A technique for evaluating optical confinement in GaN-based lasing structures

We present a technique for evaluating optical confinement in GaN-based lasi ng structures by studying their spectrally resolved near-field pattern unde r high optical excitation. Emission spectra were found to be strongly depen dent on the position of the collection optics relative to the active region when the sample was excited above the lasing threshold. The spatially reso lved spectra contain a modulation signature that can be used to deduce the optical confinement characteristics. We show that the observed index-guided modes result from multiple internal reflections at angles very close to th e critical angle for total internal reflection between the semiconductor la yers with different refractive indices. This technique was used to evaluate the degree of optical confinement in GaN epilayers and GaN/AlGaN separate confinement heterostructures. The implications of this study on the design of GaN-based laser diodes are discussed. (C) 1999 American Institute of Phy sics. [S0003-6951(99)00941-9].