Fano interference in the intersubband THz response of photoexcited GaAs/AlGaAs quantum wells

In our work, we directly observe the interplay between ponderomotive contribution and true intersubband transition (IST). To this end, we probe the conduction IST of an undoped GaAs/Al0.34Ga0.66As multiple quantum well by transmitting broadband THz pulses after resonant photoexcitation with either a broad-band or narrowband laser at the 1s heavy-hole exciton. The field oscillations of the transmitted THz pulses are directly detected by phase-matched electro-optic sampling. We observe that the differential THz transmission transient as the pump-induced change in the transmitted THz field shows a strong beating. In the frequency domain this results in an intersubband resonance with a broad additional peak at lower energy. The line shape of this intersubband resonance strongly resembles a Fano-resonance with an undershoot at the low-frequency side and an asymmetric peak to higher frequencies. However, since we are able to measure differential transmission and total THz absorption separately, we find that the absorption does not show such a Fano-asymmetry, but reveals the expected Lorentzian-like line shape of the intersubband resonance. Especially, it is a single peak and therefore the beating in the time domain cannot originate from an adjacent second absorptive resonance next to the true IST resonance.
In our microscopic theory these features can be explained unambiguously by a phase-sensitive superposition of the true THz intersubband current and the ponderomotive current.