The collisional effects in nanocell spectroscopy are investigated. Experiments performed on the well-resolved Cs D/sub 1/ line (894 nm), have enabled a simultaneous comparison of /spl lambda//2 and /spl lambda//4 thickness for various densities. Because nanocell spectroscopy, when performed under normal incidence irradiation, allows a sub-Doppler resolution owing to the velocity-selection operated by the transient regime of optical interaction, the pressure behaviour of the various hyperfine components can be studied individually. Considerable differences are found to occur for the pressure-induced shift, as already observed through selective reflection (SR) spectroscopy on the Cs D/sub 2/ line.
The collisional effects in nanocell spectroscopy are investigated. Experiments performed on the well-resolved Cs D/sub 1/ line (894 nm), have enabled a simultaneous comparison of /spl lambda//2 and /spl lambda//4 thickness for various densities. Because nanocell spectroscopy, when performed under normal incidence irradiation, allows a sub-Doppler resolution owing to the velocity-selection operated by the transient regime of optical interaction, the pressure behaviour of the various hyperfine components can be studied individually. Considerable differences are found to occur for the pressure-induced shift, as already observed through selective reflection (SR) spectroscopy on the Cs D/sub 2/ line.
Spectroscopy close toaninterface isappropriate toevaluate collisional effects, like pressure broadening and shifts, inanoptically opaquemedium, asoccurs atlarge atomic densities. Relatively tothevolumespectroscopy in comparable conditions, applicable onlyinthefarspectral wings, itprovides aninformation obtained close tothe resonance line-centre. However, itimplicitly assumes that thevicinity withthesurface hasnotmodified thecollisional features themselves. Thissupposes that theatomdynamics -i.e. thevelocity distribution andthepopulation distributionisidentical inthegasbulkandclose tothesurface, andthat soistheinteraction between anatomanditsperturber. To testsuchanassumption, we havestarted toinvestigate thecollisional effects innanocell spectroscopy [11, inthe elementary caseofaresonant transition. Indeed, therecentdevelopment ofnanocells ofatomic vapour, withthickness typically spanning from20to1000nm,offers aspectroscopic alternate tolong-time developed Selective Reflection (SR) spectroscopy ataninterface, withtheadditional possibility ofexploring avariable distance tothesurface. Presently, ourdetailed experiments, performed onthewell-resolved CsD1line(894nm),haveenabled a simultaneous comparison ofX/2andk/4thickness forvarious densities. Systematic experiments forshorter thickness areplanned forthenearfuture. Because nanocell spectroscopy, whenperformed under normal incidence irradiation, allows asub-Doppler resolution owing tothevelocity-selecti on
When a vapour is confined, the collisions to the confinement wall usually cause a broadening of the transitions. However velocity selective processes and the coherent enhancement of some transient contributions can lead to sub-Doppler contributions because of the confinement. In the optical range, this had been demonstrated for a one-dimensional confinement with micro- or nano-cells of a small thickness. A two-dimensional confinement in a hollow fiber had shown sub-Doppler contributions, but only equivalent to macrosocpic non linear saturated absorption spectroscopy. Here, we describe experiments showing that a thermal vapor confined in a three-dimensional nanostructure can exhibit sub-Doppler features in a linear regime. We perform reflection spectroscopy on a Cs vapour cell whose window is covered with a thin opal film (typically, 10 or 20 layers of ~ 1µm or 400 nm diameter spheres). We demonstrate, both experimentally and through a modeling that the sub-Doppler structures observed for a large range of oblique incidence angles (~ 30-50°) originate from the interstitial regions of the opal, hence corresponding to a three-dimensional submicrometric confinement. We also show that the partially porous nature of the glass spheres allows some LIAD effects. In a different set of experiments, an atomic vapour is confined in glass medium with large size pores (tens of µm) randomly distributed. The effect of scattered light and fluorescence have been distinguished. Also, sub-Doppler features evolving non linearly with the irradiating intensity are observed, originating in a saturated absorption process induced by the backscattered light .
