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International Laser Science Conference

Nonlinear Doppler-free spectroscopy of atoms near a surface

Pierre Simoneau, S. Le Boiteux, C. De Araujo, Daniel Bloch, M. Ducloy

Published Jan 1, 1986• 0 citations

Abstract

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.

DOI: 10.1364/ils.1986.wa4

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Conference on Lasers and Electro-Optics•Jan 1, 2000•0 citations

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.

The atomic response to resonant light irradiation near vapor-dielectric interfaces is monitored via high-resolution reflection spectroscopy. One demonstrates that this Doppler-free spectroscopic technique yields a novel probe of atomic dynamics and QED atom-surface long- range interaction. It provides for the first time a spectral evidence of van der Waals attraction, and the method is intrinsically applicable to the observation of surface interaction exerted on excited atoms. Effects related with atomic excitation level, nature of the dielectrics, and atomic collisions are discussed.

Selective reflection spectroscopy at an interface with a low-density resonant vapor, especially when combined with a frequency modulation technique, is a high-resolution Doppler-free tool for probing atoms interacting with a surface. We analyze different types of relevant surface interaction, emphasizing the spectral consequences of a van der Waals surface attraction associated to a

z^{-3}

potential dependence (

z

: distance to the wall). We present detailed results of two series of experiments at a Cs vapor/dielectric window interface on the 6S

_{1/2}

-6P

_{3/2}

(

\lambda = 852

nm) resonance line and on the 6S

_{1/2}

-7P second resonance line (

\lambda = 455

nm and 459 nm). Lineshape analysis at various pressures consistently shows that a van der Waals-type surface attraction has to be considered to interpret strong lineshape distortions and resonance shift. The attractive strengths are found to be equal respectively to

\approx 2

kHz

\mu

m

^3

and

\approx 20

kHz

\mu

m

^3

, independently of the considered hyperfine component, within the experimental accuracy. It yields also typical parameters of pressure broadening and shift, which are shown to originate in collisional processes, at densities where the medium is opaque. Theoretical expectations for the VW strength are discussed on the basis of the results of atomic theory. The predicted values are smaller, by a typical factor of 2, than those deduced from the experiments. The validity of the theory, when applied to a dielectric interface, is discussed and seems questionable when the frequency of virtual atomic transitions involved in the van der Waals attraction potential lies in the dielectric window absorption range.

Non-intrusive laser-spectroscopical methods for the diagnostics of the gas-surface interaction in the near-field of a dielectric surface have been developed both experimentally and theoretically. The techniques base on either two-photon evanescent wave excitation or combined evanescent-volume wave excitation. Spectra obtained for sodium atoms at a glass-vacuum interface are quantitatively reproduced by a rigorous theoretical approach. In the case of pure evanescent wave excitation the excitation conditions can be chosen such that either the velocity distribution of desorbing atoms is determined or that atom-surface interaction parameters (polarizability of the adsorbed atoms, desorption rate, sticking coefficient) are obtained from the Autler- Townes splitting of the two-photon lines. By the use of combined evanescent-volume wave excitation one is able to distinguish optically between different groups of atoms interacting with the surface and one can extract their two- dimensional velocity distributions. In addition, information is obtained about superelastic scattering of the atoms interacting with the dielectric substrate.

Summary form only given. Evanescent wave atomic mirrors have been shown to be very sensitive to the surface roughness of the glass substrate on which the evanescent wave is formed. Non-specular reflection of slow atoms at normal incidence has been observed for surface roughness as low as 0.1 nm. We interpret this behaviour in terms of diffraction of the atomic De Broglie wave on the modulated potential created by the interference of the evanescent wave with light scattered by the rough surface. However these experiments could not separate the specular peak of reflected atoms from the diffuse part. We have developed a new method to measure the velocity distribution of the atoms after the bounce with a much better resolution, using Raman transitions. Two phase locked counterpropagating laser beams induce a Raman transition between the ground state hyperfine levels of the /sup 85/Rb atoms falling from a magneto-optical trap. The resonance condition is only fulfilled for atoms with a transverse velocity controlled by the interaction time between the atoms and the beams. These velocity selected atoms bounce on the mirror and their velocity distribution after reflection can be measured using the same Raman beams, by scanning their difference in frequency.