Evaluation of optical confinement in GaN-based lasing structures

We present a technique for evaluating optical confinement in GaN-based lasi ng structures through their spectrally resolved near-field pattern under hi gh optical excitation. Emission spectra were found to be strongly dependent on the position of collection optics relative to the active region when th e sample was excited above the lasing threshold. The spatially resolved spe ctra contain a modulation signature that can be used to deduce the optical confinement characteristics. We show that the observed index-guided modes r esulted from multiple internal reflections very near the critical angle of total internal reflection between the semiconductor layers with different r efractive indices. This technique was used to evaluate the degree of optica l confinement in GaN epilayers and GaN/AlGaN separate confinement heterostr ucture. The implications of this study on the design of GaN-based laser dio des is discussed.