Abstract Selective reflection (SR) spectroscopy of the 6 s 2 S 1 / 2 → 7 p 2 P 3 / 2 electric dipole transition of Caesium ( λ = 456 nm) is performed for the first time using a nanometric-thin cell (L = 50–1500 nm, T = 200 ∘ C). We successfully form narrow resonances corresponding to the F g = 3 , 4 → F e = 2 , 3 , 4 , 5 hyperfine transitions. All transitions are well spectrally resolved, and the geometry of the cell allows us to obtain a strong 30 times narrowing of the Doppler width with a single beam pass. For lower thicknesses ( L < 400 nm), we observe a red shift of the SR lines, which we attribute to atom-surface interactions, and provide estimates of the C 3 interaction coefficient. The oscillator strengths of the 6 s 2 S 1 / 2 → 7 p 2 P 3 / 2 hyperfine transitions are about two orders of magnitude smaller than that of the Cs D 2 line at 852 nm. Our experimental measurements are in good agreement with theoretical calculations. We emphasize the advantage of nanocell spectroscopy compared to the well-known saturated absorption method and present possible applications.
Since the recent realization of extremely thin vapour cells (local thickness: 20-1000 nm), we investigate the optical properties of these 1-D confined vapours. Aside from their interest for Doppler-free spectroscopy, nanocells offer a new tool to evaluate collisional shift and broadening, yielding an access to the open problem of collisions under confinement. It also allows probing of the atom-surface interaction in a range of unusual short distances. The experimental exploration of the distance dependence, normally evolving according to the z −3 van der Waals (vW) dependence ( z : the atom-surface distance), is worth doing because it could be affected by imperfections of the real surface, such as roughness, adsorbed impurities or charges. A detailed lineshape analysis is now under progress, with tight constraints imposed to the fitting by the twin information brought by simultaneous reflection and transmission spectra. Another issue is a possible resonant enhancement, susceptible to induce a repulsive vW, due to the coupling between atom excitation and a surface mode.
It is demonstrated that the velocity-selective optical pumping/saturation resonances of the reduced absorption in a Rb vapor nanocell with thickness L = λ, 2λ, and 3λ (resonant wavelength λ = 780 nm) allow for the complete elimination of crossover (CO) resonances. We observe well-pronounced resonances corresponding to the F g = 3 → F e = 2, 3, and 4 hyperfine transitions of the 85Rb D2 line with line widths close to the natural width. A small CO resonance located midway between F g = 3 → F e = 3 and F g = 3 → F e = 4 transitions appears only for L ≥ 4λ. The D2 line (λ = 852 nm) in a Cs nanocell exhibits a similar behavior. From the amplitude ratio of the CO and VSOP resonances, it is possible to determine the thickness of the column of alkali vapor in the range of 1–1000 μm. The absence of the CO resonances for nanocells with L ∼ λ is attractive for the frequency reference application and for studying the transitions between the Zeeman sublevels in external magnetic fields.
We report on the investigation of narrow sub-Doppler-width resonances observed at the D2 line of Cs atoms (Fg = 4 → Fe = 3, 4, 5 hyperfine transitions), confined in thin optical cells. These cells have a thickness of L = 6λ (λ = 852 nm) and L = 700 μm, and are irradiated by two narrow-band laser radiations (one with fixed and the other with scanned frequency). We show that for zero frequency detuning of the fixed-frequency laser from the Fg = 4 → Fe = 5 hyperfine transition, its absorption spectrum in the thin cell consists of resonances, centered at the respective hyperfine transitions. The spectral widths of these resonances range from 2 to 20 MHz, and they are characterized by a good contrast and extremely small Doppler background. This spectrum is highly sensitive to the frequency offset of the fixedfrequency laser. The analysis leads to the conclusion that the spatial restriction of Cs vapor suppresses the interaction of the light with atoms with significant velocity component along the direction of the laser beam propagation.