Ultraviolet anti-Stokes photoluminescence in InxGa1-xN/GaN quantum-well structures

Ultraviolet anti-Stokes photoluminescence (PL) is observed in InxGa1-xN/GaN multiple quantum wells. The observed anti-Stokes PL exhibits a quadratic dependence on the excitation energy density. Anti-Stokes PL excitation spectrum is proportional to the optical absorption spectrum of the InxGa1-xN quantum wells. Time-resolved PL measurement shows that a decay of the anti-Stokes PL is slower than that of the GaN PL under the excitation above the band gap of the GaN barrier, and it is half the time constant of the InxGa1-xN PL decay. A two-step two-photon absorption process is directly observed by means of two-color pump-and-probe experiment. It is considered that the anti-Stokes PL is caused by a two-step two-photon absorption process involving a localized state in the InxGa1-xN quantum wells as the intermediate state, and that the second absorption step is provided by photon recycling of the InxGa1-xN PL.