Ultrafast Nonequilibrium Dynamics of Intersubband Excitations in Quasi-two-dimensional Semiconductors

Abstract

Intersubband excitations play an important role for the nonequilibrium carrier dynamics in quasi-two-dimensional semiconductors. In this chapter, optical studies of ultrafast intersubband dynamics in quantum wells and device structures are reviewed. We first discuss the optical spectra of intersubband transitions and the relevant scattering processes of carriers, many-body effects, and couplings to other elementary excitations in a tutorial way. This is followed by a brief review of experimental techniques that provide insight into ultrafast intersubband dynamics. The main part of the chapter is devoted to experimental and theoretical results on the coherent dynamics of intersubband polarizations and the subsequent incoherent carrier relaxation processes, i.e., intersubband scattering, thermalization, and cooling of carrier plasmas. We mainly consider the behavior of single-component electron plasmas in GalnAs/A1 InAs multiple quantum wells for intersubband energy spacings larger than the energy of longitudinal optical phonons. In particular, the role of carrier-carrier and carrier-phonon scattering for the different stages of the relaxation scenario is analyzed. In the final part of the chapter, the physics of optoelectronic devices making use of optical intersubband excitations, i.e., photodetectors for the mid-infrared and the quantum cascade laser, is briefly discussed.