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

Transient atom optics: reflection and transmission of atoms by a time-dependent laser field

Xia M. Zeng, Weiping Zhang

Published May 1, 1997• 0 citations
Publisher

Abstract

Although there are many kinds of different techniques developed to manipulate atomic matter waves, a laser beam is still considered as the most effective `optical element' to manipulate atomic waves in atom optics. In the laser-based atom optic devices, the spatial profiles of a laser beam play a central role in controlling the motion of atoms. By choosing different spatial profiles of laser beams, one can construct atom optic devices for different purposes. Recently experimental demonstrations of manipulation of center of mass motion of atoms by time-dependent light field were reported. In this paper we theoretically study the transient effects in atom optics by using a laser pulse train. The reflection and transmission of atoms through the laser pulse train are studied in detail. We find that the laser pulse train acts as both a spatial beam splitter and a `temporal grating' which can both reflect or transmit atoms in spatial domain and `diffract' atoms in the time domain.

DOI
10.1117/12.273758

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Although there are many kinds of different techniques developed to manipulate atomic matter waves, a laser beam is still considered as the most effective optical element' to manipulate atomic waves in atom optics. In the laser-based atom optic devices, the spatial profiles of a laser beam play a central role in controlling the motion of atoms. By choosing different spatial profiles of laser beams, one can construct atom optic devices for different purposes. Recently experimental demonstrations of manipulation of center of mass motion of atoms by time-dependent light field were reported. In this paper we theoretically study the transient effects in atom optics by using a laser pulse train. The reflection and transmission of atoms through the laser pulse train are studied in detail. We find that the laser pulse train acts as both a spatial beam splitter and a temporal grating' which can both reflect or transmit atoms in spatial domain and `diffract' atoms in the time domain.
Realization of the analogues of optical components for the manipulation of atomic de Broglie waves has recently become possible through the use of laser-based techniques. The strong fields generated by laser light have already been used to reflect atoms using travelling evanescent waves, and to diffract atomic beams through small angles using transmission gratings formed by a standing laser light wave in vacuo. Present experiments aim to combine these techniques to produce a reflection diffraction grating formed by a standing evanescent laser-light field, and to exploit the larger diffraction angles made possible theoretically by such a scheme.
Atom optics has seen fast development and vast achievements,and has wide application prospects.The basic principles of the manipulation and control of cold atoms using magnetic and laser fields are briefly introduced,followed by a detailed review of recent progress of geometric and wave atom optics as well as potential applications and the development of atom optics devices Topics include the reflection of atom beams and mirrors,the deflection(refraction) and focusing of atom beams and lenses,the diffraction of atom matter waves and gratings,atom interferometers,and atomic holography.
In this thesis, the use of pulsed laser fields to control the center-of-mass motion of neutral atoms and molecules is studied both numerically and experimentally. In particular, we consider the use of a pulsed standing wave for the deflection of atomic beams, and develop novel types of mirrors for applications in atom and molecule optics. The research work shows that pulsed laser fields can be applied to form efficient mirrors for neutral particles regardless of the details of their internal energy-level structure. Moreover, it is shown that pulsed standing waves can also be utilized to efficiently deflect atomic beams. In the first part of the thesis, we study the effects of phase fluctuations of the laser field on the momentum distribution of atoms after interaction with a standing wave. Our numerical simulations show that the coherence properties of the laser field play a major role in diffraction of atoms, especially when using pulsed lasers. It turns out that the effects caused by the limited coherence time of the optical field are clearly observable as a transfer of atoms in momentum space to the region around zero momentum rather than to high momentum states. The numerical analysis of the pulsed standing-wave diffraction was complemented by experimental measurements of the deflection profiles of a beam of thermal sodium atoms. The experimental distributions were measured for a value of the coherence time intermediate between the Rabi period and the interaction time. A comparison of the results with an expected distribution calculated for a fully coherent interaction shows that the phase fluctuations of the light field drastically affect the achievable transverse momentum values. Based on our theoretical and experimental analysis of the diffraction profiles, we conclude that large deflection angles can be reached only when laser pulses with a Fourier-limited bandwidth are applied. The second part of the thesis concentrates on the development of pulsed mirrors for neutral atoms and molecules. In particular, we consider the use of an evanescent wave, formed in total internal reflection of light on a vacuum-dielectric interface, and of a standing wave as possible mirror configurations. Our numerical simulations of the pulsed evanescent-wave mirror show that atoms with initial velocities up to several tens of meters per second can be reflected already at the moderate field intensities of a few tens of MW/cm2. This allows pulsed evanescent waves to be applied in a compact setup to control the motion of thermal atomic beams, for which the traditional atom mirrors are suited only at grazing incidence. However, since the effective interaction volume of the atomic beam and the laser field is limited by the short decay length of the evanescent wave, only a relatively small number of atoms can be reflected with a single laser pulse. Furthermore, the reflection is always inelastic, since the force exerted on the atoms varies as a function of the initial position of the particles. To solve the above problem we consider the use of a standing wave as a pulsed mirror for atoms. Our numerical simulations show that with a proper choice of the pulse duration and field intensity, each period of the standing-wave pattern functions as an elastic mirror for particles with an initial kinetic energy below the depth of the standing-wave potential. Hence, the standing-wave pattern acts as a long row of independent atom mirrors, which provides a novel route to controlling large volumes of fast atoms even with a single laser pulse. As a second application for the pulsed standing-wave mirror we consider the reflection of slow molecules, for which no efficient mirror configuration previously has been available. It is shown that pulsed standing waves of nonresonant frequencies can be used to efficiently manipulate large volumes of rotationally cold molecules. With a suitable choice of the parameter values, the molecules can be reflected nearly elastically, or with a tailored momentum distribution. This opens up a possibility to use the pulsed standing-wave mirror to control the motion of particles in various applications of atomic and molecular physics. Such a mirror could be applied, for example, to steer and image molecular beams, separate molecular species, and to form pulsed molecular beams.
We study the transient dynamics that arise during the formation of an atom laser beam in a tight waveguide. The time dependent density profile develops a series of wiggles which are related to the diffraction in time phenomenon. The apodization of matter waves, which relies on the use of smooth aperture functions, allows to suppress such oscillations in a time interval, after which there is a revival of the diffraction in time. The revival time scale is directly related to the inverse of the harmonic trap frequency for the atom reservoir. PACS numbers: Atom lasers have recently been created in the laboratory and stand up for their applications in atom lithography and interferometry. Similar to their light analogues, they consist in coherent and directed (matter-) wave beams, which are extracted from a Bose-Einstein condensate (BEC). Different schemes have been proposed to out-couple the atoms from the BEC reservoir. One of the prototypes uses two-photon Raman excitation to create “output coupled ” atomic pulses with well defined mean momentum which overlap and form a quasi-continuous beam [1, 2] (For other approaches see e.g. [3] or [4]). The features of the beam profile are of outmost importance for applications [5, 6, 7], and depend on the pulse shape, duration, emission frequency, and initial phase [8]. We study the dynamics of a beam created by multiple overlapping pulses, transversely confined in a tight waveguide and with negligible interactions between the out-coupled atoms. It is found that an oscillatory pattern appears in the density profile of the beam which is closely related to the diffraction in time phenomenon. If the switching process is slow, such oscillations are suppressed on a time window after which there is a revival of the diffraction in time. The time scale in which the spatial fringes appear is related to the the spatial width of the out-coupled pulses.