ABSTRACT
This chapter gives a brief review of the present knowledge of the optical properties of the materials, from an experimental perspective. It concentrates on recent work within the areas where considerable progress in the knowledge of optical properties has recently been made, and refers the reader to other more extensive reviews for a more complete coverage. In order to obtain reliable values for bulk unstrained GaN, one has to study very thick epilayers, where the strain has been relieved to a large extent via cracking. Alternatively one can study homoepitaxial layers, which until very recently have not been available. A different technique to measure the fundamental exciton resonances is calorimetric absorption or reflection at very low temperatures. Such data for GaN were recently reported for a thick epilayer. Recently a large number of papers have appeared, discussing experimental data on stimulated emission and gain in both GaN bulk material and related heterostructures.
