t Solid State Conzsunications, Vol.32, pp.301—304. PerganEn Press Ltd. 1979. Printed in Great Britain. RESONANT RAMAN SCATTERING BY EXCITONIC POLARITONS IN SEMICONDUCTORS Arao Nakamurat and Claude Weisbuch Laboratoire de Physique de la Matière Condens&e 5 Ecole Polytechnique, 91128 Palaiseau, France M. Balkanski. Manuscript received on April 26, 1979. Revised manuscript received on July 23, 1979. We report results on Resonant Reman Scattering (RRS) mediated by excitonic polaritons in a high—purity semiconductor (CdTe) at low temperatures. For the first time imgoing and outgoing resonances at the n — I, 2, 3 exciton states are detected on the one amd two LO— phonon RRS. The transformation at resonance of sharp Reman peaks into well—thermalized exciton luminescence bands is observed for every ingoing, outgoing or intermediate state resonance. The resonant behaviour of Reman Scattering polariton description of RRS, as these coupled— (RS) near the fundamental edge of semiconductors mode excitations have been shown by various has been wide1~’studied both experimentally and experiments” to describe exciton—associated theoretically. Depending on the hierarchy of effects in pure direct—gap semiconductors at interactions (electron—hole, electron—phonon, low temperatures. In such a description, the exciton—phonon, exciton—photon) the electronic backscatterin~efficiency per unit solid angle states mediating the scattering process are is given by5’ uncorrelated electron—hole pairs, free excitons formed by Coulomb—correlated electron—hole dP TT’k’2L pairs or excitonic polaritons, which are the V ~ <k’IVIk>12S S’ (1) ee coupled-mode excitations formed by the strongly g interacting excitons and photons. The present where, at a given energy, T is the transmission study evidences two prominent features of coefficient of photons as polaritons at the polariton-mediated Reman scattering in a high— crystal interface (or the reverse for T’), vg purity semiconductor (CdTe) at low temperatures: is the group velocity of polaritons,7 k their (i) a huge increase of the Reman efficiency2 at wavevector, L the Reman active depth of the reaonance (ii) the complete transformation of cr’stal and S~ the exciton content of the sharp Reman scattering peaks into thermalized polariton wavefunction5 (primed quantities luminescence bands at excitonic resonances.3 correspond to scattered or outgoing quantities). The experiments are performed on high— From the known parameters of CdTe8 (transverse purity n—type CdTe (IND + NAJ~1O1’~ ~ exciton energy ET — 1.5958 eV, longitudinal— p35K ~ 1O~cm2 V s~) cleaved along the [110] transverse splitting ~LT — 0.4 meV), the face, immersed in a pumped—Re cryostat exciton content is near unity in the whole (T — 1 .8 K) and e~ccitedwith a cv dye laser range reported here. The matrix element is that (linevidth ~ 0.1 A). The secondary emission of the exciton—phonon interaction between spectrum is analyzed in a double—spectrometer excitonic states with the same wavevectors as equiped with a cooled GaAs—photocathode photo— the polaritons. Therefore, the polariton multiplier and photon—counting capability. We scattering probability is given by that of concentrate here on the main feature of the excitons, the polariton character being spectrum, the “forbidden” LO—phonon back— accounted for by the prefactor of (1). The scattering caused by the Fr6hlich interaction.1 resonance behaviour is determined by the In our experimental conditions, the sample variations of the ingoing or outgoing group thickness (~1 mm) is always larger than the velocities, which are almost equal to exciton penetration depth of light so that we are deal— velocities at resonance and tend towards light ing with a semi—infinite geometry and can velocity far from resonance. The transmission characterize the Reman efficiency by a back— coefficients can be considered as constant in scattering efficiency. most of the range. The measured scattering Let us first discuss the scattering efficiencies are shown in Figure 1 and their efficiency properties. We shall use an excitonic— overall features resemble strikingly those of the Bendow—Birman description of polariton— mediated RS, which clearly provides a sound basis for the analysis of such phenomena. tPresent address : Institute for Solid Resonances are observed whenever the ingoing or State Physics, University of Tokyo, outgoing photons are resonant with the Roppongi, Minato—ku, Tokyo, Japan. exciton energies.9 The insert of Fig.Ia shows that the n—I peak is structured : this is *Groupe de Recherches du Centre National most certainly due to the existence of heavy de la Recherche Scientifique. and light excitons (i.e. polaritons) in zinc— 301