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Physical Review A

Diode-laser production of collimated slow cold potassium beams and crossover resonances

Shifang Li, John F. Clauser

Published Apr 1, 1994• 6 citations
Publisher

Abstract

We describe the results of three sets of experiments to investigate producing cold slow collimated potassium atoms for applications with no access available for the introduction of counterpropagating laser light along the exiting atomic beam's axis. Velocity profiles are obtained via fluorescence from D1 excitation at perpendicular and nearly parallel incidence by a probe laser. Spatial profiles of both the cold- and residual hot-beam components are measured with a position-scanned hot-wire detector. Velocity-component holes and decelerated cold components are observed, along with direct evidence of perpendicular heating of the cold components. A first set of experiments uses a variant of conventional chirp cooling and/or hole burning. Light is introduced by a heated inclined mirror, while atoms exit through a slit in the mirror. Loss of intensity at low velocity is observed and discussed. A second set of experiments investigates velocity-selective optical-pumping crossover resonances that occur near the mirror, where the incident and reflected beams overlap for several alternative geometries. A third set of experiments investigates cold-beam production using laser-induced velocity selection and deflection. It provides a higher cold-atom intensity, along with the means to perform high-resolution spectroscopy.

DOI
10.1103/physreva.49.2702

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We describe the results of three sets of experiments to investigate producing cold slow collimated potassium atoms for applications with no access available for the introduction of counterpropagating laser light along the exiting atomic beam's axis. Velocity profiles are obtained via fluorescence from D1 excitation at perpendicular and nearly parallel incidence by a probe laser. Spatial profiles of both the cold- and residual hot-beam components are measured with a position-scanned hot-wire detector. Velocity-component holes and decelerated cold components are observed, along with direct evidence of perpendicular heating of the cold components. A first set of experiments uses a variant of conventional chirp cooling and/or hole burning. Light is introduced by a heated inclined mirror, while atoms exit through a slit in the mirror. Loss of intensity at low velocity is observed and discussed. A second set of experiments investigates velocity-selective optical-pumping crossover resonances that occur near the mirror, where the incident and reflected beams overlap for several alternative geometries. A third set of experiments investigates cold-beam production using laser-induced velocity selection and deflection. It provides a higher cold-atom intensity, along with the means to perform high-resolution spectroscopy.
Results of experiments to cool and decelerate potassium atoms are described. D2 light from one RF modulated, from two CW, or from two ramped diode laser(s) is counter-directed on a potassium - effusing oven`s exit slit via a heated inclined mirror, with a 60{mu} slit cut through it to extract the cooled beam. The pumping light (or RF amplitude) is AC modulated to allow phase sensitive detection of the cooled/decelerated velocity components. Velocity profiles are obtained via fluorescence from D1 excitation at near parallel incidence by a probe laser. Fluorescence excited perpendicularly resolves the K{sup 39} and K{sup 41} hyperfine structures and calibrates the probe wavelength. Velocity component holes and decelerated peaks are observed in all three modes. AC and DC spatial profiles of the beam are measured with a position scanned hotwire detector. While the hole component maintains the spatial profile of the parent thermal beam, perpendicular heating of the decelerated peak component is evidenced by its broader spatial profile.
Results of atom interferometry experiments in generalized Talbot-vonLau configurations are presented. AC modulated D2 light from diode lasers is counter-directed on a thermal potassium beam. A pA AC current of cold, slow potassium atoms is extracted through a slit in a heated mirror, transmitted through an interferometer, and ionized by a hotwire. The interferometer consists of a sequence of three planar vacuum slit diffraction gratings, microfabricated from silicon nitride membranes. Interference fringes form a Moire with the third grating and are sensed by measuring the transmission as a function of grating relative position. Different spatial Fourier components (up to 5th) in the diffraction pattern, resonant in the interferometer at different atomic velocities, may be selected via the tuning of the cooling lasers. Shifts of the patterns by gravity and rotation are evident when the vacuum chamber is tilted.
