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Terahertz induced intra-excitonic Autler-Townes effect in semiconductor quantum wells

Wagner, M.; Schneider, H.; Stehr, D.; Winnerl, S.; Helm, M.; Roch, T.; Andrews, A. M.; Schartner, S.; Strasser, G.

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 [1]. 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 [2] and electron [3] intersubband transitions in semiconductor quantum wells.
Here, we report clear evidence of the intra-excitonic AT effect. In our experimental work we study the NIR transmission at low temperature of a GaAs/AlGaAs multiple quantum well film (substrate etched away) exposed to strong picosecond THz pulses from the free-electron laser (FEL) at FZD. NIR spectra are recorded for a series of different THz frequencies and intensities. When tuning the THz photon energy in the range from 6 to 17 meV 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 off resonance. This behavior is consistent with the AT effect. In Fig. 1 the measured absorption around the heavy-hole 1s exciton is displayed for different THz photon energies, showing the two dressed states and their expected anticrossing behavior. We discuss our experimental evidence of a coupling between the NIR “bright” 1s state and the NIR “dark” 2p state on the basis of a two-level model. Near resonance (10.5 meV) we find that our simplified model describes the situation surprisingly well up to a THz field strength of 10 kV/cm (I = 650 kW/cm2), corresponding to a Rabi energy of 0.6 times the 1s-2p transition energy. This is already well beyond the limits of the rotating-wave approximation. At the highest field strengths, the Rabi sidebands appear to start interacting with other exciton states. A full many-body theory would have to deal with the complete manifold of excitonic states as well as with the possibility of exciton field ionization. Note that for the above parameters in our case the ponderomotive energy is 3 meV and thus of the same order as the exciton transition/ionization energy (i.e. Keldysh parameter near unity) and the Rabi energy, an extremely non-trivial regime.
Using picosecond THz pulses we finally demonstrate that the induced absorption change occurs adiabatically only during the THz pulse. This ultrashort change that corresponds to an up to 20-fold increase in transmission can in principle be exploited in NIR modulators or switches.

[1] S. H. Autler and C. H. Townes, Phys. Rev. 100, 703 (1955).
[2] S. G. Carter et al., Science 310, 651 (2005).
[3] J. F. Dynes, M. D. Frogley, M. Beck, J. Faist, and C. C. Phillips, Phys. Rev. Lett. 94, 157403 (2005).

Keywords: AC Stark effect; Autler Townes effect; excitons; terahertz; sideband generation; nonlinear optics

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Publ.-Id: 14446