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APL Photonics

Spatially resolved atomic dichroism and CPT-induced coherence-enhanced transmission in vector beam fields

Wenlei Zhao, Xinxiu Zhou, Zhaoyang Cao, Wenxiang Xie, Le Wang, Jingcheng Shang

Published Oct 1, 2025• 0 citations
Publisher

Abstract

In our previous studies, we reported polarization-dependent profile variations in modulation transfer spectroscopy, including enhanced error signals and modified absorption profiles, which could not be fully explained by conventional models. In this work, we propose a unified physical framework to interpret these phenomena by introducing the concept of atomic spatial polarization-dependent transmission. Using hybrid polarized vector beams as both pump and probe in a counter-propagating configuration, we demonstrate that polarization-induced anisotropic absorption arises from optical pumping dynamics mediated by the vector beam structure. We further show that, when coherent population trapping (CPT) is introduced via dual-frequency laser excitation, the formation of an atomic dark-state population markedly enhances transmission contrast and gives rise to pronounced spectral features. We observed that narrowing the full width at half maximum of the saturated absorption spectrum nearly doubled the MTS signal amplitude. These effects manifest as spatially modulated intensity patterns in the probe beam, thereby revealing CPT-enhanced, polarization-selective transmission in atomic media. Our results not only explain previously observed polarization-induced signal enhancements but also offer new insights into spatially coherent manipulation of atomic populations, with implications for precision spectroscopy and laser stabilization.

