Terahertz induced intra-excitonic Autler-Townes effect in semiconductor quantum wells up to room temperature

When light is resonant with a material excitation the optical Stark or Autler-Townes (AT) effect couples the involved energy states and alters their energy, i.e. the states get "dressed" by the light-matter interaction. This fundamental quantum-mechanical feature of light-matter interaction was originally observed in atomic spectroscopy. However, despite some theoretical work, it took a long time to the first observation of the AT effect for terahertz (THz) light coupled to hole and electron intersubband transitions in semiconductor quantum wells. Recently we have reported clear evidence of the intra-excitonic AT effect at low temperature. In that work the 1s to 2p transition of an exciton, i.e. an hydrogen-like electron-hole pair in a quantum well, was driven resonantly with strong THz light from the HZDR free-electron laser (FEL). A distinct power- and wavelength dependent splitting of the 1s absorption line has been observed in the near-infrared (NIR) transmission. Here we extend our study up to room temperature. NIR transmission spectra of the GaAs/AlGaAs multiple quantum well are recorded for a series of different temperatures, THz frequencies and THz intensities. When tuning the THz photon energy around the 1s-2p intra-excitonic transition energy that lies at 9 meV, we observe a line splitting when pumping near resonance, and low- and high-energy shoulders, respectively, when pumping above and below resonance. This behavior, the AT effect, is still observable up to a temperature of 200 K where the thermal energy is 17 meV and exceeds the exciton ionization energy of 10 meV. Hence, the excitonic system is quite robust. By delaying the NIR pulse in time with respect to the THz pulse, we find that the induced absorption change occurs adiabatically during the THz pulse. This ultrashort absorption modulation that is present even at elevated temperatures can in principle be exploited for NIR modulators or switches.