The Reststrahlen effect, or Reststrahlen band, refers to a phenomenon observed in the infrared spectrum of certain materials, particularly crystalline substances. It occurs when there are specific vibrational modes of the material's lattice that strongly absorb infrared radiation, leading to distinct spectral features. In essence, when infrared light interacts with a crystal, some wavelengths are absorbed due to the resonant frequency of the lattice vibrations (phonons), while others are reflected.
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In a polar insulating crystal, a band with negative real permittivity can occur between the transverse and longitudinal optical resonances. A lossless bulk transverse wave is evanescent there, producing high normal-incidence reflectance. Damping rounds the edges and permits absorption. The reflection minimum beyond the band can occur where the refractive index approaches one; it is not exactly the zero of permittivity.