Exciton-polariton propagation in AlGaN/GaN quantum-well waveguides probed by time-resolved photoluminescence

The propagation properties of light in AlGaN/GaN multiple-quantum-well (MQW) waveguides have been studied by time-resolved photoluminescence (PL) spectroscopy. The waveguides were patterned with fixed width of 0.5 μm but orientations varying from -30° to 60° relative to the a-axis of GaN by electron-beam lithography and inductively-coupled plasma (ICP) dry etching. The peak position and line-width of the emission peak were found to vary systematically with orientations of the waveguides and followed the six-fold symmetry of the wurtzite structure. This is explained in terms of anisotropy of the exciton/carrier diffusion coefficient along the different crystal orientations of the semiconductor materials. We also observed a remarkable decrease in the PL intensity as well as increase in time delay of the temporal response as the location of the laser excitation spot on the waveguide is varied. These observations can be understood in terms of exciton-polariton propagation in the waveguides. The speed of generated polaritons with energy corresponding to the well transitions in the waveguides was determined from the time delay of the temporal response to be approximately (1.26 ± 0.16) × 10⁷ m/sec. The optical loss in the waveguides was determined to be about 5 - 8 cm^-1 for different excitation intensities. The implications of these results to waveguiding in optical devices based on the III-nitride semiconductors are discussed.