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We report the observation of optical polarization switching in ${\text{In}}_{x}{\text{Ga}}_{1\ensuremath{-}x}\text{N}/\text{GaN}$ quantum well active layers, using semipolar ${11\overline{2}2}$ planes. When the In composition is less than $\ensuremath{\sim}30%$, the emissions related to the top and second valence bands are polarized along the $[1\overline{1}00]$ and perpendicular $[\overline{1}\overline{1}23]$ directions, respectively, similar to earlier studies. On the contrary, as the In composition increases above 30%, the polarizations switch, indicating a crossover between the two valence bands. Because the polarization degree is less sensitive to the well width, the observed polarization switch is ascribed to the InN deformation potentials.

The well width dependence of internal electric field effects was investigated using nondegenerate pump and probe spectroscopy at low and room temperature in four types of InGaN-based semiconductors of active layer thicknesses (a) 30 nm (single layer), (b) 10 nm (3 periods), (c) 5 nm (6 periods), and (d) 3 nm (10 periods). For sample (a) and (b) photoinduced absorption was observed due to screening of the internal electric field at low and room temperature. We observed two competing effects, exciton localization and the internal electric field, in sample (c) at low temperature. For sample (d) only photobleaching was observed due to occupation at the localized states. The results show that the field screening effects are more important for increasing well width after carrier generation. Furthermore, the carrier density to observe the photoinduced absorption due to screening the internal electric field is much less than the carrier density for stimulated emission at room temperature.

The formation of not only A biexcitons ${(XX}_{\mathrm{AA}})$ but also heterobiexcitons that consist of A and B excitons ${(XX}_{\mathrm{AB}})$ in a free-standing bulk GaN is identified by polarization-sensitive spectrally resolved FWM measurements. The FWM spectra and delay-time dependence show that the interaction between A and B exciton gives rise to the energy shifts of the spectra and the phase shifts of the quantum beating, which is considered as the effect of the unbound state of ${\mathrm{XX}}_{\mathrm{AB}}$ (i.e., ${\mathrm{XX}}_{\mathrm{AB}}^{*})$ and ${\mathrm{XX}}_{\mathrm{AB}}^{*}$ is found to play an important role in the FWM signals for all polarizations. The unbound A biexciton ${(XX}_{\mathrm{AA}}^{*})$ is also observed clearly in spectral and temporal domains.

We have studied the properties of coherent exciton states in a free standing wurtzite GaN using the degenerate four-wave-mixing (FWM) technique with subpicosecond time resolution. The polarization dependence of spectrally resolved FWM signals has demonstrated that excitation-induced dephasing is the dominant FWM process. The exciton energy renormalization has been found to dependent on the polarization configuration. FWM signals due to local field effect have been successfully discriminated from those due to other effects. These reveal an importance of four-particle correlations in two-exciton continuum states even for the low excitation power limit where no bound biexciton signal is detected. The temperature dependence of the homogeneous broadening of excitons has also been studied up to 60 K, to obtain the exciton-acoustic phonon interaction coefficient of $9 \ensuremath{\mu}\mathrm{eV}/\mathrm{K}.$