Ultrafast carrier relaxation in GaN, ${\mathrm{In}}_{0.05}{\mathrm{Ga}}_{0.95}\mathrm{N},$ and an ${\mathrm{In}}_{0.07}{\mathrm{Ga}}_{0.93}\mathrm{N}/{\mathrm{In}}_{0.12}{\mathrm{Ga}}_{0.88}\mathrm{N}$ multiple quantum well

Room-temperature, wavelength-nondegenerate ultrafast pump/probe measurements were performed on GaN and InGaN epilayers and an InGaN multiple quantum well (QW) structure. Carrier relaxation dynamics were investigated as a function of excitation wavelength and intensity. Spectrally resolved sub-picosecond relaxation due to carrier redistribution and QW capture was found to depend sensitively on the wavelength of pump excitation. Moreover, for pump intensities above a threshold of $100\mu {\mathrm{J}/\mathrm{c}\mathrm{m}}^2,$ all samples demonstrated an additional emission feature arising from stimulated emission (SE). SE is evidenced as accelerated relaxation $(<\mathrm{}10\mathrm{}\mathrm{ps})$ in the pump-probe data, fundamentally altering the redistribution of carriers. Once SE and carrier redistribution is completed, a slower relaxation of up to 1 ns for GaN and InGaN epilayers, and 660 ps for the multiple QW sample, indicates carrier recombination through spontaneous emission.