AC Stark effect of the intraexciton 1s-2p quantum well transition


AC Stark effect of the intraexciton 1s-2p quantum well transition

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

Driving a two-level system with intense light can lead to non-perturbative phenomena like Rabi oscillations. The frequency-domain equivalent is the Autler-Townes or AC Stark effect where dressed light-matter states of a resonantly driven two-level scheme occur. Known from molecular spectroscopy [1] the effect exhibits an absorption line splitting whose magnitude is proportional to the field strength, whereas the symmetry of the splitting depends on the detuning from the resonance case. For atomic or molecular resonances the line widths are much smaller compared to solid state systems like semiconductors, making it much easier to observe the effect in the first case. While only recently the effect has been investigated in quantum well intersubband transitions [2, 3], we present strong evidence for the THz induced intraexciton AC Stark effect using the hydrogen-like 1s and 2p levels of the quantum well heavy-hole exciton [4]. Despite its similarity to the hydrogen problem (except the energy is scaled down by a factor of 1000) we can reach a regime where the Keldysh parameter is comparable to the transition energy and the Rabi energy, a regime not easily accessible for atomic systems.In the experiment an undoped GaAs/AlGaAs multiple quantum well (substrate etched away) is illuminated under normal incidence with intense THz pulses from the Dresden Free-Electron Laser FELBE. At the same time we probe the low-temperature transmission with a broadband femtosecond laser. Tuning the THz photon energy around the measured 1s-2p resonance energy of 9 meV (at 3 meV line width) we observe characteristic absorption changes at the hh(1s) exciton line (see Fig. 1). Below resonance (6.1 meV) an additional high-energy peak or Rabi sideband occurs, while above resonance (14 meV) a low-frequency shoulder is seen. On resonance we have a nearly symmetric splitting. From the two-line fit of Fig. 1 we find an anticrossing for various THz photon energies around the undriven exciton line (marked as dashed vertical line). Moreover, with increasing THz field strength the splitting on resonance increases linearly up to a field of 10 kV/cm with a Rabi energy as large as 60% of the resonance energy. We explain our findings qualitatively on the basis of a simple two-level model, though the underlying rotating-wave approximation breaks down in this regime. Observed deviations would have to be addressed within a full many-body theory. Shifting the NIR probe pulses in time with respect to the THz pulses has been found to change the transmission adiabatically on a time scale of several picoseconds only during the THz pulse. In connection with the observed up to 20-fold transmission change at the hh(1s) exciton, this temporal behavior promises ultrafast optical modulation based on the AC Stark effect.
In conclusion, our experimental data provides the first unambiguous evidence for the intraexciton Autler-Townes effect with its characteristic intensity-dependent line splitting on resonance and its frequency-dependent anticrossing behavior.

[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 et al., Phys. Rev. Lett. 94, 157403 (2005).
[4] M. Wagner et al., Phys. Rev. Lett. 105, 167401 (2010).

Keywords: Terahertz; Semiconductor; Nonlinear Optics; Light-Matter Coupling; Free Electron Laser

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