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arXiv:2406.15607 · Physics Letters A

Features of alkali D2 line magnetically-induced transitions excited under π-polarized laser radiation

Armen D. Sargsyan, Emmanuel Klinger, A. Amiryan, David G. Sarkisyan

Published Feb 20, 2025• 7 citations
arXivHTMLPDF

Abstract

The impact of the optical field polarization on the spectrum of magnetically-induced transitions, a class of transitions forbidden at zero magnetic field, is studied with a weak-probe sub-Doppler technique. The high spectral resolution of the technique combined with the simplicity in interpreting the observed spectra allows to follow the behavior of individual transitions as a function of the magnetic field amplitude. We observe only one intense transition (out of 2 F g + 1 , where F is the quantum number associated with the total angular momentum of the atom) in the case of π -polarized laser radiation (a configuration where the applied magnetic field is parallel to the electric field from the laser radiation) in the F g → F g + 2 manifolds of 85 Rb, 87 Rb and 133 Cs for fields above a few hundreds of gauss. We show that this behavior is in agreement with a model based on the diagonalization of the Zeeman Hamiltonian matrix. With the rapid development of micro-machined vapor-cell-based sensors, these results will be of use to magnetometers operating above Earth field, wide-range laser frequency stabilization systems and atomic Faraday filters. • Magnetically-induced transitions of Rb and Cs D2 lines are studied with π -polarization. • An anomalous behavior of magnetically-induced transition intensities is observed. • These features are studied with a high spectral resolution technique based on nanocell. • Observed features, well described by theory, are expected for all other alkali transitions.

