Thermal scattering of light in crystals. Part III. Theory for birefringent crystals

Abstract

For the first time, the theory of Doppler shifts in thermal scattering of light in birefringent crystals is worked out and the magnitude of the shift Δv of the components is given by Δv/v = ±v_e√n_i²+ n_s²-2n_sn_s cosθ, where Δv is the frequency of the incident light, c the velocity of light in vacuum, v_e, the velocity of the elastic wave effective in scattering andn i andn s are either of the refractive indices of the crystal for the incident and observation directions. Sincen i andn s can each take two values, there are four pairs of values (n_i, n_s ) and further v_e takes three values. Therefore, there mustin general betwelve pairs of Doppler components in the light scattered along a particular direction. The twelve pairs can be divided into four species each with a specific pair of values (n_i, n_s ) and consequently specific polarisation character. They can be studied individually by the use of proper polarising devices in the incident and scattered paths. Each species consists of three pairs of components arising from the elastic waves of wavelength λ_e = λ√n_s² + n_s² - 2n_sn_s cosθ, where λ is the wavelength of the incident light in vacuum and propagated along a specific direction. For any particular species, the scattering must be appropriately regarded as "coherent reflection" or "coherent refraction" of light waves by the effective elastic waves according as cosθ<n_i/n_s and n_s/n_i or cosθ>n_s/n_i or n_s/n_i. There can in general be 3 pairs of Doppler components with finite shifts in the exactlyforward scattering. In singly refracting crystals (n_s/n_i = n_s/n_s=n) the expression for shift reduces to the familiar expression Δv/v =±(n_s/v_e/c)2n sinθ/2 and in this case there could only be three pairs of Doppler components arising from "specular reflection" of light by elastic waves.