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We have observed photoinduced negative optical conductivity, or gain, in theterahertz frequency range in a GaAs multiple-quantum-well structure in a strongperpendicular magnetic field at low temperatures. The gain is narrow-band: itappears as a sharp peak (linewidth $<$0.45 meV) whose frequency shifts withapplied magnetic field. The gain has a circular-polarization selection rule: astrong line is observed for hole-cyclotron-resonance-active polarization.Furthermore, the gain appears only when the exciton $1s$ state is populated,which rules out intraexcitonic transitions to be its origin. Based on theseobservations, we propose a possible process in which the stimulated emission ofa terahertz photon occurs while two free excitons scatter into one biexciton inan energy and angular-momentum conserving manner.

Observation of Photoinduced Terahertz Gain in GaAs Quantum Wells: Evidence for Radiative Two-Exciton-to-Biexciton Scattering