We demonstrate that a Rayleigh surface acoustic wave ͑SAW͒ propagating on the surface of a ͑001͒-oriented GaAs quantum well structure induces different optical properties for propagation directions along the ͓110͔ and the ͓1 10͔ symmetry axes. The nonequivalence of the two directions is attributed to the lift of the fourfold rotation symmetry of the optical and acoustic properties around the ͓001͔ growth axis by the finite wave vector of the SAW, a spatial dispersion effect which is also expected for bulk modes in zinc blende semiconductors.The symmetry properties of elementary excitations in a crystal are mainly determined by the crystal point group. In addition, nonlocal effects make these properties also dependent on the wave vector q of the excitation, a phenomenon called spatial dispersion. 1 For excitations with wavelengths much larger than the lattice constant, such as elastic waves and photons in the visible range, spatial dispersion is normally small; as a result, the qϭ0 limit applies and their symmetry becomes imposed by the crystal point group. Under appropriate conditions, however, a nonvanishing q may lower the symmetry relative to the crystal. Interesting examples of the reduction in symmetry of the optical properties of cubic semiconductors include the intrinsic optical anisotropy 2 and the pressure-induced optical activity, 3 which are of second and first order in q, respectively. Spatial dispersion also affects the symmetry of vibrational modes. 4 These effects are particularly interesting in polar crystals, where vibrations with a nonvanishing q may induce a macroscopic electric polarization field. The electronphonon interaction becomes mediated not only by the ordinary deformation potential but also by the piezoelectric ͑for acoustic phonons͒ or Fröhlich ͑for optical phonons͒ mechanisms associated with the polarization field. Recently, Ilisavskii et al. 5 reported differences in acoustoelectric properties induced by a reversal in propagation direction of elastic modes in La 0.67 Ca 0.33 MnO 3 layers on LiNbO 3 , which were attributed to an interference between the deformation and piezoelectric mechanisms. In zinc blende semiconductors, the interference between the deformation potential and the Fröhlich interactions induced by a LO mode propagating along the zϭ͓001͔ direction leads to different Raman polarizabilities along the xЈϭ͓110͔ and yЈϭ͓1 10͔ axes. 6 In this Brief Report, we show that the superposition of the two mechanisms induced by an acoustic mode in ͑Al,Ga͒As quantum well ͑QW͒ structures also removes the invariance of the optical properties under a 90°rotation around the z ϭ͓001͔ growth axis. Specifically, we demonstrate that the phase relationship between the acoustic ͑u͒ and the piezoelectric (⌽) fields composing the wave function of elastic modes differs by 180°for modes propagating along the xЈ and yЈ axes, thus leading to different optical properties. The phase difference, which is also expected in zinc blende crystals, arises from the distinct transformation properties of the ...