AMPLIFICATION OF LO PHONONS IN PHOTOEXCITED POLAR SEMICONDUCTORS

We consider the photoproduction of optical phonons in n-doped polar semiconductors. It is shown that they are produced in measurable excess of equilibrium only in a small off-centre and restricted region of reciprocal space, corresponding to small values of wavenumber. We also show that nonlinear effects are negligible in the weak to intermediate laser intensities but come to play a role in the range of strong excitations. Nonlinear effects associated to relaxation processes to the carrier system allows to recover steady state values for the Lo-phonon population that would otherwise (in the linear regime) grow indefinitely in time. © 1990.