P. Todorov

Sub-Doppler spectroscopy of cesium vapor layers with nanometric and micrometric thickness

The saturation behavior of absorption and fluorescence spectra on the D2 line of Cs is presented, demonstrating a significant difference between open and closed transitions. Cs vapor is confined in an extremely thin cell (ETC) with widely tunable thickness L=(0.5-3)λ, where λ is the light wavelength. In the saturation regime, the closed transition demonstrates enhanced absorption in a narrow spectral interval due to the Dicke effect, while the open one demonstrates only a velocity-selective dip in the absorption. The fluorescence of open transitions shows reduced fluorescence dips, enhancing their contrast with ETC thickness. The closed transition exhibits only a small plateau around the optical transition center. Applying two-level theoretical modeling based on optical Bloch equations, a qualitative agreement with experimental observations is achieved. The rate of contrast enhancement with cell thickness is larger for the theoretical than for the experimental dips. In addition, for the closed transition a tiny peak in the fluorescence is theoretically predicted, with the first experimental confirmation presented. The sub-Doppler spectra of vapor layers with a thickness of several light wavelengths show potential for realization of precise frequency references and photonics sensors.

Observation and theoretical simulation of electromagnetically induced transparency and enhanced velocity selective optical pumping in cesium vapour in a micrometric thickness optical cell

Observation of electromagnetically induced transparency (EIT) and enhanced velocity selective optical pumping (VSOP) signals in a micrometric cell with cesium is reported. The line shape and non-linear features observed in the case of fluorescence in the direction parallel to the cell windows and the transmission spectra observed along the propagation direction of the probe beam show considerable differences in the spectral profile. A theoretical model based on five level optical Bloch equations is used to simulate the spectra. The Doppler convolution includes all possible orientations of atomic velocities with respect to the laser beam direction. Atoms moving nearly parallel to the windows and perpendicular to the collinear pump and probe beams have much lower Doppler shift and hence produce considerable narrowing of the Doppler background in the fluorescence spectra. The coherence decay rate is also low for such atoms as they do not meet with the cell walls. The simulated curves reproduce the observed sharp EIT peaks and enhanced broad VSOP signals for the closed probe transition in the fluorescence and absorption spectra. The observed effect of detuning of the pump frequency on the non-linear features is also reproduced by the simulation.

Absorption and fluorescence in saturation regime of Cs-vapor layer with thickness close to the light wavelength

Absorption and fluorescent spectra are obtained in saturation regime of a single beam laser spectroscopy of Cesium D2 line, in a vapor layer with thickness close to the light wavelength. We compare experimentally and theoretically obtained spectra distinguishing between open and closed, in terms of optical pumping, hyperfine transitions. In absorption, we observe a persisting difference between open and closed transitions, in terms of Dicke narrowing, when increasing laser light intensity. In fluorescence, for open transitions we note saturation dips which do not change significantly when increasing intensity. In case of closed transition, a small feature at the fluorescence line center appears for relatively high light intensity.

Experimental test of the distribution of atomic trajectories at grazing incidence in a micrometric thin vapour cell

For a gas at thermal equilibrium, the velocity distribution is usually assumed to follow an isotropic 3-dimensional Maxwell-Boltzmann (MB) law. Several sub-Doppler laser spectroscopy techniques for confined gases (including selective reflection spectroscopy at an interface under normal incidence, micro and nano-cell of vapors, …) rely on the specific contribution of atoms moving nearly parallel to the boundary of a vapour container. For these techniques, a dilute gas regime is considered, with atomic trajectories essentially governed by wall collisions, rather than by atom-atom collisions. In spite of speculations of a metrological sub-Doppler resolution with microcells for experiments on extremely narrow lines (see e.g. [1]), based upon the expected response of atoms following truly grazing trajectories (i.e. very slow “normal” velocities), experimental demonstrations have been restricted to sub-Doppler lines as narrow as 1/50th of the Doppler width [2, 3], owing to the relatively strong transitions used until now. The presence of such very slow atoms is highly questionable, for technological reasons (such as the local roughness of the surface itself), but also because of more fundamental issues[4], such as the atom-surface interaction (notably van der Waals type [3]), and mostly to the law itself of desorption from the surface. Indeed, the assumption of a MB velocity distribution implies a “cos θ” law for the flux of atoms leaving the surface, an hypothesis which cannot be justified by surface physics [5]. Moreover, the very few experimental investigations on this topic [6] were not suitable to test grazing incidences.

Forty years after the first dark resonance experiment: an overview of the COSMA project results

COSMA: Coherent Optics Sensors for Medical Application is an European Marie Curie Project running from 2012 to March 2016, with the participation of 10 teams from Armenia, Bulgaria, India, Israel, Italy, Poland, Russia, UK, USA. The main objective was to focus theoretical and experimental research on biomagnetism phenomena, with the specific aim to develop all-optical sensors dedicated to their detection and suitable for applications in clinical diagnostics. The paper presents some of the most recent results obtained during the exchange visits of the involved scientists, after an introduction about the phenomenon which is the pillar of this kind of research and of many other new fields in laser spectroscopy, atomic physics, and quantum optics: the dark resonance.

Velocity anisotropy effect in pump-probe spectra of cesium in a micrometric thickness optical cell

The pump-probe spectra in a cell of micrometric thickness containing cesium vapor are reported. The line shape and nonlinear features observed in the case of fluorescence in the direction parallel to the cell windows and the transmission spectra observed along the propagation direction of the probe beam show considerable differences in the spectral profiles. We observed Electromagnetically Induced Transparency (EIT) and enhanced Velocity Selective Optical Pumping (VSOP) signals. Atoms moving nearly parallel to the windows and perpendicular to the collinear pump and probe beams will see much lower Doppler shift of incident frequencies and hence will lead to considerable narrowing of the Doppler background in the fluorescence spectra. The coherence decay rate is also low for such atoms as they do not meet with the cell walls. A theoretical model based on five level optical Bloch equations is used to simulate the spectra. The Doppler convolution includes all possible orientation of atomic velocities with respect to the laser beam direction. The simulated curves reproduce the observed sharp EIT peaks and enhanced broad VSOP signals for the closed probe transition in the fluorescence and absorption spectra. The observed effect of the light intensity and temperature change on the non-linear features is reproduced by the simulation.

High-resolution spectroscopy of a cesium-vapor layer with micrometric thickness for the development of frequency reference

Experimental results are presented related to high-resolution atomic spectroscopy of a cesium-vapor layer confined in extremely thin cells (ETCs) with thicknesses equal to the wavelength ? (852?nm) of the irradiating light and 6?. It is shown that increasing the ETC thickness by a few micrometers results in a strong enhancement of the sub-Doppler resonance contrast observed in the center of the fluorescence profile of the hyperfine transition.