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 re?normalization of the binding energy of a high-density exciton gas in GaAs/AlGaAs 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 10-14 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.

The density-driven crossover of electron-hole pairs from insulating to conducting states is observed via the internal 1s-2p exciton resonance. Decreasing interparticle distance induces strong shifts and broadening, and ultimately the disappearance of the excitonic resonance.

We report the generation of ultrabroadband pulses spanning the 50-130 THz frequency range via phase-matched difference frequency mixing within the broad spectrum of sub-10 fs pulses in LiIO_3. Model calculations reproduce the octave-spanning spectra and predict few-cycle THz pulse durations less than 20~;fs. The applicability of this scheme is demonstrated with 9-fs pulses from a Ti:sapphire oscillator and with 7-fs amplified pulses from a hollow fiber compressor as pump sources.