Ultrafast coherent and incoherent dynamics of intersubband excitations in quasi-two-dimensional semiconductors

Intersubband excitations play an important role for ultrafast carrier dynamics in quasi-two-dimensional semiconductors and for device applications. We present a study of the ultrafast coherent and incoherent dynamics of intersubband excitations in a pure electron plasma by means of femtosecond spectroscopy in the mid-infrared. The different relaxation processes following intersubband excitation of electrons in GaInAs/AlInAs quantum wells are observed in real-time and the relevant microscopic scattering mechanisms are identified. We find a decay of coherent intersubband polarizations on a time scale of several hundreds of femtoseconds which is governed by electron-electron scattering. Electrons excited to the n=2 conduction subband undergo intersubband scattering to the n=l subband by emission of longitudinal optical phonons with characteristic time constants of l ps. This is followed by thermalization of the backscattered electrons on a similar time scale, involving both electron-electron and electron-phonon scattering. Eventually, the hot electron distribution cools down to lattice temperature within about 50 ps.