Exciton--Exciton Interactions in Tensile-Strained GaN

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We report on the four-wave mixing (FWM) spectroscopy of excitons in tensile-strained GaN films. Three excitonic resonances (A-, B-, and C-excitons) were clearly observed with energy separations of \Delta_{\text{AB}}\sim 4.7 meV and \Delta_{\text{BC}}\sim 14.5 meV. Owing to tensile strain, these values are much smaller than those of free-standing GaN, allowing the simultaneous excitations of A--B and B--C excitons pairs. The time evolutions of the FWM in different polarization configurations resolved the differences in the simultaneous excitonic transitions; the excitation of A--B excitons showed an oscillation with a critical \pi-phase shift in the orthogonal polarization configuration as expected in the quantum beat, and the oscillation of B--C excitons pair is independent of the polarization configuration, indicating polarization interference. The polarization-dependent FWM spectra of A--B excitons revealed peaks associated with biexciton transitions with binding energies of E_{\text{AA}} = 4.6 meV, E_{\text{AB}} = 2.1 meV, and E_{\text{BB}} = 1.7 meV.
2013-08-25