Conservation of intraband coherence upon carrier relaxation by LO-phonon emission

The evolution of the quantum mechanical phase of electronic wave packets on seattering-with phonons is an issue of great relevance in solid-state physics. In semiconductor superlattices, evidence for phase conservation on scattering was obtained when time-resolved THz-emission experiments indicated that the dephasing of the probed intraband coherence may proceed slower than the relaxation of carriers via emission of LO-phonons estimated to occur on a 200-fs time scale as in bulk GaAs.1 To corroborate this interpretation, the unknown LO-phonon emission times in the superlattices must be measured for the experimental conditions of the THz-emission experiments. In this paper, a combined investigation of the coherent and incoherent dynamics of optically excited charge carriers in a superlattice structure is presented. A comparison of the characteristic time constants provides clear evidence for the conservation of the phase of the intraband coherence on LO-phonon relaxation.