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Physical Review Letters

Multiphoton transitions in a spin-polarized 3D optical lattice

Andreas Hemmerich, Claus Zimmermann, Theodor W. Hänsch

Published Jan 31, 1994• 44 citations
Publisher

Abstract

We have observed novel resonances occuring at subharmonic frequencies in probe transmission spectra of ultracold atoms confined in the antinodes of a 3D optical standing wave by means of dipole forces. We interpret these resonances in terms of multiphoton Raman transitions between vibrational states of the trapped atoms.
DOI
10.1103/physrevlett.72.625

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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.
A subharmonic resonance occurs on irradiation of a three-level (\ensuremath{\Lambda}) system by two bichromatic fields when an integer multiple of the common frequency difference matches the Raman detuning between the two lower levels. The occurrence of these multiphoton resonances requires saturation of the Raman transition. The three-level model is applied in order to understand recent experimental studies of subharmonic Raman resonances between vibrational states of optically trapped atoms. I explain the difference between these resonances and the well-known velocity-tuned (Doppleron) resonances in two-level atoms by considering a two-state model with the excited state eliminated. I finally point out a Sisyphus cooling mechanism that is entirely dependent on an additional relaxation of the Raman coherence, comparable to collision-induced resonances.
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 demonstrate a new type of optical lattice which confines atoms in nearly dark states. The lattice is formed by cooperation of an optical standing wave detuned to the blue with respect to an atomic $F\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}1\ensuremath{\rightarrow}F\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}1$ transition and a homogeneous magnetic field. The atoms are predominantly localized where the light is purely $\ensuremath{\pi}$ polarized such that they are optically pumped to the mF=0{m}_{F}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}0mF​=0 Zeeman component which is decoupled from the light field. We have observed quantized vibrations of the localized atoms by means of probe transmission spectroscopy. Our ``dark'' optical lattice works in two and three dimensions.
We have investigated higher order multiphoton Raman resonances with two pulsed optical frequencies. Multiphoton transfer with up to 50 photons is observed with milliwatts of laser power. We demonstrate that the spectral width of the multiphoton resonances can be far below the Fourier transform linewidth of the driving optical pulses. The functional dependence of the transition linewidth on the number of exchanged photons is found to vary with the pulse shape. Our experiment is performed with laser-cooled rubidium atoms confined in a CO2-laser optical dipole trap.
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.