NONLINEAR EFFECTS IN THE PRODUCTION OF HIGHLY PHOTOEXCITED POLAR VIBRATIONS

We consider how dissipative nonlinear processes influence the photoproduction of optical phonons in n-doped polar semiconductors. It is shown that such nonlinear effects may lead to interesting and remarkable results. First we show that through free-carrier absorption processes LO phonons in excess of equilibrium are produced in a small off-center region of reciprocal space. Next we show that nonlinear relaxation effects are unimportant at weak to intermediate laser intensities but come to play a role in the range of strong excitations. On the one hand, they reduce the amplification of the preferentially pumped modes, and at very strong levels of photoexcitation the nonlinear contributions are responsible for a channeling of energy from the pumped modes to the modes with the lowest frequency that grow enormously: it follows a phenomenon akin to a nonequilibrium Bose-Einstein condensation. Even though this effect is hardly accessible experimentally in real semiconductors under laser-light excitation, it may follow in other systems, like biological polymers.