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IEEE Transactions on Nuclear Science

Nonlinear Fast-Beam Laser Interactions

U. Nielsen, Ove Poulsen

Published Apr 1, 1983• 7 citations
Publisher

Abstract

High-resolution spectroscopy performed on fast-moving absorbers has several desirable features. First, in a collinear geometry, the residual Doppler width of the fast atoms is low due to the velocity bunching taking place during acceleration. Second, the velocity of these cooled atoms can easily be changed very precisely. Third, this makes possible Doppler tuning of a laser into resonance with a particular atomic transition. Doppler shifts tip to ≃80 Å are present, dependent on the beam energy and the atom used. This, in turn, makes it possible to 'create' completely harmonic three-level atoms, using the relativistic transformation between laboratory and atom rest frame and to perform optical pumping in well defined spatial regions. Applications to velocity control, resonant three-level nonlinear field-atom interactions, high-resolution laser-r.f. double-resonance spectroscopy, and lifetime determinations are discussed.

DOI
10.1109/tns.1983.4332448

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High-resolution spectroscopy performed on fast-moving absorbers has several desirable features. First, in a collinear geometry, the residual Doppler width of the fast atoms is low due to the velocity bunching taking place during acceleration. Second, the velocity of these cooled atoms can easily be changed very precisely. Third, this makes possible Doppler tuning of a laser into resonance with a particular atomic transition. Doppler shifts tip to ≃80 Å are present, dependent on the beam energy and the atom used. This, in turn, makes it possible to 'create' completely harmonic three-level atoms, using the relativistic transformation between laboratory and atom rest frame and to perform optical pumping in well defined spatial regions. Applications to velocity control, resonant three-level nonlinear field-atom interactions, high-resolution laser-r.f. double-resonance spectroscopy, and lifetime determinations are discussed.
Two-photon spectroscopy has been performed in a fast, accelerated atomic beam, with a resolution limited only by the natural linewidth of the upper excited level. The main feature of the experiment is the "creation" of a completely harmonic three-level atom, by using the relativistic transformation between the laboratory and atom rest frame. Thus resonant two-photon absorption, with a strength comparable to electricdipole transitions, takes place. The systematic effects of importance in spectroscopy are investigated.
Resonant-saturation spectroscopy has been carried out in both the VVV configuration and the inverted-VVV configuration in fast, accelerated atom beams of Ne I* and Ca I*. Hole (peak) creation in the velocity distributions are shown to yield resonances Doppler free to first order in a collinear geometry, which allows high spectral stability and resolution. Wave numbers for three optical transitions in 20Ne^{20}\mathrm{Ne}20Ne are reported.
A collection of stationary or nearly stationary atoms would be an ideal sample on which to perform high-resolution spectroscopy. Spectral width, shifts, and distortions that are due to second-order-Doppler and transit-time effects, which now limit high-resolution Doppler-free spectroscopies, would be virtually eliminated. Such low-energy atoms would also be useful for loading into the shallow optical1,2 or magnetic3,4 traps that have been proposed. By irradiating an atomic beam with a near-resonant counter- propagating laser, we have produced such a sample of atoms. The average velocity of the atoms is nearly zero, and the spread in velocities is equivalent to that of a gas at a temperature of about 1 K.
Abstract : The goal of our work has been to investigate a few well-chosen effects in laser spectroscopy under clean experimental conditions which facilitate a sharp and meaningful contact with theory. The initially-defined task was to study large pulse-area effects in single upward transitions from metastable atoms in a fast, monovelocity Ne* beam. It proved possible as well to investigate multiple-pulse transitions on a cw basis. Ramsey fringe resonance signatures were clearly observed and for the first time in the optical domain, using three pulse excitation. Basically the first two zones of light provide initially a coherence and secondly an atomic population in the excited state. The counter-running fields in the second zone and third zones convert excited state population into a coherence again and finally into a nonlinear (with intensity) change in the excited and ground state populations. In view of the time gap tau in the preparation phase, one could anticipate and did observe narrow Ramsey fringes which vary as cos (delta, omega, tau) in the middle of the single zone excitation linewidth function ( Rabbi pedestal ). The neon beam employed was rather fast (v/c approx. .001) and, being produced by charge transfer had an exceptionally an narrow velocity distribution, delta, approx .0001. These characteristics permitted unambiguous measurements of the fringe shapes and facilitated useful contact with theory. A second extremely successful experiment was the combination of laser excitation of two-photon Doppler-free transitions in the 5S yields nS series in Rubidium with precise Lambdameter interferometer techniques to accurately measure the transition wavelength.
Up to now, Doppler-free nonlinear spectroscopy has been applied to atoms inside a gas medium (volume spectroscopy). We have extended these methods to atoms in the vicinity of a surface. The basic principle consists of monitoring reflectivity changes produced when a laser beam, incident on a glass–vapor interface, is brought into resonance with an atomic transition. For Doppler-free spectroscopy, two counterpropagating beams are used to operate velocity selection. If these two beams are under normal incidence in the glass—velocity selection perpendicular to the surface—one observes saturated dispersion (selective reflection sensitive to the real index). For incidence angles in the glass (prism) larger than the critical angle (total internal reflection), a standing evanescent wave is produced in the gas (velocity selection along the surface), and saturated absorption is detected. Experimental demonstration was performed on the Na resonance line; high sensitivity (10−4 relative reflectivity change) and spectral resolution as good as 20 MHz were obtained. The main limitation to better resolution comes from the short atomic transit time in the evanescent wave (typical extension ~λ). Relevant applications of these techniques should also concern the diagnostics of optically thick media, as analysis of interaction between gas-phase atoms and the surface, notably atom–wall collisions, and modification of the spontaneous emission characteristics.
This chapter shows that several non-linear features can be demonstrated and the line shape of sub-natural line width can be obtained by manipulation of the powers of the two interacting laser radiations. The interaction of the laser radiation can produce a coherent superposition of the atomic states such that atoms are trapped in one state so that the atomic system may be transparent even in the presence of radiation frequency that is on resonance. Saturation spectroscopy is more conveniently performed in three level atomic systems interacting with two laser radiations. One of the radiations, called pump beam, has higher intensity and frequency on resonance or close to resonance with the transition frequency across two levels of the atom. The second laser radiation, called probe beam, is weaker and interacts with two other energy levels, one of which is common to the pump transition. A few hyperfine transitions are within the envelope of one Doppler profile.