Femtosecond THz studies of intra-excitonic transitions

Few-cycle THz pulses are employed to resonantly access the internal fine structure of photogenerated excitons in semiconductors, on the femtosecond time scale. This technique allows us to gain novel insight into many-body effects of excitons and reveal key quantum optical processes. We discuss experiments that monitor the density-dependent renormalization of the binding energy of a high-density exciton gas in GaAs/Al_0.3Ga_0.7As quantum wells close to the Mott transition. In a dilute ensemble of 3p excitons in Cu₂O, stimulated THz emission from internal transitions to the energetically lower 2s state is observed at a photon energy of 6.6 meV, with a cross section of 10¹⁴ cm². Simultaneous interband excitation of both exciton levels drives quantum beats, which cause efficient THz emission at the difference frequency. By extending this principle to various other exciton resonances, we develop a novel way of mapping the fine structure by two-dimensional THz emission spectroscopy.