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 $<0.45\mathrm{meV}$) 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.

中文翻译：

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GaAs量子阱中光诱导的太赫兹增益的观察：辐射的两个激子到双激子散射的证据。

我们已经在低温下在强垂直磁场中的GaAs多量子阱结构中的太赫兹频率范围内观察到光诱导的负光导率或增益。增益为窄带：显示为尖峰（线宽$<0.45\mathrm{病毒}$），其频率随施加的磁场而变化。增益具有圆极化选择规则：对于空穴回旋共振活性极化，观察到一条强线。此外，仅当激子$\mathrm{1个}s$填充状态，排除了兴奋内跃迁是其起源。基于这些观察，我们提出了一个可能的过程，其中发生太赫兹光子的受激发射，而两个自由激子以能量和角动量守恒的方式散射成一个双激子。