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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE

Dynamic atom optics to produce ultraslow, ultracold helium atoms: design study and possible applications

R. Bruce Doak, K. Kevern, Andrew V. G. Chizmeshya, Rudolf David, George Comşa

Published May 1, 1997• 2 citations
Publisher

Abstract

Atoms, unlike photons, travel at velocities readily attainable in the laboratory, making possible novel atom optical devices based on optical elements which move relative to the atomic beam. We report design studies of a device to accelerate or decelerate and simultaneously monochromatize a helium atomic beam. A helium supersonic free-jet (v equals 950 m/s, (Delta) v equals 5 m/s, FWHM) is reflected from a moving surface to produce an output beam is continuously tunable in velocity from epithermal (2000 m/s) to ultra-slow (ca. 1 m/s, De Broglie wavelength 1000 angstroms). The decelerator is highly dispersive in the low velocity regime, making the collimated ultra-slow output beam nearly monochromatic, i.e. ultra-cold. Measured properties of an actual supersonic helium free-jet expansion were used in Monte Carlo simulations of the decelerator to predict the attainable decelerated beam properties. Ultra- slow beams of usable intensity at T < 100 nK appear attainable. Finite element stress calculations for a magnetically suspended rotor verify that the necessary reflector velocities (475 m/s) can be safely achieved. Possible applications of an ultra-cold helium beam might include gas-surface scattering experiments to probe `quantum reflection' of atoms from surfaces and to measure gas- surface bound states of exceptionally low binding energy, thereby testing the contributions of retardation to the gas- surface potential.

