Intraband terahertz emission from coupled semiconductor quantum wells: A model study using the exciton representation

Intraband emission resulting from the second-order polarization induced by a femtosecond pump field in semiconductor nanostructures is calculated. The two-band model is transformed to the exciton representation, and exciton-phonon coupling represented by arbitrary spectral densities is incorporated into the equations of motion for the relevant exciton variables through Redfield superoperators. Time-resolved coherent and incoherent terahertz radiation as well as the incoherent optical fluorescence are calculated for a one-dimensional model of coupled quantum wells in an external electric field with on-site Coulomb interaction between electrons and holes and an overdamped Brownian oscillator spectral density representing the electron-phonon coupling. A doorway-window picture based on the reduced single-electron density matrix is developed for the analysis of both signals. Disorder and phonon-induced dephasing effects on the various signals are discussed and compared.