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Experimental investigation of the reflection of thermal sodium atoms from a moving evanescent grating

B. W. Stenlake

Published Jan 1, 1997• 0 citations
Publisher

Abstract

This thesis reports an investigation of the interaction of a thermal sodium beam with a variety of near-resonant evanescent field configurations which was performed between March 1990 and September 1993.The evanescent fields were tuned above the sodium D2 transition, in order to repel atoms at glancing incident angles through the gradient force.Three evanescent field configurations, corresponding to various optic elements, were employed.The first configuration was a travelling evanescent field, from which reflection of the beam was observed at glancing angles up to 4.5mrad.This configuration corresponds to a mirror for atoms.The second configuration was a standing evanescent wave generated by retroreflecting the travelling evanescent wave.The resulting evanescent wave consisted of closely spaced reflective regions separated by non-reflecting nodes of the standing wave.This configuration corresponds to a mirror grating with periodicity 7J2.The atomic beam was reflected from the evanescent field.No diffraction was observed, however.The final configuration was a moving standing evanescent grating, generated by interference between two counterpropagating laser beams.The relative velocity between the atoms and the grating could be continuously varied by adjusting the frequency of one laser.Coherent exchange of photons between the two fields in the moving evanescent grating was observed.This exchange exhibited velocity dependent resonant behaviour corresponding to doppleron resonances.The results are explained in terms of a dressed-state model for diffraction of atoms from such a grating.A parameter search was performed in an attempt to observe the diffraction of the thermal sodium beam, with null results.However, deflections which were dependent on grating velocity are reported.

DOI
10.25911/5d67b687b228e

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This thesis reports an investigation of the interaction of a thermal sodium beam with a variety of near-resonant evanescent field configurations which was performed between March 1990 and September 1993.The evanescent fields were tuned above the sodium D2 transition, in order to repel atoms at glancing incident angles through the gradient force.Three evanescent field configurations, corresponding to various optic elements, were employed.The first configuration was a travelling evanescent field, from which reflection of the beam was observed at glancing angles up to 4.5mrad.This configuration corresponds to a mirror for atoms.The second configuration was a standing evanescent wave generated by retroreflecting the travelling evanescent wave.The resulting evanescent wave consisted of closely spaced reflective regions separated by non-reflecting nodes of the standing wave.This configuration corresponds to a mirror grating with periodicity 7J2.The atomic beam was reflected from the evanescent field.No diffraction was observed, however.The final configuration was a moving standing evanescent grating, generated by interference between two counterpropagating laser beams.The relative velocity between the atoms and the grating could be continuously varied by adjusting the frequency of one laser.Coherent exchange of photons between the two fields in the moving evanescent grating was observed.This exchange exhibited velocity dependent resonant behaviour corresponding to doppleron resonances.The results are explained in terms of a dressed-state model for diffraction of atoms from such a grating.A parameter search was performed in an attempt to observe the diffraction of the thermal sodium beam, with null results.However, deflections which were dependent on grating velocity are reported.
We report on the observation of velocity-tuned (doppleron) resonances in the grazing-incidence reflection of thermal sodium atoms from a moving evanescent light grating created by interference between two counterpropagating laser beams. The relative velocity between the atoms and the grating can be varied continuously by adjusting the frequency of one laser. We observe an exchange of photons between copropagating and counterpropagating fields in an evanescent light grating. The results are in general agreement with a dressed-state model for the diffraction of atoms from such a grating.
In order to diffract atoms from an evanescent light reflection grating the Rabi frequency must be greater than the Doppler shift associated with the atomic velocity. For thermal atoms this cannot be achieved using dye lasers. Rather than slow the atoms we have produced a moving light grating and observe resonances in the reflectivity of the grating for certain grating velocities. The position of these resonances fulfils the Doppleron condition Δ’=nΔD’, which is consistent with a dressed state model for diffraction from such a grating.
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.
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.
In this paper we present some new results concerning the observed anomalous frequency properties of the process of the neutral atom scattering in the standing light wave field near the reflecting mirror. The investigations were carried out using the method of scattering of the sodium atomic strip beam, intersecting at right angle the region of the standing light wave field. The standing light wave field was formed by two counterpropagating waves of resonant laser radiation -- incident and reflected from the mirror, mounted at a distance of about 1 cm from the atomic beam. The coherence length of the laser radiation was about 10 cm, that's why the interaction of atoms with the field of two counterpropagating light waves was coherent. The width of the strip atomic beam (a equals 0.02 cm) was much less than the distance to the mirror. The specific feature of the given setup is the use of pulsed laser radiation with the duration (tau) less than 1/(gamma) ((gamma) -1- the time of resonant level spontaneous decay), so the atoms are scattered only due to the induced light pressure force. In this paper the detailed investigation of the distance R influence on the scattering process was carried out. With this purpose the mirror was mounted at different distances from the atomic beam. (truncated)