We discuss the implementation and characterization of the source of a slow, intense, and collimated beam of rubidium atoms. The cold atomic beam is produced by two-dimensional magneto-optical trapping in directions transverse to the atomic beam axis and unbalanced Doppler cooling in the axial direction. The vacuum design allows use of relatively low laser power and a considerably simplified assembly. The atomic beam has a high flux of about $2\ifmmode\times\else\texttimes\fi{}{10}^{10}\phantom{\rule{0.3em}{0ex}}\mathrm{atoms}∕\mathrm{s}$ at a total cooling laser power of 55mW55\phantom{\rule{0.3em}{0ex}}\mathrm{mW}55mW. It has a narrow longitudinal velocity distribution with mean velocity 15m∕s15\phantom{\rule{0.3em}{0ex}}\mathrm{m}∕\mathrm{s}15m∕s with full width at half maximum 3.5m∕s3.5\phantom{\rule{0.3em}{0ex}}\mathrm{m}∕\mathrm{s}3.5m∕s and has a low divergence of 26mrad26\phantom{\rule{0.3em}{0ex}}\mathrm{mrad}26mrad. The high flux enables ultrafast loading of about 1010{10}^{10}1010 atoms into a three-dimensional (3D) magneto-optical trap within 500 ms. The variation of the atomic beam flux was studied as a function of the rubidium vapor pressure, cooling laser power, transverse cooling laser beam length, detuning of the cooling laser, and relative intensities of the cooling beams along the atomic beam axis. We also discuss a detailed comparison of our measurements of the cold atomic beam with a 3D numerical simulation.
Results of high flux atom interferometry experiments with potassium in generalized Talbot-vonLau configurations are presented. The interferometer consists of a sequence of three planar vacuum slit diffraction gratings, microfabricated from silicon nitride membranes. Interference fringes are sensed by measuring the transmission of atoms on a hot-wire as a function of grating relative position. Different spatial Fourier components (up to fifth) in the diffraction pattern are resonant in the interferometer at different atomic velocities. When a laser cooled slow beam is incident, various different diffraction patterns are observed as a function of atomic velocity, selected via the tuning of D2 cooling lasers. In an alternative “Heisenberg Microscope” configuration an incident thermal beam produces a velocity average over different fringe Fourier components. Fringe patterns for each HFS component are revealed by their selective destruction by AC modulated weak Dl laser light passing through the interferometer near the middle grating. Since imaging of the fluorescent light could determine which slit an atom passes, the laser removes the fringe pattern contribution by Doppler shifted atoms at its wavelength. That contribution is thus AC modulated and detected. Shifts of the patterns by gravity and rotation are evident when the vacuum chamber is tilted.
We report the use of frequency-chirped diode lasers to decelerate a thermal atomic beam of Rb and, in a separate experiment, the use of frequency-stabilized diode lasers to collimate the beam with optical molasses. For this we used a 100-mW cw laser-diode array injection locked to light from a low-power, frequency-narrowed, single-stripe diode laser. Also, a novel scheme for imaging the neutral-atom beam profile has been developed for monitoring the collimation experiments in two dimensions. We have increased both beam brightness and intensity by a factor of more than 20.
In this paper we report on the first purely ``pump-probe'' nonlinear laser spectroscopy results in a slow atomic beam. We have observed Raman, Rayleigh, and recoil-induced resonances (RIR) in a continuous beam of slow and cold cesium atoms extracted from a two-dimensional (2D) magneto-optical-trap (MOT) with the moving molasses technique. The RIR enabled us to measure the velocity distribution, therefore the average speed (0.6--4 m/s) and temperature (50--500 \ensuremath{\mu}K) of the atomic beam. Compared to time of flight, this technique has the advantage of being local, more sensitive in the low-velocity regime (v<1m/s),(v<1 \mathrm{m}/\mathrm{s}),(v<1m/s), and it gives access to transverse velocities and temperatures. Moreover, it may be extended to measure atomic velocities in the 2D MOT source of the atomic beam.