Stimulated emission induced by two-photon absorption in GaAs quantum wells

We report evidence of stimulated emission induced by two-photon optical pumping in properly designated GaAs/${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As multiple quantum wells. The two-photon absorption coefficient is measured in order to evaluate the electron-hole pair density photogenerated in the crystal around the stimulated emission threshold. The determined threshold electron-hole density of ${\mathit{n}}_{\mathit{p}}$=3×${10}^{11}$ ${\mathrm{cm}}^{\mathrm{-}2}$ is in good agreement with the value reported for one-photon excitation.