Two-photon photoluminescence and excitation spectra of InGaN∕GaN quantum wells

We report an observation of efficient two-photon photoluminescence (TPL) of InGaN∕GaN multi-quantum-well (MQW) structures using broadband femtosecond near-infrared excitation laser. Near quadratic excitation-intensity dependence and asymmetric collinear interferometric autocorrelation trace of the TPL signal unambiguously verify the nonlinearity of the TPL process. We also measured the excitation spectrum of the TPL signal and found that it can be fitted well with the theoretical two-photon absorption coefficient formula for direct wide gap semiconductors. The decay time of the TPL signal was determined using a time-resolved photoluminescence technique. These results demonstrate the strong nonlinear optical property of InGaN∕GaN MQWs.