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Physical Review A

Atom trapping and spectroscopy in cavity-generated optical potentials

Yun Jhih Chen, Stefan Zigo, Georg A. Raithel

Published Jun 10, 2014• 15 citations
Publisher

Abstract

We use a near-concentric optical cavity at 1064 nm to generate trapping potentials for cold atoms. The cavity exhibits nondegenerate Hermite-Gaussian modes. Using just a few-milliwatt trap laser power, the cavity readily generates one- and higher-dimensional optical traps that replicate the mode functions. We spectroscopically characterize the optical trapping potentials and laser-cooling limits under continuous loading conditions. We use absorption images to measure atom densities and to compare the spatial profiles of trapped-atom samples with calculated mode functions. Steady-state fluorescence images reveal bright radiation emerging from the ends of the elongated, cavity-trapped atom clouds, providing evidence for radiation guiding.

DOI
10.1103/physreva.89.063409

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•Oct 12, 2007•1 citation
We use a near-concentric optical cavity at 1064 nm to generate trapping potentials for cold atoms. The cavity exhibits nondegenerate Hermite-Gaussian modes. Using just a few-milliwatt trap laser power, the cavity readily generates one- and higher-dimensional optical traps that replicate the mode functions. We spectroscopically characterize the optical trapping potentials and laser-cooling limits under continuous loading conditions. We use absorption images to measure atom densities and to compare the spatial profiles of trapped-atom samples with calculated mode functions. Steady-state fluorescence images reveal bright radiation emerging from the ends of the elongated, cavity-trapped atom clouds, providing evidence for radiation guiding.
We have developed a technique to fully characterize an arbitrary potential energy of an atom trap.The characterization allows one to measure the symmetry of the trap, a determining factor in many trapped atom applications.Created through a magneto-optical trap and evaporative cooling, a cold atom cloud is first loaded into our weakly-confined time-orbiting potential (TOP) trap.The whole atom cloud is then optically pumped into a dark state, an energy level that does not interact with the probe laser light.A selected part of the atom cloud is reactivated by a repump light which optically pump the atoms back into a state that can be probed.The repump light is shaped into a light sheet 168 µm thick.The reactivated region interacts with the probe laser light to create a fluorescence image.Since the light sheet is much thinner than the atom cloud, which is roughly 2 mm wide, the fluorescence image obtained is a cross-section of the atom cloud.A movable light sheet allows us to generate crosssection images of the cloud at different positions.A composite image of all the crosssection images shows the complete potential profile of our atom trap.This is similar to tomographic imaging used in medical imaging.We have verified the technique with two other methods: direct oscillation measurement at varying amplitude and numerical simulation of the atom trap.The technique is able to measure the potential up to the fourth-order terms in spatial coordinates.A complete characterization of the atom trap's symmetry will allow us to develop an atomic Foucault pendulum, a novel application for trapped atoms.5.3 Acceptable level of anharmonicity . . . . . . .
This thesis reports on the design, setup, and operation of a new atom trap apparatus, which is developed to confine few rubidium atoms in ultrahigh vacuum by means of taylored and dynamically variable light potentials (dipole traps), thus making the atoms available for spectroscopy and controlled manipulations. To maintain low background pressure, atoms of a vapour cell are transferred into a cold atomic beam by laser cooling techniques, and accumulated by a magneto-optic trap (MOT) in a separate part of the vacuum system. The laser cooled atoms are then transferred into dipole traps made of focused far-off-resonant laser fields in single or crossed-beam geometry, which are superimposed with the center of the MOT. Gaussian as well as hollow Laguerre-Gaussian (LG01) beam profiles are used with red-detuned and blue-detuned light. Microfabricated dielectric phase objects allow the efficient and robust mode conversion of Gaussian into Laguerre-Gaussian laser beams. Trap geometries can easily be changed due to the highly flexible experimental setup. The dipole trap laser beams have powers up to several hundred milliwatts and are focused to around 10 m. At a detuning of several tens of nanometers from the atomic resonance, the resulting traps are a few millikelvin deep, and the trapped atoms exhibit motional frequencies near 30 kHz, trap light scattering rates of a few hundred photons per second per atom, and storage times of several seconds. The number of dipole-trapped atoms ranges from more than ten