Coherent nonlinear optical spectroscopy of InGaN disks in GaN nanowires
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This study explores the potential for coherent control of the quantum state of quantum confined InGaN nanostructures, known for large exciton binding energies, using optical fields for future applications such as quantum information processing up to room temperature. Here we specifically report on the coherent nonlinear optical spectrum of InGaN disks grown in GaN nanowires at both cryogenic temperatures and room temperature. Two different samples are featured in this study: a self-assembled nanowire sample grown on a silicon substrate and a selective area sample grown on GaN/sapphire. Self-assembled DINWs grown on silicon are important for on-chip silicon photonics applications, however the morphological properties of the nanowires are much more difficult to control compared to selective area samples. Using frequency domain nonlinear spectroscopy methods, we find resonances in an ensemble of self-assembled InGaN disks in GaN nanowires that persist without significant broadening up to room temperature that we assign to quantum-confined excitons. Furthermore, the nonlinear optical spectrum as a function of excitation frequency depends only on the disorder state dominated photoluminescence excitation spectrum. Under these growth conditions, we find that the kinetics are dominated by metastable traps with strongly temperature dependent lifetimes corresponding to an activation energy between 5 and 12 meV. The metastable traps are mostly likely formed due to the coalescence of multiple nanowires during growth. The selective area sample is grown free of nanowire coalescence; therefore we do not observe effects from metastable states in the system. The nonlinear optical spectrum in the selective area shows some evidence of spectral hole burning from exciton states without significant additional broadening at room temperature, where the spectral holes have homogeneous linewidths ~20-30 meV at both low temperature and room temperature corresponding to ultrafast decoherence processes. The ultrafast decoherence could be associated with fast electron-hole transfer within a large density of background disorder states that is observed in the nondegenerate nonlinear absorption spectrum. In summary, although the samples show evidence of excitons with high temperature stability, the presence of material disorder and metastable trap states severely limits the use of InGaN disks-in-nanowires for coherent control applications without further material engineering.