Resonant terahertz excitation of tunnel-coupled quantum wells: Nonlinear absorption and photovoltage

The redistribution of nonequilibrium electrons between tunnel-coupled double quantum wells under terahertz pump is described by using a self-consistent two-level basis with “rigid” orbitals. The photoinduced component of the self-consistent potential, that changes both energy spectra (i.e., transition energies and oscillator strengths) and efficiency of tunnel relaxation, causes such redistribution. The intersubband photoexcitation and relaxation of electrons due to quasielastic scattering are described by the system of balance equations for electron concentrations over the tunnel-coupled pair of levels. In order to calculate the electron population numbers we use self-consistent wave functions, which take into account the transverse potential obtained from Poisson equation. Such equations describe also the transverse voltage induced by the redistribution of electrons between the quantum wells under pumping (rectification effect) and nonlinear modifications of intersubband absorption peak. Parameters of nonequilibrium electron redistribution significantly depend on the intersubband transitions energies, broadening and intensity of pumping (because of resonant excitation). Calculating them, we have found the photoinduced level splitting and considered the transition from the linear regime excitation to saturation one. We have a bistable solution in the transition region. Also, we present results for the nonlinear absorption and for the photoinduced voltage. Comparisons to experimental data are presented as well.