Shedding Light on the Emission Mechanisms of In$_{0.54}$Ga$_{0.46}$N Disks in GaN Nanowires Using C.W. Non-Linear Spectroscopy

Linear and non-linear spectroscopy has been performed on an ensemble of 3 nm thick In$_{0.54}$Ga$_{0.46}$N disks grown in self-assembled GaN nanowires ($\sim$ 30 nm diameter). PLE measurements nearly resonant with the PL show a mostly broad, featureless spectrum with a linear increase in absorption as a function of energy, similar to InGaN/GaN quantum wells. Unlike InGaN quantum wells, the centroid of the PL spectrum shows a negligible intensity-dependent shift in PL emission wavelength. Further, the non-linear optical spectrum is dominated by excitonic resonances with line widths $\sim$ 20-30 meV in the same region as the PLE data. Distinguishable peaks in the PL spectrum overlap with the non-linear resonances. Continuous -wave nearly degenerate pump-probe absorption measurements show no evidence of spectral hole burning within the resonances; however there is evidence of population pulsations in the 3$^{\mathrm{rd}}$ order signal. This data shows evidence consistent with regular excitonic saturation and two beam coupling similar to that expected in discrete (e.g. 2 or 3 level systems). The excitonic behavior is also consistent with anti-bunching seen in g2 measurements from single dots.