Exciton–Exciton Interactions in Tensile-Strained GaN

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 ΔAB∼4.7 meV and ΔBC∼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 π-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 EAA = 4.6 meV, EAB = 2.1 meV, and EBB = 1.7 meV.

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