Reststrahlung

Reststrahlung, or the Reststrahlen band, is a spectral region in polar dielectric crystals where infrared radiation is strongly reflected due to coupling between electromagnetic waves and optical phonons. The term comes from German and describes the persistence of radiation in this band because transmission through the material is suppressed.

The phenomenon arises from the dielectric response of polar crystals. Each crystal has a transverse optical (TO) phonon frequency and a longitudinal optical (LO) phonon frequency. In the frequency range between ω_TO and ω_LO, the real part of the dielectric function becomes negative, causing incident infrared light to be reflected with very high efficiency. In this band, bulk waves cannot propagate in the crystal, and the reflectance approaches unity. Outside the band, the material can transmit or absorb infrared light more readily.

The exact position and width of the Reststrahlen band depend on the material and its phonon spectrum. Common Reststrahlen materials include certain oxides and fluorides such as magnesium oxide, lithium fluoride, silicon carbide, and aluminum oxide. The band lies in the mid- to far-infrared and is often used to characterize phonon frequencies and dielectric properties through infrared spectroscopy and ellipsometry.

Applications of Reststrahlen phenomena include infrared filters, coatings, and mirrors that rely on high reflectivity in the infrared. It also provides a tool for studying lattice dynamics and phonon–photon interactions in solid-state systems.