Transmission spectra of thin CdS and CdSe crystals near higher-order isotropic points

Abstract

(ricevuto il 12 Lugiio 1993; revisionato il 29 Gennaio 1994; approvato il 12 Febbraio 1994) Summary. --For photon energies below the absorption edge in CdS, CdSe and other II-VI crystals the polariton dispersion curves for E • c and E II c (c-axis in a wurtzite crystal) cross at some points called isotropic points (IP). The occurrence of isotropic points provides the possibility of mode coupling between ordinary and extraordinary waves. Since the consequences of mode coupling on the optical properties for photon energies near the lowest IP lying much below the f'u'st excitonic state were widely discussed in recent years, more attention is now paid to isotropic points lying near the band gap and related to the n = 2, 3, ... excitonic states (,,higher isotropic points,0. Making use of Stahrs real density matrix approach we derive the polariton dispersion relations k• (~o), and kl, (oJ), for CdS and CdSe bulk crystals and determine the positions of IP's due to the crossing of the B-polariton with higher A-excitonie resonances. By the method of multiple internal reflection we calculate the transmission spectra for various crystal thicknesses (between 3 ~m and 0.5 mm) and coupling mechanisms. The calculated transmission shows sharply peaked structures centred at the isotropic points. PACS 71.35 -Excitons and related phenomena (including electron-hole drops). PACS 71.36 -Polaritons (including photon-photon and photon-magnon interactions). PACS 78.20 -Optical properties of bulk materials.