Abstract
The modulational instability of an intense laser beam is investigated in a doped semiconductor by use of four-wave parametric coupling theory. The crystal is subjected to an intense magnetic field along the direction of the input laser field. The origin of the modulational instability process is assumed to lie in the third-order nonlinear optical susceptibility of the medium. The effects of the optically induced electromechanical and acousto-optical strain are considered. These include piezoelectric and deformation potential coupling in noncentrosymmetric (NCS) semiconductor systems and electrostrictive and acousto-optic strain in centrosymmetric (CS) systems. The threshold intensity required for inciting the instability process and the subsequent modulation amplification process are investigated. The effect of the carrier-density modulation that is due to doping is also considered. The electron collision frequency is also found to modify the output profile of the modulated wave significantly. The enhanced diffraction of the modulated wave that is due to the finite second-order susceptibility retards the amplification process in a NCS medium, resulting in a lower growth rate of the signal as compared with that for a CS medium. The growth rate of the unstable mode can be enhanced by an increase in the carrier density of the doped semiconductor. The external magnetic field also appreciably enhances the growth rate of the signal at lower threshold fields.
© 1994 Optical Society of America
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