Effects of off-diagonal radiative-decay coupling on electron transitions in resonant double quantum wells

Density-matrix equations for electrons in laser-coupled quantum wells are derived in second quantization, including an off-diagonal radiative-decay coupling between a pair of electron transitions. Calculations of spontaneous photoluminescence and time-resolved optical absorption for the probe field are formulated. The zero absorption of the pump-laser field within an overlapping region between two absorption peaks is found in a resonant asymmetric double-quantum-well system and explained as the quantum interference between two nearly degenerate electron transitions. Quantum interference is clearly demonstrated through phase cancellation between the two statistically averaged transition dipole moments. The laser frequency for zero absorption can be tuned within a tunneling gap by applying a small dc bias field. The $k_{\Vert }$-dependent energy-level separation is found to be a crucial factor for destroying quantum interference. The optical gain of the probe field is seen as a hole in the weak absorption peak for the resonant asymmetric double quantum wells selectively coupled by a laser field and shown to be a result of the partial inversion of the electron occupation probabilities in momentum space after laser excitation. The probe-field gain increases with the strength of the pump laser. The effects of transition blocking, induced quantum coherence, and off-diagonal radiative-decay coupling are quantitatively analyzed for this gain.