THz quantum optics with dark excitons in Cu₂O: From stimulated emission to nonlinear population control

Fundamental quantum optical processes in photogenemted excitons in Cu ₂O 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-para-exciton gas is prepared by two-photon generation of electronhole pairs and subsequent phonon cooling. Intense multi-terahertz fields of order MV/cm coherently promote 70% of the quasipaiticles from the 1s to the 2p state via a partial intentai Rabi oscillation. Electro-optic detection monitors the Larmor precession of the Bloch vector directly in the time domain, with femtosecond resolution.