Publikationsrepositorium - Helmholtz-Zentrum Dresden-Rossendorf

THz spectroscopy on semiconductor heterostructures has revealed new insight in low-energy excitations such as intersubband transitions (ISTs). A microscopic analysis has to consider the true THz transitions, but also the so-called ponderomotive response as a charge current of carriers generated by the alternating classical electric field. In former experiments the ponderomotive contribution could only be seen rather indirectly [1].
Here, we directly study the interplay between ponderomotive contribution and true IST [2]. In our experiment we create electrons in the conduction band of an undoped GaAs/Al0.34Ga0.66As multiple quantum well by resonant interband excitation at the 1s heavy-hole exciton, using either a broad- or narrowband laser. Broadband THz pulses probe the IST and are detected by electro-optic sampling. The observed 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 low-frequency peak. 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 intersubband 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.
[1] J. R. Danielson et al. ,”Interaction of Strong Single-Cycle Terahertz Pulses with Semiconductor Quantum Wells“, Phys. Rev. Lett. 99, 237401 (2007).
[2] D. Golde et al., “Fano Signatures in the Intersubband Terahertz Response of Optically Excited Semiconductor Quantum Wells”, Phys. Rev. Lett. 102, 127403 (2009)