DOI
10.1063/5.0291497

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In our previous studies, we reported polarization-dependent profile variations in modulation transfer spectroscopy, including enhanced error signals and modified absorption profiles, which could not be fully explained by conventional models. In this work, we propose a unified physical framework to interpret these phenomena by introducing the concept of atomic spatial polarization-dependent transmission. Using hybrid polarized vector beams as both pump and probe in a counter-propagating configuration, we demonstrate that polarization-induced anisotropic absorption arises from optical pumping dynamics mediated by the vector beam structure. We further show that, when coherent population trapping (CPT) is introduced via dual-frequency laser excitation, the formation of an atomic dark-state population markedly enhances transmission contrast and gives rise to pronounced spectral features. We observed that narrowing the full width at half maximum of the saturated absorption spectrum nearly doubled the MTS signal amplitude. These effects manifest as spatially modulated intensity patterns in the probe beam, thereby revealing CPT-enhanced, polarization-selective transmission in atomic media. Our results not only explain previously observed polarization-induced signal enhancements but also offer new insights into spatially coherent manipulation of atomic populations, with implications for precision spectroscopy and laser stabilization.
Vector beams (VBs) with potential applications are successfully utilized in many fields as light sources with a spatially-varying polarization profile in recent years. Here, we study the transmission of a VB by manipulating atomic polarization via the optical pumping effect. By using hybridly and radially polarized beams as pump and probe beams in a counter-propagating configuration, we observe a four-petal pattern intensity distribution of probe beam, and the four-petal pattern rotates with the polarization state orientation of the pump beam. The results show a polarization dependent absorption in the atomic media. We experimentally demonstrate the absorption characteristics under different polarization combinations of pump and probe beams. The Jones matrix method is used to explain this phenomenon and the simulations are consistent with the experimental observation. Our results may provide a sound foundation for applications in optical manipulation and quantum information in atomic ensembles.
Coherent population trapping (CPT) is extensively studied for future vapor cell clocks of high frequency stability. In the constructive polarization modulation CPT scheme, a bichromatic laser field with polarization and phase synchronously modulated is applied on an atomic medium. A high contrast CPT signal is observed in this so-called double-modulation configuration, due to the fact that the atomic population does not leak to the extreme Zeeman states, and that the two CPT dark states, which are produced successively by the alternate polarizations, add constructively. Here, we experimentally investigate CPT signal dynamics first in the usual configuration, a single circular polarization. The double-modulation scheme is then addressed in both cases: one pulse Rabi interaction and two pulses Ramsey interaction. The impact and the optimization of the experimental parameters involved in the time sequence are reviewed. We show that a simple seven-level model explains the experimental observations. The double-modulation scheme yields a high contrast similar to the one of other high contrast configurations like push-pull optical pumping or crossed linear polarization scheme, with a setup allowing a higher compactness. The constructive polarization modulation is attractive for atomic clock, atomic magnetometer, and high precision spectroscopy applications.
In a spin-exchange relaxation-free (SERF) atomic magnetometer, the pump and probe laser frequencies are tuned to the D 1 transition line of alkali metal atoms to enhance measurement sensitivity. The D 1 line, a non-cycling transition, exhibits a low absorption coefficient. This study theoretically analyzes the influence of different parameters on the absorption coefficient of the D 1 line and proposes a polarization-dependent approach to enhance it. A frequency stabilization method based on modulation transfer spectroscopy (MTS) with an enhanced absorption coefficient is introduced. Compared with the conventional method, optimizing the probe beam’s polarization increases the absorption coefficient, raising the amplitude of the MTS signal. The results show that using a stronger MTS signal for frequency stabilization reduces the laser's frequency noise. Using heterodyne measurement, we characterize the linewidth-narrowing effect of MTS locking and evaluate the frequency stability of the locked laser. The linewidth of each laser is reduced from the free-running 2.2 MHz to 885 kHz after MTS stabilization. The Allan deviation measurements of the beat signal between two locked lasers are 2.6 × 10 −11 / τ . This work provides a stable optical frequency reference for SERF atomic magnetometers and offers potential applications in extracting spatial information from optically pumped atoms.
We show that optical activity in atomic media can generate multidimensional correlations in frequency, polarization and spatial degrees of freedom, allowing us to observe frequency shifts as image rotations. Specifically, we demonstrate a pump-probe scheme, where optical pumping with circularly polarized light generates macroscopic magnetization in an atomic vapor, associated with frequency-dependent circular dichroism and birefringence. A vortex probe beam, characterized by spatially varying polarization, maps this optical activity onto the spatial structure of the transmitted light. Measuring the intensity profile in a suitable polarization component then allows us to perform spatially resolved polarization spectroscopy. On resonance we observe a rotation with a slope in the order of 98 mrad per MHz. These findings may find applications in image-based laser-locking, high-precision spectroscopy, magnetometry, and the generation of hybrid entanglement.
Vector beams with spatially varying polarization distribution in the wavefront plane have received increasing attention in recent years. The manipulation of vector beams both in intensity and polarization distributions is highly desired and under development. In this work, we study the transmission property of vector beams through warm rubidium vapor and realize controllable transmission of vector beams based on atomic dichroism. By utilizing the linearly polarized beam and vector beams as the pump and probe beams in a pump–probe configuration, a spatially-dependent dichroism can be obtained, which leads to spatially varied absorption of the probe beam. The controllable intensity distribution of the probe beam, as a two-petal pattern, can rotate with the variation of the pump beam's polarization states. We experimentally demonstrate the mechanism of dichroism with linear polarization light and provide an explanation based on the optical pumping effect. Alternatively, the varying trend of the probe beam's intensity is also interpreted by utilizing the Jones matrix. Our results are thus beneficial for providing potential applications in optical manipulation in atomic ensembles.
We show that optical activity in atomic media can generate multidimensional correlations in frequency, polarization and spatial degrees of freedom, allowing us to observe frequency shifts as image rotations. Specifically, we demonstrate a pump-probe scheme, where optical pumping with circularly polarized light generates macroscopic magnetization in an atomic vapor, associated with frequency-dependent circular dichroism and birefringence. A vortex probe beam, characterized by spatially varying polarization, maps this optical activity onto the spatial structure of the transmitted light. Measuring the intensity profile in a suitable polarization component then allows us to perform spatially resolved polarization spectroscopy. On resonance we observe a rotation with a slope in the order of 98 mrad per MHz. These findings may find applications in image-based laser-locking, high-precision spectroscopy, magnetometry, and the generation of hybrid entanglement.