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Optics Express

Confinement-enhanced polarization spectroscopy in ultrathin alkali vapors

Yali Carmon, Golan Ben-Ari, Amir Shemer, Yossef Danan, Eliran Talker

Published Jul 2, 2026• 0 citations
Publisher

Abstract

We report measurements of the anisotropic optical response of an atomic medium confined in submicron-thick vapor cells using polarization spectroscopy. The technique enables systematic characterization of anisotropy as a function of cell thickness, temperature, and probe intensity. Owing to the extremely low optical density of these submicron cells, the medium remains optically transparent even at temperatures up to 250 °C, allowing operation far beyond the conventional thermal limits of vapor-cell spectroscopy. Under these conditions, we observe a dramatic enhancement of the anisotropic signal, reaching up to three orders of magnitude compared to cells of identical thickness operated at 150 °C. This giant anisotropy gives rise to pronounced sub-Doppler spectral features in the probe transmission, persisting even for subwavelength cell dimensions (L⁄λ≈0.64). The observed behavior is attributed to the evolution of Zeeman population imbalance among ground-state sublevels in the strongly confined geometry. Our results demonstrate that submicron vapor cells provide a powerful platform for high-contrast anisotropy spectroscopy and for studying coherence phenomena in extreme confinement regimes.

DOI
10.1364/oe.597451

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We report measurements of the anisotropic optical response of an atomic medium confined in submicron-thick vapor cells using polarization spectroscopy. The technique enables systematic characterization of anisotropy as a function of cell thickness, temperature, and probe intensity. Owing to the extremely low optical density of these submicron cells, the medium remains optically transparent even at temperatures up to 250 °C, allowing operation far beyond the conventional thermal limits of vapor-cell spectroscopy. Under these conditions, we observe a dramatic enhancement of the anisotropic signal, reaching up to three orders of magnitude compared to cells of identical thickness operated at 150 °C. This giant anisotropy gives rise to pronounced sub-Doppler spectral features in the probe transmission, persisting even for subwavelength cell dimensions (L⁄λ≈0.64). The observed behavior is attributed to the evolution of Zeeman population imbalance among ground-state sublevels in the strongly confined geometry. Our results demonstrate that submicron vapor cells provide a powerful platform for high-contrast anisotropy spectroscopy and for studying coherence phenomena in extreme confinement regimes.
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We study electromagnetically induced transparency (EIT) in a heated potassium vapor cell, using a simple optical setup with a single free-running diode laser and an acousto-optic modulator. Despite the fact that the Doppler width is comparable to the ground state hyperfine splitting, transparency windows with deeply sub-natural line widths and large group indices are obtained. A longitudinal magnetic field is used to split the EIT feature and induce magneto-optical anisotropy. Using the beat note between co-propagating coupling and probe beams, we perform a heterodyne measurement of the circular dichroism (and therefore birefringence) of the EIT medium. The observed spectra reveal that lin‖lin polarizations lead to greater anisotropy than lin⊥lin. A simplified ‖analytical model encompassing sixteen Zeeman states and eighteen Λ subsytems reproduces the experimental observations.
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.
We present here the new behavior of Electromagnetically Induced Transparency (EIT), Velocity Selective Optical Pumping (VSOP) and Velocity Selective Excitation (VSE) resonances observed in Cs vapor confined in unique cells with thicknesses L = 1.5λ and L = 6λ. It is shown experimentally that in both cells, the EIT resonance is significantly narrower than would be expected from the ground state dephasing rate due to atomic collisions with the cell windows. The enhanced absorption (fluorescence) narrow VSOP resonance at the closed transition transforms into reduced absorption (fluorescence) one with small increase of atomic concentration or light intensity. A striking difference appears between the VSE resonance broadening due to excited atom thermalization, in L = 6λ and conventional L = 2.5 cm cells.
The detection of narrow and high-amplitude optical resonances in atomic cells is of crucial importance for the development of compact and high-stability lasers or vapor-cell optical references. In this work, using monochromatic light and appropriate polarization settings for the counterpropagating pump and probe beams, we demonstrate the detection of enhanced-absorption sub-Doppler resonances on the D1{D}_{1}D1​ line of the Rb\mathrm{Rb}Rb atom. We report a theoretical model showing that the Zeeman coherence induced between magnetic sublevels---especially in the $F\ensuremath{\rightarrow}{F}^{\ensuremath{'}}=F\ensuremath{-}1$ systems, with FFF and ${F}^{\ensuremath{'}}$ being the hyperfine quantum numbers of the ground and excited states, respectively---plays a key role in the observed enhanced-absorption process. The theoretically predicted behavior matches the experimental observations well. The impact of light polarization, laser intensity, magnetic field, and cell temperature on the Doppler-free resonance features is experimentally investigated. For application, two identical diode lasers are frequency-stabilized onto enhanced-absorption sub-Doppler resonances. The fractional frequency stability of the laser beatnote is measured to be $1.8\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}12}$ at 1s1\phantom{\rule{0.1em}{0ex}}\mathrm{s}1s and $8.4\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}12}$ at 104s{10}^{4}\phantom{\rule{0.1em}{0ex}}\mathrm{s}104s. These results demonstrate the potential of this scheme for the implementation of a compact or even chip-scale optical frequency reference, which might find applications in instrumentation, navigation, and metrology.
Alkali vapors confined in nano-scale cells are promising tools for future integrated atom-based sensor. In this thesis, we investigate the interaction between light and an ensemble of atoms confined in a nano-geometry. We focus on the different processes that can modify the optical response of the atomic ensemble and possibly affect the sensitivity of a sensor based on that technology. First, we study the non-local response of atoms to a light excitation due the atomic motion in thermal vapors. When the distance over which the atoms relaxes is larger than the size of the cell, the optical response depends on the size of the system. We have observed that for transmission spectroscopy, this leads to a periodic modification of the optical response with a period equal to the wavelength of the optical transition. Subsequently we showed that when the density of atom increases, the atomic response becomes local again. In this dense regime, the resonant dipole-dipole interaction in a sub-wavelength geometry leads to collective frequency shifts of the spectral lines. We demonstrate that these shifts were induced by the cavity formed by the cell walls, hence clarifying a long-standing issue. We developed a model to extract the density shifts deconvolved from the cavity effects. Close to a surface, the optical response is also affected by the van der Waals atom-surface interaction. We introduced a new method to extract precisely the strength of this interaction. We also developed a new generation of super-polished glass nano-cells and we presented promising spectroscopic signals. Finally, using these cells, we have compared transmission and off-axis spectroscopic techniques.