . We can thus associate this molecular oscillator resonance frequency with a particular feature in the phonon spectrum of the crystal. The combined excitation of EM wave with phonon traveling through the crystal is named polariton. Using the analogy of the mechanical oscillator, which is accelerated by the electric vector of an EM wave, the classical Lorentz model dielectric function is derived [2] by ˜ "(!) = "∞ + Ne 2