Line width narrowing of polaritons P. Kinsler, D.M. Whittaker Department of Physics, University of Sheffield, Sheffield S3 7RH, United Kingdom. Phys. Rev. B 54, 4988 (1996). Polaritons are formed when the electromagnetic field couples to excitons in a semiconductor. In their high quality semiconductor quantum microcavities, Fisher et al \cite{Fisher-ASRHP-1995pp} observe an unexpected decrease in the polariton linewidth (the FWHM) near resonance. This paper uses Greens function methods to show how the exciton linewidth and the cavity losses combine together, and that they can give a polariton linewidth reduced below that of either of its components. The well disorder is described as a spatially varying well potential due to both alloy fluctuations and well width variation which causes the exciton component to become broadened. Email: Dr.Paul.Kinsler@physics.org