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arXiv:physics/9611019

Band Population Measurements in a Purely Optical Dark Lattice

F. Sander, Tilman Esslinger, Theodor W. Hänsch, Herwig Stecher, Helmut Ritsch

Published Nov 22, 1996• 0 citations
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Abstract

We create a dark optical lattice structure using a blue detuned laser field coupling an atomic ground state of total angular momentum F simultaneously to two excited states with angular momenta F and F-1, or F and F+1. The atoms are trapped at locations of purely circular polarization. The cooling process efficiently accumulates almost half of the atomic population in the lowest energy band which is only weakly coupled to the light field. The populations of the two lowest energy bands reaches 70%. Kinetic energies on the order of the recoil energy are obtained by adiabatically reducing the optical potential. The band populations are directly mapped on free particle momentum intervals by this adiabatic release. In an experiment with subrecoil momentum resolution we measure the band populations and find good absolute agreement with the theoretically calculated steady state band populations.

DOI
10.48550/arxiv.physics/9611019

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We create a dark optical lattice structure using a blue detuned laser field coupling an atomic ground state of total angular momentum F simultaneously to two excited states with angular momenta F and F-1, or F and F+1. The atoms are trapped at locations of purely circular polarization. The cooling process efficiently accumulates almost half of the atomic population in the lowest energy band which is only weakly coupled to the light field. The populations of the two lowest energy bands reaches 70%. Kinetic energies on the order of the recoil energy are obtained by adiabatically reducing the optical potential. The band populations are directly mapped on free particle momentum intervals by this adiabatic release. In an experiment with subrecoil momentum resolution we measure the band populations and find good absolute agreement with the theoretically calculated steady state band populations.
We create a gray optical lattice structure using a blue detuned laser field coupling an atomic ground state of angular momentum J simultaneously to two excited states with angular momenta J and J-1. The atoms are cooled and trapped at locations of purely circular polarization. The cooling process efficiently accumulates almost half of the atomic population in the lowest-energy band, which is only weakly coupled to the light field. Very low kinetic temperatures are obtained by adiabatically reducing the optical potential. The dynamics of this process is analyzed using a full quantum Monte Carlo simulation. The calculations explicitly show the mapping of the band populations on the corresponding momentum intervals of the free atom. In an experiment with subrecoil momentum resolution we measure the band populations and find excellent absolute agreement with the theoretical calculations.
We study a new type of optical lattice in which the localized atoms experience a much reduced optical pumping and fluorescence rate. An optical standing wave is tuned to the blue of the F = 2 ? F = 2 transition of the (87)Rb D(2) line and induces periodic optical potentials by coupling the F = 2 ground state to both the F = 2 and F = 3 excited states. A Sisyphus mechanism efficiently cools the atoms into the lattice sites. We adiabatically release the atoms from the optical lattice and measure their momentum distribution with a resolution of one third of a single photon recoil. This allows us to determine the population of the two lowest energy bands in the optical lattice (44% and 20%).
We show that by using an atomic-transition scheme including a common angular momentum J g = 1 atomic ground state and two excited states with angular momenta J e 1 = J e 2 = 1 or J e 1 = 0 and J e 2 = 1, one can construct an all-optical trap, where atoms are trapped and cooled towards a single nearly dark quantum state with a spatial dimension of the order of an optical wavelength. Single-atom calculations for two blue detuned counterpropagating σ + and σ − polarized light fields in one dimension predict that close to 100% of the atomic population will be transferred to the lowest quantum state.
The \ensuremath{\Lambda} system resulting from the interaction of counterpropagating orthogonally polarized beams of light with a Jg{\mathit{J}}_{\mathit{g}}Jg​=1\ensuremath{\rightarrow}Je{\mathit{J}}_{\mathit{e}}Je​=1 transition is known to exhibit laser cooling without the presence of a damping force, through velocity selective coherent population trapping. This occurs independent of the sign of the laser's detuning from the atomic resonance frequency. We present measurements and calculations that show a detuning-dependent transient peak or dip in the ground-state momentum distribution centered at pg{\mathit{p}}_{\mathit{g}}pg​=0, however, which is reminiscent of laser cooling by a damping force. We explain this in terms of the character of the quantum-mechanical eigenstates of the total ground-state Hamiltonian of the system, using both analytic expressions and numerical calculations.
The quantum-mechanical transport properties of cold atoms in an optical lattice are described in terms of an analytically tractable model. The atoms are cooled down to the lowest energy band in the periodic optical potential, in the tight- binding limit. The transport mechanisms are coherent tunneling through potential barriers, and momentum recoil at optical pumping. Expressions are obtained for the time-dependent population distributions over the energy eigenstates and over localized states. These expressions are modified in the presence of a dark atomic state, that is uncoupled to the field.
The probe-transmission spectrum of a one-dimensional optical lattice is calculated. The optical lattice consists of atoms that are cooled by a pair of counterpropagating, linearly polarized fields, whose frequency \ensuremath{\omega} is detuned to the blue side of an atomic transition. A weak probe field of frequency ${\mathrm{\ensuremath{\omega}}}^{\ensuremath{'}}$ is assumed to be propagating in the same direction as one of the cooling fields. It is shown that, owing to the existence of dark and gray states in the lattice, the probe-transmission spectrum exhibits interesting features that are qualitatively different from those of a ``normal'' optical lattice with red-detuned cooling fields. Examples of these features include subrecoil linewidths and reversed absorption-amplification sidebands. It is shown that by monitoring the magnitudes of resonance signals in the spectrum, one can gain information on the evolution of the dark-state population in the lattice.