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Physical Review A

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

Andreas Hemmerich, Theodor W. Hänsch

Published Sep 1, 1993• 11 citations
Publisher

Abstract

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.

DOI
10.1103/physreva.48.r1753

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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.
We have observed quantized motion of rubidium atoms spatially confined by dipole forces inside microscopic light traps. A cubic lattice of such traps is formed by three mutually orthogonal standing waves. We compare 2D and 3D confinement in the same experiment by observing the absorption of a weak probe laser beam. In both cases we observe Raman transitions between vibrational levels and a central Rayleigh resonance. When we switch to 3D confinement the line width of this Rayleigh resonance dramatically decreases to values below 1 kHz.
We have observed the vibrational levels of rubidium atoms spatially confined in two dimensions by two crossed optical standing waves (oriented along the x and y axes) with mutually orthogonal linear polarization (in the x-y plane). When the two standing waves oscillate with a 90\ifmmode^\circ\else\textdegree\fi{} time phase delay, the atoms are confined by dipole forces to the intensity maxima forming a two-dimensional collimated array of linear de Broglie waveguides, spaced by half an optical wavelength. When the standing waves oscillate in phase no vibrational energy structure can be observed.
We present a calculation of the probe-transmission spectrum of a two-dimensional optical lattice, in which atoms are cooled by a pair of standing-wave fields with perpendicular polarizations within the plane of incidence. A probe laser is directed along the axis of one of the cooling fields, with a polarization parallel to the incident plane of the cooling fields. The calculated probe-transmission spectrum exhibits both sidebands, which originate from stimulated Raman transitions between different vibrational states of the atoms in the molasses, and a central resonance structure, which originates from Rayleigh resonances among these vibrational states. We also include in our calculation a possible misalignment of the probe field relative to the axis of the cooling field. It is shown that the effrects of the probe misalignment can lead to significant modifications to the central resonance signal.
In a one-dimensional polarization-gradient cooling scheme, with two counter-propagating laser beams having mutually orthogonal linear polarization, a linear array of potential wells for cold atoms are formed. These optical potential wells are created by the spatially varying light shift of the atomic ground state. Transitions between quantized vibrational states in such wells have been observed by stimulated1 and spontaneous2 Raman spectroscopy. Recently, this was extended to three dimensions using both four beams3 and six beams.4 In this work, we intersect four travelling-wave laser beams, all polarized in the same plane, as indicated in Fig. 1. This produces a threedimensional body-centered cubic lattice of potential wells for cesium atoms. The laser wavelength is tuned 25 linewidths below the 62S1/2(F = 4) → 62P3/2(F = 5) transition at 852 nm; and the resultant lattice constants (distances between wells of the same character) are λ / 2 perpendicular to the plane of polarization, and λ × 2 in the plane of polarization.
A sodium atomic beam is deflected by two optical standing-wave fields which excite near resonance Raman transitions. Observed deflections are consistent with theoretical predictions of a long-range dc component of the force on a three-level atom in the \ensuremath{\Lambda} configuration. The low laser intensities (and small detunings) used, combined with observed evidence of damping in the three-level \ensuremath{\Lambda} system, may ultimately lead to the design of high-density, all optical traps wherein the atoms would be mostly in the dark state.