1. See, for example, M. Cardona inModulation Spectroscopy (Academic, New York, 1969) and Refs. therein.
2. See, for example, D. E. Aspnes inHandbook on Semiconductors, ed. T. S. Moss (North Holland), Amsterdam, 1980) Vol. 2, p. 109 and Refs. therein.
3. See, for example, Y. Hamakawa and T. Nishino inOptical Properties of Solids: New Development, ed. by B. 0. Seraphin (North Holland, N.Y. 1976) p. 259.
4. See, for example, F. H. Pollak inProc. S.P.I.E. (Bellingham, 1981)276, 142 (1981).
5. Y. R. Lee, A. K. Ramdas, L. A. Kolodziejski and R. L. Gunshor, Phys. Rev.B38, 13143(1988).