Reflectance and emission spectra of excitonic polaritons in GaN

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Abstract
High-resolution reflectance and emission spectra have been measured for high-quality free-standing GaN nearly free from residual strains and impurities. They have been analyzed based on a model exciton-polariton picture in which free A, B, and C excitons couple simultaneously to an electromagnetic wave, where the effective-mass anisotropy and the optical anisotropy are taken into account. Taking account of the free-exciton damping, we have calculated not only the dispersion relations but also the energy dependence of the imaginary part of the wave vectors for the excitonic polaritons. Furthermore, the lifetime of each polariton branch has been calculated combining the imaginary part of the polariton wave vectors and the group velocity obtained from the polariton dispersion relations. It is demonstrated that information on the polariton lifetime is indispensable for interpreting the emission spectra. A brief discussion will be given on obtained values for some physical parameters, including hole parameters.