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.
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.
Understanding the processes underlying the optical excitation of atomic vapor in the vicinity of solid surfaces is of interest both from physical and technical points of view. Selective reflection spectra of laser light from an interface of a transparent dielectric material and resonant vapor are known to be Doppler-free and to provide important spectroscopic information about the atom-wall interaction. On the other hand, atomic vapors have such a large and fast nonlinear response on or near resonances that they are capable of processing signals and images with milliwatt lasers at a time scale of submicroseconds. In the present paper we investigate linear and nonlinear optical properties of a thin film of atomic vapor confined between two solid surfaces. Assuming two-level atoms, we solve the Maxwell- Bloch equations for light reflection from resonant atoms. The interaction between the resonant atoms and the wall is accounted for through the boundary conditions for all elements of density matrix. The transient polarization is shown to make an important contribution not only to the field structure in the vicinity of the wall but also to the interference pattern of the reflected field. It is found that the spectrum of reflection depends remarkably on the film thickness. Strong enhancement of signal is also found from this study and possible devices for the detection 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 (
λ=852 nm) resonance line and on the 6S
1/2-7P second resonance line (
λ=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
≈2 kHz
μm
3 and
≈20 kHz
μm
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.
The high sensitivity of Laser Spectroscopy has made possible the exploration of atomic resonances in newly designed "nanometric" gas cells, whose local thickness varies from 20nm to more than 1000 nm. Following the initial observation of the optical analogous of the coherent Dicke microwave narrowing, the newest prospects include the exploration of long-range atom surface van der Waals interaction with spatial resolution in an unprecedented range of distances, modification of atom dielectric resonant coupling under the influence of the coupling between the two neighbouring dielectric media, and even the possible modification of interatomic collisions processes under the effect of confinement.
Selective reflection spectroscopy at normal incidence provides signals with sub-Doppler resolution, linear with optical intensity. Frequency modulated (FM) selective reflection probes atomic vapours at distances comparable to the excitation wavelength (~λ/2π) and is used extensively to probe the Casimir-Polder interactions between an excited state atom and a macroscopic surface. Extending selective reflection spectroscopy to molecular gases allows probing a thin layer of molecular gas next to a surface. This is a very attractive prospect that allows envisaging high-resolution molecular spectroscopy and molecular frequency references in a compact and miniaturised apparatus, such as a thin cell [1] or a photonic crystal [2]. Additionally, it paves the way for spectroscopic probing of the Casimir-Polder interaction with molecules. The molecule-surface interaction has been the object of extensive theoretical investigations, focusing on the effects of molecular orientation and chirality. However, experimental tests are few and comparison with theoretical predictions has been challenging [3].