  We have observed photoinduced negative optical conductivity, or gain, in the
terahertz frequency range in a GaAs multiple-quantum-well structure in a strong
perpendicular magnetic field at low temperatures. The gain is narrow-band: it
appears as a sharp peak (linewidth $<$0.45 meV) whose frequency shifts with
applied magnetic field. The gain has a circular-polarization selection rule: a
strong 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 these
observations, we propose a possible process in which the stimulated emission of
a terahertz photon occurs while two free excitons scatter into one biexciton in
an energy and angular-momentum conserving manner.
