Effect of nonequilibrium LO phonons and hot electrons on far-infrared intraband absorption inn-type GaAs

We report far-infrared transient-grating measurements in $n$-type GaAs in which we observe that nonequilibrium longitudinal-optical (LO) phonons, emitted by hot electrons, directly couple in the infrared $\left(\sim 17\mu \mathrm{m}\right)$ intraband absorption process. We find that a few picoseconds after the far-infrared optical excitation, the time evolution of the 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 distribution.