We have observed the Raman scattering from surface polaritons in a thin film (« 2500 A thick) of GaAs on a sapphire substrate. The mode frequency varies as a function of the scattering angle from the TO phonon frequency of GaAs, a> TO = 270 cm" 1 , corresponding to forward scattering, to the asymptotic value of 283 cm" 1 , corresponding to a large scattering angle. The dispersion of the mode frequency was found to be appropriate for the surface polariton of a GaAs film bounded by a sapphire substrate on one side and by air on the other side.In recent years, considerable interest has developed in the surface polaritons of infrared-active optical phonons in crystals. Surface polaritons are admixtures of optical phonons and photons and, like the bulk polaritons, show strong dispersion in the long-wave length limit. Several theoretical studies on this subject have appeared in the literature in the past few years. 1 ' 2 Experimentally, surface polaritons with frequencies in the infrared have been observed by the attenuated total reflection (ATR) method 3 " 5 and by the lowenergy electron-diffraction (LEED) method. 6 Since bulk optical phonons in opaque semiconductors and metals can be observed by the surfacereflection Raman scattering (SRRS) method, 7 * 8 which samples only the optical penetration region of several hundred angstroms, and since the surface-polariton fields extend deeper into a crystal surface than a typical optical penetration depth, we have been convinced that surface polaritons should be observable by means of Raman scattering.In this Letter, we report the first measurement of the dispersion of surface polaritons by Raman scattering. The Raman spectra of a thin film (-2500 A) of GaAs on a sapphire substrate were measured for different scattering angles, and the dispersion curve of the surface polaritons of the GaAs film was obtained. Comparison of the experimental dispersion curve with the theory of Mills and Maradudin 9 shows that the modes observed in our experiment correspond to the surface polaritons of a layered structure composed of vacuum-GaAs-sapphire.In Fig. 1, for the purpose of comparison, we show three dispersion curves for surface polaritons: a, at a GaAs-vacuum interface; 6, at a GaAs-sapphire interface; and c, in a vacuumGaAs-sapphire layered structure. Curve a was obtained from the dispersion relation 2 C 2^| 1 2 /C0 2 =€(a))/[1 + €(C0)](1) for a vacuum-GaAs interface, where q^ is the component of the surface-polariton wave vector parallel to the surface, e(co) is the frequency-dependent dielectric function of GaAs, and c is the speed of light in vacuum. The dielectric function e(a>) for GaAs has the form €(u>) = Coo + K -€oo)w T oV(a>TO 2 -^2) ?(2)where €oo = ll.l, e 0 = 13.1, and ^ TO ~ 270 cm The dispersion curve b for the film-substrate interface, when the substrate is a (0001) sapphire surface, is given by 10 ' 11 c 2 ffi, 2^€ ||€(co)[€(a?)~eJ (JO 2 € 2 (Ct))-€x£,| '(3)