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Excitonic optical properties of GaN have been studied by means of time-resolved nonlinear luminescence spectroscopy, which was based on an excitation correlation technique under excitation at which a transition of the dominant radiative recombination process occurred from donor-bound excitons to biexcitons. Nonlinear luminescence observed at a donor-bound-exciton line resulted from the superposition of three components: a faster-decay component of a superlinear signal, a slower-decay component of a sublinear signal, and a positive base; that is, a much slower-decay component of a superlinear signal as compared with the time range employed in the present measurement (∼600 ps). The sublinear signal was attributed to the state-filling effect of donor-bound excitons, and the positive base was attributed to the saturation of nonradiative recombination centers. In addition, the superlinear signal was attributed to the stimulation of biexciton luminescence, which appeared close to the donor-bound-exciton luminescence. This type of time-resolved nonlinear spectroscopy enabled us to observe separately the contributions of bound excitons and biexcitons even at the same excitation level, through the difference in optical nonlinearity. © 2004 American Institute of Physics.