Physical Review Letters ( IF 8.385 ) Pub Date : 2020-10-16 , DOI: 10.1103/physrevlett.125.167401
Xinwei Li; Katsumasa Yoshioka; Qi Zhang; Nicolas Marquez Peraca; Fumiya Katsutani; Weilu Gao; G. Timothy Noe; II; John D. Watson; Michael J. Manfra; Ikufumi Katayama; Jun Takeda; Junichiro Kono

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 $<0.45\text{\hspace{0.17em}}\text{\hspace{0.17em}}\mathrm{meV}$) 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.

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