Detection of slow atoms in laser spectroscopy of a thin vapor film

Abstract

We report on high-resolution single-light-beam transmission spectroscopy through an ultra thin vapor cell (thickness 10-100 mm). In addition to the expected Doppler-broadened absorption, a novel sub-Doppler structure is observed under normal incidence irradiation. This structure originates from the optical response of atoms with very small velocity components perpendicular to the cell walls, and is connected with the transient atom excitation regime during the wall-to-wall time of flight. In experiments performed in Cs vapor, the slow mechanism of optical pumping leads, for weak light intensities (10-100 mW/cm 2 ) and a 10 mm cell, to a velocity selection equivalent to an effective 1D temperature in the sub-mK range.

Laser spectroscopy of nanometric gas cells

“…When the incident light is normal to the surface, the Doppler effect cancels for those atoms moving at grazing incidence: the light absorption becomes free of Doppler broadening, as has been demonstrated in thin cells of Cs gas at the D 2 resonance line, A= 852nm (Fig.4, from Ref. [7]). …”

Testing the limits of the Maxwell distribution of velocities for atoms flying nearly parallel to the walls of a thin cell

“…Velocity selection can also be operated through transient optical pumping technique (pumping towards a third-level for which the irradiation is non resonant), and "slow" atoms are thus defined as those travelling less than the observation zone during the time ( pump ) required for the optical pumping to take place. This time can be quite long time at low power [27,28] Experimentally, these techniques were implemented only for alkali-vapor lineson resonance lines, or sometimes on weak and more excited transitions [22,25,26,31,33]-, but always in a regime where collisions impose a notable broadening to the optical width .…”

“…large values of ). Note also that in the two works mentioned above, the surface was simple polished glass, and the experiments are far from providing "universal" evidences of the effectiveness of a "cos" law.A blooming of sub-Doppler resolution spectroscopy techniques in a thin film of dilute vapor at thermal equilibrium has been noted in the last decades, including selective reflection spectroscopy at an interface [22,26], and confined vapor spectroscopy in microor nano-cells [22,[27][28][29][30][31][32][33][34]. The Doppler broadening, associated to the distribution of random 7 atomic velocities, can be eliminated for well-chosen signals note by the way that the early observation of a sub-Doppler contribution in selective reflection [35], before the laser era, was originally attributed to the contribution of specular atomic collisions onto the surface, so that the first modern theory [36] of selective reflection spectroscopy also includes the specific contributions of atoms having undergone specular reflection.…”

“…Velocity selection can also be operated through transient optical pumping technique (pumping towards a third-level for which the irradiation is non resonant), and "slow" atoms are thus defined as those travelling less than the observation zone during the time ( pump ) required for the optical pumping to take place. This time can be quite long time at low power [27,28] Experimentally, these techniques were implemented only for alkali-vapor lineson resonance lines, or sometimes on weak and more excited transitions [22,25,26,31,33]-, but always in a regime where collisions impose a notable broadening to the optical width .Hence, the gain in resolution, relatively to Doppler-broadened spectroscopy, has always remained well below 2 order of magnitudes, and the selected atomic (normal) velocities were at best on the order of 5-10 m/s -to be compared with a thermal velocity ~200 m/s for Cs. The possibility of benefiting of a more stringent velocity selection through narrow transitions, in view of references of a metrological interest, has been considered in a theoretical proposal (on the intercombination line of Ca) [38], or briefly attempted experimentally (on the intercombination line of Ba -see page 128 of ref.…”

“…Velocity selection can also be operated through transient optical pumping technique (pumping towards a third-level for which the irradiation is non resonant), and "slow" atoms are thus defined as those travelling less than the observation zone during the time ( pump ) required for the optical pumping to take place. This time can be quite long time at low power [27,28] apparently implying a sharp velocity selection. The effective weighting of slow (<u th /) or very slow (< [k pump ] -1 ) atoms, analyzed in detail in [30], remains actually dependent on the optical linewidth of the transition.…”