EFFECT OF NONEQUILIBRIUM LO PHONONS AND HOT-ELECTRONS ON FAR-INFRAREDINTRABAND ABSORPTION IN N-TYPE GAAS

We report far-infrared transient-grating measurements in n-type GaAs i n which we observe that nonequilibrium longitudinal-optical (LO) phono ns, emitted by hot electrons, directly couple in the infrared (similar to 17 mu m) intraband absorption process. We find that a few picoseco nds after the far-infrared optical excitation, the time evolution of t he induced intraband absorption change is in fact completely dominated by these nonequilibrium phonons. This observation is possible because intraband absorption, contrary to optical interband absorption, is a second-order LO-phonon-assisted process, which is directly affected by changes in both the electron distribution and the LO-phonon distribut ion. [S0163-1829(98)51408-7].