thousand to below ten. The dipole-trapped atoms are detected either by a photon counting system with efficient straylight discrimination, or by recapture into the MOT, which is imaged onto a sensitive photodiode and a CCD-camera. Due to the strong AC-Stark shift imposed by the high-intensity trapping light, energy-selective resonant excitation and detection of the stored atoms is possible. The measured energy distribution is consistent with a harmonic potential shape and allows for the determination of temperatures and heating rates. From initial measurements, the thermal energy is found to be about 10 % of the trap depth. Using a crossed-beam geometry with red-detuned laser light, efficient transfer of atoms between the beams is observed. Optimum transfer occurs when the two beams cross at a radial offset, which can qualitatively be understood when the particle energy and geometrical properties of the two-beam trapping potential are considered. Numerical simulations reproduce the general features of the measured transfer efficiency vs. radial beam offset. This experiment is a first step towards dynamically variable potentials. Atoms have been radially confined in a blue-detuned hollow beam, and their decay time has been determined. This configuration is currently extended to a three-dimensionally confining blue-detuned dipole trap. For advanced laser cooling, state manipulation and spectroscopy of trapped atoms, a system of two diode lasers has been set up which are phase-coherently locked with a difference frequency near 6.834 GHz, to drive Raman transitions between the hyperfine-split ground states of 87Rb atoms. Resonances of such a transition have been recorded in a buffer gas loaded rubidium vapour cell with linewidths below 100 Hz.
We report on the stable effective trapping of cold rubidium atoms by a method for a single optical trap in the near-optical resonant regime. An optical trap with the near-optical resonance condition consists of not only the dipole but also the radiative forces, while a trap using a far-off resonance dominates only the dipole force. We measure the spatial behaviors of the center-of-mass positions and the loading efficiencies of the trapped atoms in the near-optical resonant trap, by changing the detuning over the range of $\ensuremath{-}0.373$ to $\ensuremath{-}2.23$ THz from the D2{\mathrm{D}}_{2}D2​ resonance. The time dependence of the spatial behavior in the trap indicates that the suitable positions for stably holding atoms are altered because the equilibrium condition between the optical dipole and the radiative forces depends on the trap laser detuning. The stable position, which is not a primary position of focus in the Gaussian beam optics, depends only on the laser detuning due to the change in the radiative force, while it is independent of the change in the laser intensity, which results in a balance between the radiative and dipole forces.
Experiments are performed on loading cold atoms into a far-off resonance optical dipole trap that is spatially separated from the laser-cooling region. The atoms are delivered using a far-off resonant optical dipole guide and enter the trap through a switchable blue-detuned light sheet or ``trap door.'' This scheme provides a means to transfer atoms from a dissipative to a non-dissipative trap without a significant loss of phase-space density. We study the dynamics of atoms within the trap and measure their lifetime. Preliminary results on multiple loading are presented. As the technique does not rely on near-resonant light, the atom number limit characteristic of laser-cooling experiments can, in principle, be exceeded.
NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document. Investigations of optical processes for laser cooled and trapped atoms are described. Fluorescence from Cs atoms in a magneto-optical trap is detected under conditions of very low atomic density. Discrete steps are observed in the fluorescent signal versus time and are associated with the arrival and departure of individual trapped atoms. Histograms of the frequency of occurrence of a given level of fluorescence exhibit a series of uniformly spaced peaks that are attributed to the presence of N = 0, 1, 2 atoms in the trap. In addition, numerical absorption and emission spectra for three-level [...], [...], and V systems under intense radiations are calculated. Absorption spectra for a [...] system is used to explain the probe-wave amplification and absorption spectra recorded for Cs atoms cooled and confined in a magneto-optical trap, in which novel spectral features of narrow frequency widths with single-pass gain exceeding 20% are observed. The consequence of the optical gain is demonstrated to lead to negative radiation pressure, which is investigated together with other mechanical forces in the trap. Various alternative trapping schemes in three-level and two-level atoms are proposed as possible means to compress an atomic sample and demonstrated for a two-level magneto-optical trap.