The metrological capability of the picosecond four-wave mixing (FWM) technique for evaluation of the photoelectrical properties of GaN heterostructures grown on sapphire, silicon carbide, and silicon substrates as well as of free-standing GaN films is demonstrated. Carrier recombination and transport features have been studied in a wide excitation, temperature, and dislocation density (from similar to 10(10) to 10(6) cm(-2)) range, exploring non-resonant refractive index modulation by a free carrier plasma. The studies allowed to establish the correlations between the dislocation density and the carrier lifetime, diffusion length, and stimulated emission threshold, to reveal a competition between the bimolecular and nonradiative recombination, and to verify the temperature dependence of bimolecular recombination coefficient in the 10-300 K temperature range. It was shown that the FWM technique is more advantageous than the time-resolved photoluminescence technique for determination of carrier lifetimes in high quality thick III-nitride layers. (c) 2006 Elsevier B.V. All rights reserved.