A reflectivity spectrum of a LiF crystal shows for wavelengths between and

PHYSICS · Multi-part · Fermi University Physics · Condensed Matter, Materials & Soft Matter · Dielectric and optical properties of solids · Dielectric and optical properties of solids

Solid State Physics, Ch Collective and coupled excitations: plasmons, polaritons and polarons

Problem

A reflectivity spectrum of a LiF crystal shows

R>0.9

for wavelengths between

15.3\,\mu\mathrm{m}

and

32.6\,\mu\mathrm{m}

, falling away sharply outside this range. LiF has static and high-frequency permittivities

\varepsilon_s=9.01

and

\varepsilon_\infty=1.96

. ((a)) Translate the two band edges into the transverse and longitudinal optical phonon frequencies

\omega_{TO}

and

\omega_{LO}

, in THz, stating which edge is which. ((b)) Check the assignment using the Lyddane-Sachs-Teller relation.

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