Excitons in strong, narrowband terahertz fields: Rabi splitting and beyond

Excitons possess a hydrogen-like internal excitation spectrum with a characteristic energy scale in the THz range. We explore these internal degrees of freedom by using an intense THz free-electron laser to pump the 1s-2p intraexcitonic transition in GaAs and InGaAs multiquantum wells. The induced absorption changes are probed via interband absorption using a near-infrared femtosecond laser. We observe a splitting of the 1s exciton line, which can be explained by the Autler-Townes effect, also called AC Stark or Rabi splitting [1]. The behavior is, however, much more complex than for an ideal two-level system. Since for electric fields in the 10 kV/cm range the Rabi energy is already of the same order of magnitude as the 1s-2p transition energy, we are in fact clearly beyond the validity of the rotating wave approximation. At the highest fields, when also the ponderomotive energy (e2F2/4mw2) approaches the exciton binding energy, signatures of exciton field ionization are observed. Microscopic calculations based on the semiconductor Bloch equations including THz interaction show that the spectra can be reproduced, if the full set of higher exciton states is taken into account [2].
The experiments would not have been possible without the high-quality multiquantum well samples grown and provided to us by A. M. Andrews and G. Strasser (TU Vienna, Austria), and by G. Khitrova and H. M. Gibbs (Univ. Arizona, Tucson, USA), whose contributions are gratefully acknowledged.