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Abstract:
Fundamental quantum optical processes in photogenerated excitons in Cu2O are demonstrated using few-cycle terahertz pulses and field-sensitive electro-optic sampling: (i) In a dilute ensemble of 3p excitons, stimulated terahertz 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. (ii) An optically dark, dense, and cold 1s-paraexciton gas is prepared by two-photon generation of electron-hole pairs and subsequent phonon cooling. Intense multi-terahertz fields of order MV/cm coherently promote 70% of the quasiparticles from the 1s to the 2p state via a partial internal Rabi oscillation. Electro-optic detection monitors the Larmor precession of the Bloch vector directly in the time domain, with femtosecond resolution.
