Line strengths in vibrational spectra of a two-dimensional optical crystal

We have studied Raman transitions between vibrational levels of atoms confined in the antinodes of a two-dimensional (2D) optical standing wave by means of dipole forces. These transitions are excited by the cooperation of the confining 2D field, with a weak probe laser beam directed through the atomic sample. We experimentally find that the relative line strengths corresponding to transitions from the vibrational ground state to the first and second excited states depend on the polarization of the probe beam. The excitation mechanism is based on a time-modulated distortion of the potential wells. We discuss how the spatial geometry of this distortion depends on the probe polarization, thus qualitatively explaining the experimental observations.