Self-induced birefringence of infrared light inn-Ge

Abstract

We report the experimental proof of the self-induced birefringence of infrared light in many-valley cubic semiconductors. The effect is connected with the redistribution of free electrons between the equivalent valleys on account of heating by the infrared wave. The agreement of the obtained data with the theoretical prediction is very good.PACS 72.20.Ht, 72.80.Cw Nonlinear optical effects associated with free carriers in semiconductors have been investigated, e.g., in [1][2][3][4][5]. The frequency mixing and the optical phase conjugation of CO2 laser beams arising from the nonparabolicity of the conduction band were investigated in III-V compounds and in Hgi-^Cd^Te [1,2] . The third harmonic in the far IR region due to both the nonparabolicity of the energy band and the energy dependence of the scattering events was observed in Ge and Si [5].In the cubic many-valley semiconductors strong nonlinearity of optical characteristics in the IR region is connected with the redistribution of the carriers between the equivalent valleys. There are two reasons for this redistribution: (a) the energy displacement of a valley, which is caused by the oscillatory character of the carrier motion in the electric field E of the light wave and therefore depends on the orientation of E to the axes of the valley; (b) the different carrier heating in the various valleys. As shown theoretically the latter mechanism dominates over the former one [6].As a consequence of the redistribution, the contribution of the free carriers to the dielectric function becomes anisotropic and dependent on the intensity of the light, causing self-induced effects of laser beams.Some phenomena in n-Ge for E of a CO2 laser wave parallel to (111) were theoretically investigated in [7]. As well known for this orientation of the electric field the redistribution of electrons in Ge has a maximum. Nonetheless, it is reasonable to consider the other directions of E with respect to the crystallographic axes. For such nonsymmetric directions self-induced birefringence of light waves takes place. The investigation of this effect is the subject of the present paper.The self-induced change of the light polarization in n-Ge has been measured for the linearly polarized IR light with A = 10.6 /im at 300 and 80 K. The samples were cut from Sb-doped Ge single crystals in the form of plates of known thickness. Slight wedge angles were used between the opposite surfaces of the plate to avoid Fabry-Perot effects. The carrier concentration was measured by the Hall effect and was about 3 x 10 12 cm -3 (intrinsic) and 5 x 10 16 cm -3 , respectively.The multimode TEA-CO2 laser (where TEA is transversely excited at atmospheric pressure) was used to provide the IR radiation. The laser produced a maximal peak power of about 1 MW in a 100 nsec FWHM single pulse. Pyroelectric detectors with a response time of about 3 nsec were used to measure the peak power of the incident and transmitted light pulses and their shapes. The variation of the output peak power was about ±15% from o...