DOI
10.1117/12.273750

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Atoms, unlike photons, travel at velocities readily attainable in the laboratory, making possible novel atom optical devices based on optical elements which move relative to the atomic beam. We report design studies of a device to accelerate or decelerate and simultaneously monochromatize a helium atomic beam. A helium supersonic free-jet (v equals 950 m/s, (Delta) v equals 5 m/s, FWHM) is reflected from a moving surface to produce an output beam is continuously tunable in velocity from epithermal (2000 m/s) to ultra-slow (ca. 1 m/s, De Broglie wavelength 1000 angstroms). The decelerator is highly dispersive in the low velocity regime, making the collimated ultra-slow output beam nearly monochromatic, i.e. ultra-cold. Measured properties of an actual supersonic helium free-jet expansion were used in Monte Carlo simulations of the decelerator to predict the attainable decelerated beam properties. Ultra- slow beams of usable intensity at T < 100 nK appear attainable. Finite element stress calculations for a magnetically suspended rotor verify that the necessary reflector velocities (475 m/s) can be safely achieved. Possible applications of an ultra-cold helium beam might include gas-surface scattering experiments to probe `quantum reflection' of atoms from surfaces and to measure gas- surface bound states of exceptionally low binding energy, thereby testing the contributions of retardation to the gas- surface potential.
We demonstrate an intense, continuous cold atom beam generated via post nozzle seeding of a supersonic helium jet with 7^77Li atoms. The nozzle is cooled to about 4.4 K to reduce the forward velocity of the atoms. The atomic beam is brought to a focus 175 cm from the nozzle by a 10 cm bore diameter magnetic hexapole lens. Absorption and fluorescence imaging of the focus show a flux of 2.3(4)×10122.3(4) \times 10^{12}2.3(4)×1012 atoms/s, brightness of 4.1(7)×10194.1(7)\times 10^{19}4.1(7)×1019 m−2s−1sr−1\textrm{m}^{-2}\textrm{s}^{-1}\textrm{sr}^{-1}m−2s−1sr−1, forward velocity of 211(2) m/s, and longitudinal temperature of 7(3) mK. Results agree with a Monte Carlo simulation of the seeding dynamics and a particle tracing simulation of the atom lens. We project that 10 times higher flux would be possible with improved vacuum system design. Our method should provide a useful high-brightness source for atom-optical and other atomic and molecular physics applications.
This dissertation describes a general approach of generating a high flux of cold atoms that can be confined in a magneto-optical trap. As an alternative to the state-of-the-art laser cooling method, this approach widens our ability to cool and control atoms without relying on a specific atomic transition and availability of laser. In this dissertation I will discuss the design, construction of the experiment and characterization of a pulsed cold atom source in detail. This work is based on a new paradigm: entrainment of atoms in the carrier gas of a supersonic beam, followed by the magnetic deceleration and trapping. Our methodology is based on the supersonic beam created by the expansion of a dense carrier gas from a pulsed release of gas through a small aperture. Cold noble gas emerging from this pulsed high pressure supersonic nozzle acts as a carrier, into which atoms of interest are then entrained. In our studies with the lithium atoms, up to 10¹¹ atoms per pulse could be entrained into a supersonic beam of helium at a translation temperature of below 100 mK. The supersonic valve is typically operated at a rate of below 1 Hz in the experiment. A much larger flux can be achieved at a higher repetition rate. These atoms are moving at a speed of 500 ms⁻¹ and need to be decelerated almost to a complete stop, in order to be trapped and cooled to quantum degeneracy. A 2.5-meter long moving-trap magnetic decelerator with 480 coil pairs was built and characterized. Atoms moving at a speed around 500 ms⁻¹ were trapped and decelerated to various final velocities ranging from 400 ms⁻¹ to 50 ms⁻¹, at a resulting temperature of 30 mK and a flux of 10⁸ ~ 10⁹ atoms per pulse. This whole process takes place within only 10 ms, at a repetition rate of 100 Hz 10¹⁰ ~ 10¹¹ atoms can be delivered per second. This approach is very general compared to the laser cooling, since most atoms in the periodic table have a magnetic moment in their ground state or can be optically pumped to a long-lived para-magnetic metastable state. In this dissertation, I show the working principle of each component of our experiment and characterize the atom beam at multiple stages. I describe in detail the construction and optimization of our magnetic decelerator, and demonstrate the control and monitoring system with the experiment results. Also implemented in this work is a flexible laser system that is composed of a reference diode laser and two tapered amplifiers to control and probe the internal states of atoms, as well as provide the trapping force. I also explore the optical molasses and chirped cooling techniques which help load the atoms into a magneto-optical trap. The successful demonstration of this method of creating a cold atom source leads us to believe that the magnetically decelerated supersonic atoms will play an important role in the area of cold atom physics
A surface-electrode decelerator consisting of 44 electrodes has been used to decelerate supersonic beams of helium Rydberg atoms moving with an initial velocity of 1200 m/s. Prior to the deceleration, the helium atoms were excited from the 1s2s1S01s2s{\phantom{\rule{0.16em}{0ex}}}^{1}{S}_{0}1s2s1S0​ metastable state to selected Rydberg-Stark states with a narrow-band tunable UV laser. Complete deceleration could be achieved over a distance of 36 mm and in 60 $\ensuremath{\mu}$s, corresponding to an acceleration of $\ensuremath{-}2.0\ifmmode\times\else\texttimes\fi{}{10}^{7}$ m/s2{}^{2}2. After deceleration, the atoms were held in stationary electric traps above the chip surface, reaccelerated off the chip, and detected by pulsed electric-field ionization. The decelerator was also used to generate helium Rydberg-atom beams with a final velocity tunable between 0 and 1200 m/s. Transitions between low- and high-field-seeking Rydberg-Stark states were observed during trap loading and are attributed to adiabatic Landau-Zener dynamics at electric fields exceeding the Inglis-Teller field. By comparing the experimental results with the results of particle-trajectory simulations, the velocity distribution of the decelerated atoms was found to be characterized by temperatures ranging between 50 and 200 mK, depending on the magnitude of the electric dipole moment of the Rydberg-Stark states selected prior to the deceleration.
A supersonic beam of noble gas atoms is a source of unprecedented brightness. A novel short pulse supersonic nozzle is developed with beam intensity that is higher by at least an order of magnitude than other available sources. We show how this beam can be coherently slowed and focused using elastic reflection from single crystals. Simulations show beam fluxes of 10 11 atoms s −1 at velocities of 50 m s −1 and temperatures of less than 20 μK in the longitudinal direction. Possible applications of this slow beam to the study of atom–surface interactions and atom interferometry are discussed.
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.