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Physical Review A

Bichromatic force on helium

M. Cashen, H. Metcalf

Published Jan 17, 2001• 36 citations
Publisher

Abstract

We have observed the bichromatic force on metastable He using fiber amplifiers to produce the high power of light needed at $\ensuremath{\lambda}=1.083 \ensuremath{\mu}\mathrm{m}.$ We have done deflection experiments to confirm that the very large magnitude of the force scales to He, and preliminary deceleration experiments for eventual trap loading. Our measurements support earlier experiments in Rb. Helium is ideally suited for this kind of beam slowing because its slowing length with the usual radiative force is more than a factor of 100 larger than with the bichromatic force. Thus, loading a MOT will be much more effective because of the much larger angular capture range associated with the shorter distance.

DOI
10.1103/physreva.63.025406

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•Jan 1, 2006•0 citations
We have observed the bichromatic force on metastable He using fiber amplifiers to produce the high power of light needed at \ensuremathλ=1.083\ensuremathμm.\ensuremath{\lambda}=1.083 \ensuremath{\mu}\mathrm{m}.\ensuremathλ=1.083\ensuremathμm. We have done deflection experiments to confirm that the very large magnitude of the force scales to He, and preliminary deceleration experiments for eventual trap loading. Our measurements support earlier experiments in Rb. Helium is ideally suited for this kind of beam slowing because its slowing length with the usual radiative force is more than a factor of 100 larger than with the bichromatic force. Thus, loading a MOT will be much more effective because of the much larger angular capture range associated with the shorter distance.
We examine two approaches for significantly extending the velocity range of the optical bichromatic force (BCF), to make it useful for laser deceleration of atomic and molecular beams. First, we present experimental results and calculations for BCF deceleration of metastable helium using very large BCF detunings and discuss the limitations of this approach. We consider in detail the constraints, both inherent and practical, that set the usable upper limit of the BCF. We then show that a more promising approach is to utilize a BCF profile with a relatively small velocity range in conjunction with chirped Doppler shifts, to keep the force resonant with the atoms as they are slowed. In an initial experimental test of this chirped BCF method, helium atoms are slowed by $\ensuremath{\sim}$370 m/s using a BCF profile with a velocity width of $\ensuremath{\lesssim}$125 m/s. Straightforward scaling of the present results indicates that a decelerator for He* capable of loading a magneto-optical trap can yield a brightness comparable to a much larger Zeeman slower.
Optical forces using lasers allow precise control over the motion of atoms. The bichromatic optical force (BF) is unique in its large magnitude and velocity range, arising from the absorption and stimulated emission processes. These properties were used to transversely collimate a beam of metastable helium (He∗ ) using the 2^{3}S − 2^{3}P transition. The collimation created a very bright beam of He∗ , allowing experiments of neutral atom lithography. The He∗ beam was used to pattern material surfaces using a resist-based lithography technique, where the pattern was determined by either mechanical or optical masks. The optical masks produced features with a separation of half the wavelength of the light used. Patterning was successfully demonstrated with both IR and UV optical masks. The etched pattern resolution was ∼ 100 nm and limited by the material surface. Further experiments were performed studying the ability of the bichromatic force to cool. The finite velocity range of the BF allows estimation of a characteristic cooling time which is independent of the excited state lifetime, only depending on the atomic mass and optical transition energy. Past experiments, including the helium collimation used for neutral atom lithography, have demonstrated that the BF can collimate and longitudinally slow atomic beams, but required long interaction times that included many spontaneous emission (SE) events. The effect of SE can be mitigated, and is predicted to not be necessary for BF cooling. Since the BF cooling time is not related to the excited state lifetime, a transition can be chosen such that the cooling time is shorter than the SE cycle time, allowing direct laser cooling on atoms and molecules that do not have cycling transitions. Experiments using the helium 2^{3}S− 3^{3}P transition were chosen because the BF cooling time (285 ns) is on the order of the average SE cycle time (260 ns). Numerical simulations of the experimental system were run predicting compression of the atomic velocity distribution. Our experimental results demonstrate the stimulated nature of the force through many atomic recoils from the light in less than one SE cycle time. A large range of the atomic velocity distribution is accelerated and accumulates at the velocity limit of the force. This accumulation results in an increase in the velocity space density, demonstrating cooling. | 112 pages
The bichromatic force has been used to both slow and collimate a beam of metastable 23S{2}^{3}S23S He atoms (He*). The collimation capture range is an extraordinary $\ifmmode\pm\else\textpm\fi{}85\text{ }\mathrm{m}/\mathrm{s}$ corresponding to $\ensuremath{\sim}0.18$ radians from our source. Both slowing and collimation were accomplished in the unprecedented short distance of $\ensuremath{\sim}5\text{ }\text{ }\mathrm{c}\mathrm{m}$ each. The overall brightness increase is $\ifmmode\times\else\texttimes\fi{}3200$, and there is potential for considerably more.
The spontaneous decay rate $\ensuremath{\gamma}$ of excited atomic states limits the radiative force on atoms to $\ensuremath{\Elzxh}k\ensuremath{\gamma}/2.$ This bottleneck in the dissipative exchange of momentum between atoms and light can be overcome by coherent control of the momentum exchange interactions. The bichromatic force provides such control by using stimulated instead of spontaneous emission to produce the force, while at the same time exploiting spontaneous emission to enable a dissipative interaction. It is implemented with two counterpropagating light beams, each containing two frequencies, whose phases, amplitudes, and frequency difference are carefully chosen. We have made precise measurements of the extremely large magnitude and velocity range of this bichromatic force, and have shown that its velocity dependence near the edge of its range is suitable for atomic beam slowing and laser cooling. Our measurements have corroborated various models and calculations of this bichromatic force.
The limit of $\ensuremath{\Elzxh}k\ensuremath{\gamma}/2$ imposed on the magnitude of radiative forces by the spontaneous decay rate $\ensuremath{\gamma}$ of the excited state can be overcome by coherent control of the momentum exchange between atoms and the light field. This can be implemented with two light beams, each containing two frequencies whose phases, amplitudes, and frequency difference are carefully chosen. We have made precise measurements of the extremely large magnitude and velocity range of this bichromatic force, and have shown that its velocity dependence near the edge of its range is suitable for atomic beam slowing and laser cooling. Our measurements have corroborated various models and calculations of this bichromatic force.
In this work I report on the development of a Magneto Optic Trap (MOT) for metastable helium atoms (He*). The metastable helium atoms are produced in a discharge nozzle source and collimated, slowed and compressed to provide a slow bright beam for loading the trap. The trap confines approximately 107 atoms, has a diameter of about 3 mm and with temperature approximately 1 mK. The trap is used for intra-trap and electron-atom scattering experiments. The results from these two experiments are reported. The electron scattering experiment is unique and employs a He* MOT for the first time, in combination with a new diagnostic technique (Phase Modulation Spectroscopy) to measure the trap loss. The results of these experiments have yielded the first total electron-metastable atom collision cross section measurements at intermediate (10-100 eV) electron energies.