Absorption of electromagnetic field of the millimeter-wave band in perfect dielectric crystals

The dielectric losses in high-quality crystals of sapphire, ruby, fluorite, and quartz are measured at a frequency of 36 GHz in the temperature range 4.2-300 K by the spherical dielectric resonator method. The absorption coefficient of the electromagnetic field per unit wavelength Gamma in the crystals is found to agree both in order of magnitude and in its temperature dependence with the Gurevich theory, which treats the absorption as being the result of an electrophonon interaction of the field with acoustical phonons as a result of the anharmonicity of the crystal lattice. It is found that the crystals studied have extremely low absorption in the millimeter-wave band even at moderately low temperatures (e.g., for sapphire Gamma approximate to 10(-8) at T approximate to 40 K). Measurements are made of the residual absorption of the electromagnetic field at low temperatures, which is due to a single-phonon process of field absorption at defects of the crystal structure. It is found that the temperature dependence of the absorption of an electromagnetic field in a crystal is correlated with that for the absorption of hypersound. (C) 2000 American Institute of Physics. [S1063-777X(00)01411-0].