Interaction of intersubband transitions and ponderomotive responce in doped GaAs/AlGaAs multiple quantum wells at the THz regime

In the present work we investigate the line shape of the broadband terahertz (THz) response in doped multiple quantum wells by means of field-resolved detection. In an optically excited structure we recently observed a Fano-like shape of the THz response [1]. This results from the superposition of the broad continuous ponderomotive response and the sharp intersubband transition. The first originates from the force that takes effect on carriers in an oscillating electromagnetic field. The applied spectroscopy technique is time-resolved ultrabroadband THz spectroscopy. The THz radiation is generated by phase-matched optical rectification of 10 fs near-infrared pulses in 50 μm thin GaSe crystals. The pulses are tuneable in a range from 15 to 40 THz with a width (FWHM) of up to 15 THz. The field-resolved detection is done by phase-matched electro optic sampling. The applied detection method is crucial for observing the effect since the ponderomotive current can only be seen as a lossless phaseshift of the transmitted THz radiation while the intersubband transition leads to an absorption. Thus we are able to observe directly the superposition of ponderomotive current and intersubband transition in the time-domain.
[1] D. Golde et al., Phys. Rev. Lett. 102, 127403 (2009).