In this article we review the experimental and theoretical investigations of the linear and nonlinear optical properties of semiconductor quantum well structures, including the effects of electrostatic fields, extrinsic carriers and real or virtual photocarriers.

Linear and nonlinear optical properties of semiconductor quantum wells

The valley degree of freedom in two-dimensional (2D) crystals recently emerged as a novel information carrier in addition to spin and charge. The intrinsic valley lifetime in 2D transition metal dichalcogenides (TMD) is expected to be markedly long due to the unique spin-valley locking behavior, where the intervalley scattering of the electron simultaneously requires a large momentum transfer to the opposite valley and a flip of the electron spin. However, the experimentally observed valley lifetime in 2D TMDs has been limited to tens of nanoseconds thus far. We report efficient generation of microsecond-long-lived valley polarization in WSe2/MoS2 heterostructures by exploiting the ultrafast charge transfer processes in the heterostructure that efficiently creates resident holes in the WSe2 layer. These valley-polarized holes exhibit near-unity valley polarization and ultralong valley lifetime: We observe a valley-polarized hole population lifetime of more than 1 μs and a valley depolarization lifetime (that is, intervalley scattering lifetime) of more than 40 μs at 10 K. The near-perfect generation of valley-polarized holes in TMD heterostructures, combined with ultralong valley lifetime, which is orders of magnitude longer than previous results, opens up new opportunities for novel valleytronics and spintronics applications.

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Observation of ultralong valley lifetime in WSe2/MoS2 heterostructures.

The temperature-dependent optical absorption and luminescence spectra of GaAs/AlGaAs and InGaAs/InAlAs n-doped modulation-doped quantum wells is discussed with emphasis on the peak seen at the edge of the absorption spectra of these samples A many-body calculation of the electron-hole correlation enhancement is presented, which identifies this peak with the Mahan exciton-the result of the Coulomb interaction between the photoexcited hole in the valence band and the sea of electrons in the conduction band This calculation accounts for the strong dependence of the absorption edge peak on both the temperature and carrier concentration, in good qualitative agreement with experimental data and with previously published results The changes induced by the carriers on the subband structure through self-consistent calculations are also analyzed, and it is concluded that in these symmetric structures, the changes are small for achievable carrier densities >

Free carrier and many-body effects in absorption spectra of modulation-doped quantum wells

We present results on the picosecond dynamics of spin relaxation and thermalization of excitons in GaAs quantum wells. The spin relaxation time constant is ≊50 ps for resonant excitation and less than 10 ps for nonresonant excitation, considerably shorter than those for free electrons in the bulk. Photoexcited cold, nonthermal excitons thermalize in ≊50 ps at 10 K and less than 10 ps at 35 K. Exciton‐acoustic phonon energy exchange rates are determined from these measurements.

Spin relaxation and thermalization of excitons in GaAs quantum wells

Fabrication and optical properties of semiconductor quantum wells and superlattices

We investigate the physical processes which determine the optical spectra of modulation-doped semiconductor quantum-well structures as a function of doping. We argue that even at low doping concentrations, excitons are heavily dressed by charge- and spin-density excitations of the Fermi sea, implying, already in that limit, a Stokes shift between emission and absorption. For all doping concentrations, the charge and spin polarization of the Fermi sea due to many-body effects leads to an enhancement of the symmetry breaking in quantum-well structures, which may explain polarization anomalies observed in luminescence.

A. E. Ruckenstein

Papers

Infrared and polarization anomalies in the optical spectra of modulation-doped semiconductor quantum-well structures