MODULATION INSTABILITY IN OPTICALLY STRAINED MAGNETOACTIVE SEMICONDUCTORS

The modulational instability of an intense laser beam is investigated in a doped semiconductor by use of four-wave parametric coupling theor y. The crystal is subjected to an intense magnetic field along the dir ection of the input laser field. The origin of the modulational instab ility process is assumed to lie in the third-order nonlinear optical s usceptibility of the medium. The effects of the optically induced elec tromechanical and acousto-optical strain are considered. These include piezoelectric and deformation potential coupling in noncentrosymmetri c (NCS) semiconductor systems and electrostrictive and acoustooptic st rain in centrosymmetric (CS) systems. The threshold intensity required for inciting the instability process and the subsequent modulation am plification process are investigated. The effect of the carrier-densit y modulation that is due to doping is also considered. The electron co llision frequency is also found to modify the output profile of the mo dulated 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 ra te of the signal as compared with that for a CS medium. The growth rat e of the unstable mode can be enhanced by an increase in the carrier d ensity of the doped semiconductor. The external magnetic field also ap preciably enhances the growth rate of the signal at lower threshold fi elds.