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Bulletin of the American Physical Society

Diode laser cooling of a potassium beam through a mirrored slit

S. Li, M. Reinsch, John F. Clauser

Published May 1, 1993• 0 citations
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Abstract

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.

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We have cooled and stopped a beam of cesium atoms using frequency chirped diode lasers. In this technique, a moving atom absorbs and reemits many photons from a counter propagating laser that is frequency swept to keep it in resonance with an atomic transition. The resulting momentum transfer slows the atom. The basic approach is identical to that developed by Hall and co-workers, however, the use of diode lasers allows us to obtain the necessary frequency modulation simply by sweeping the laser injection current. We use two diode lasers tuned to each of the 6s hyperfine ground state levels to drive the 6s—6p3/2 transitions. Each laser is simultaneously swept over the Doppler profile of the 100°C thermal cesium beam to bring a substantial number to a stop. Using additional fast frequency modulation, the same lasers are used to probe the resulting velocity distribution. This technique allows us to bring ~106 atoms/cm3 to a temperature of 1 K. This method is an extremely simple and inexpensive way to produce cold atoms.

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Single-Slit Diffraction Pattern of a Thermal Atomic Potassium Beam

J.A. Leavitt, Francis Anthony Bills

The diffraction of a full thermal neutral atomic potassium beam by a single slit has been observed. Four experimental diffraction patterns of a 23.0(±0.4)μ diffracting slit were obtained with an atomic-beam apparatus which had a 2.5(±1)μ source slit, a 3.0(±0.1)μ detector wire, a source-to-slit distance of 96 cm, a slit-to-detector distance of 100 cm, and an oven temperature of 533° K. The experimental pattern was reproducible and is in good general agreement with the prediction of de Broglie's hypothesis and simple scalar Fresnel diffraction theory. The effect of diffraction is clearly evident; two prominent diffraction fringes are present on the top corners of the beam profiles. The theoretical prediction, calculated with the aid of a computer, included the effects of finite source and detector widths and the Maxwellian speed distribution of the K beam. Additional theoretical calculations showed that no major improvement in the clarity of the diffraction fringes could be obtained by using a velocity-selected beam, a Li7 beam, or a narrower diffracting slit.

American Journal of Physics•Sep 1, 1969•32 citations
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.
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 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 have laser cooled an atomic beam of rubidium with diode lasers and have used these slow atoms to load a magneto-optical trap. The cooling is performed with the frequency-chirp method,1,2 in which the laser's frequency is ramped linearly (by the injection currant) to maintain resonance with the decelerating atoms. Two synchronously chirped lasers are required to prevent optical hyperfine pumping of the ground state. The chirp typically covers 500 MHz in 10 ms. At the end of each ramp, the resulting velocity distribution is measured with a pulsed probe laser. By varying the end point of the chirp, we are able to obtain slow atoms with average velocities ranging from 200 to −50 m/s. The widths of the velocity distributions are measured to be approximately 40 m/s. This relatively large width results from the inherent linewidth (~30 MHz) of the free-running diode lasers used for both cooling and probing.
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 have cooled and stopped a beam of cesium atoms using frequency chirped diode lasers. In this technique, a moving atom absorbs and reemits many photons from a counter propagating laser that is frequency swept to keep it in resonance with an atomic transition. The resulting momentum transfer slows the atom. The basic approach is identical to that developed by Hall and co-workers, however, the use of diode lasers allows us to obtain the necessary frequency modulation simply by sweeping the laser injection current. We use two diode lasers tuned to each of the 6s hyperfine ground state levels to drive the 6s—6p3/2 transitions. Each laser is simultaneously swept over the Doppler profile of the 100°C thermal cesium beam to bring a substantial number to a stop. Using additional fast frequency modulation, the same lasers are used to probe the resulting velocity distribution. This technique allows us to bring ~106 atoms/cm3 to a temperature of 1 K. This method is an extremely simple and inexpensive way to produce cold atoms.
The diffraction of a full thermal neutral atomic potassium beam by a single slit has been observed. Four experimental diffraction patterns of a 23.0(±0.4)μ diffracting slit were obtained with an atomic-beam apparatus which had a 2.5(±1)μ source slit, a 3.0(±0.1)μ detector wire, a source-to-slit distance of 96 cm, a slit-to-detector distance of 100 cm, and an oven temperature of 533° K. The experimental pattern was reproducible and is in good general agreement with the prediction of de Broglie's hypothesis and simple scalar Fresnel diffraction theory. The effect of diffraction is clearly evident; two prominent diffraction fringes are present on the top corners of the beam profiles. The theoretical prediction, calculated with the aid of a computer, included the effects of finite source and detector widths and the Maxwellian speed distribution of the K beam. Additional theoretical calculations showed that no major improvement in the clarity of the diffraction fringes could be obtained by using a velocity-selected beam, a Li7 beam, or a narrower diffracting slit.