= Reststrahlen effect
= Reststrahlen band
{synonym}
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.
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