DOI
10.1016/j.physleta.2025.130372

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The impact of the optical field polarization on the spectrum of magnetically-induced transitions, a class of transitions forbidden at zero magnetic field, is studied with a weak-probe sub-Doppler technique. The high spectral resolution of the technique combined with the simplicity in interpreting the observed spectra allows to follow the behavior of individual transitions as a function of the magnetic field amplitude. We observe only one intense transition (out of 2 F g + 1 , where F is the quantum number associated with the total angular momentum of the atom) in the case of π -polarized laser radiation (a configuration where the applied magnetic field is parallel to the electric field from the laser radiation) in the F g → F g + 2 manifolds of 85 Rb, 87 Rb and 133 Cs for fields above a few hundreds of gauss. We show that this behavior is in agreement with a model based on the diagonalization of the Zeeman Hamiltonian matrix. With the rapid development of micro-machined vapor-cell-based sensors, these results will be of use to magnetometers operating above Earth field, wide-range laser frequency stabilization systems and atomic Faraday filters. • Magnetically-induced transitions of Rb and Cs D2 lines are studied with π -polarization. • An anomalous behavior of magnetically-induced transition intensities is observed. • These features are studied with a high spectral resolution technique based on nanocell. • Observed features, well described by theory, are expected for all other alkali transitions.
It has been shown that the D 1 line of atomic vapors of alkali metals excited by π-polarized radiation in a strong transverse magnetic field includes specific “guiding” (indicating) atomic transitions between the magnetic sublevels of the hyperfine structure. The dependence of the frequency shift of the guiding transitions on the magnetic field, as well as on their dipole moments, is asymptotic for all other transitions. An experiment with a nanocell with Rb vapor with a thickness of half the wavelength (λ/2 method) for ensuring a sub-Doppler spectral resolution has completely confirmed the presence of guiding transitions. Two groups of six transitions for 85 Rb and two groups of four transitions for 87 Rb have been detected in the absorption spectra in magnetic fields above 4 kG. A guiding transition has been identified in each of four groups. Four transitions forbidden at B = 0 have been also detected; with an increase in the magnetic field, their probabilities also approach the probabilities of the guiding transitions.
Atomic transitions of D 2 line of Cs atoms in strong transverse magnetic fields up to 7 kG with the high spectral resolution were studied for the first time. The λ/2 method based on the nanocell with the half-wavelength thickness L = λ/2 = 426 nm is used, which allows one to trace the behavior of each of the individual atomic transitions. When the laser radiation has the linear π polarization and magnetic fields B is more than 5 kG, only 16 atomic transitions, which have almost the same amplitudes, remain in the absorption spectrum (at lower fields there are 56 transitions). There is a good agreement between the experiment and the theory.
Abstract Atomic transitions of 39K, D2 line are studied in strong longitudinal magnetic fields, up to 1000 G, with a high spectral resolution. The saturated absorption (SA) method was used in a micro-cell filled with K vapors with a thickness of L= 30 µm. A double differentiation of the SA spectrum was carried out, which made it possible to obtain narrow (~50 MHz) atomic lines and study individual transition frequency and probabilistic behavior. Among alkali metals, the K atoms have the smallest hyperfine splitting of the ground levels. This allows one to observe the break of the coupling between the electronic total angular momentum J and nuclear I angular momentums at relatively low magnetic fields B > 300 G. In the case of using circularly polarized radiation (σ+ or σ–), two groups (each consisting of four) equidistantly located total eight atomic transitions are recorded in strong magnetic fields. The experimental results are well described by a known theoretical model.
Transitions F e − F g = Δ F = ±2 between the excited and ground levels of the hyperfine structure of the Cs D 2 atomic line in an external magnetic field of 300–3000 G have been studied for the first time with the use of σ + and σ − circularly polarized radiation. Selection rules forbid these transitions in zero magnetic field. At the same time, the probabilities of these transitions in a magnetic field increase significantly; for this reason, we refer to these transitions as magnetically induced transitions. The following rule has been found for the intensities of 24 magnetically induced F g = 3 → F e = 5 and F g = 4 → F e = 2 transitions: the intensities of magnetically induced transitions with Δ F = +2 are maximal (the number of such magnetically induced transitions is also maximal) in the case of σ + polarized radiation, whereas the intensities of magnetically induced transitions with Δ F = −2 are maximal (the number of such transitions is also maximal) in the case of σ − σ + and σ − polarized radiation can reach several orders of magnitude; i.e., anomalous circular dichroism is observed. For an experimental test, absorption spectra of a Cs-filled nanocell with the thickness equal to half the wavelength of resonant laser radiation = 852 nm have been analyzed in order to separately detect magnetically induced transitions. The experiment is in good agreement with the theory. Possible applications have been discussed.
We report the first use of an extremely thin vapor cell (thickness ~ 400 nm) to study the magnetic-field dependence of laser-induced-fluorescence excitation spectra of alkali atoms. This thin cell allows for sub-Doppler resolution without the complexity of atomic beam or laser cooling techniques. This technique is used to study the laser-induced-fluorescence excitation spectra of Rb in a 50 G magnetic field. At this field strength the electronic angular momentum J and nuclear angular momentum I are only partially decoupled. As a result of the mixing of wavefunctions of different hyperfine states, we observe a nonlinear Zeeman effect for each sublevel, a substantial modification of the transition probabilities between different magnetic sublevels, and the appearance of transitions that are strictly forbidden in the absence of the magnetic field. For the case of right- and left- handed circularly polarized laser excitation, the fluorescence spectra differs qualitatively. Well pronounced magnetic field induced circular dichroism is observed. These observations are explained with a standard approach that describes the partial decoupling of I and J states.
Abstract Interest in magnetically induced (MI) transitions of alkali metal atoms is caused by the fact that their intensities can exceed the intensities of ‘regular’ atomic transitions in a wide magnetic field range (200–4000 G). The goal of this work was to form and study, for the first time, electromagnetically induced transparency (EIT) resonance in a strong magnetic field using probe radiation tuned to | F g = F , m F = 0 ⟩ → | F e = F , m F = 0 ⟩ MI transition, which is forbidden in zero magnetic fields. Two narrow-band linearly polarized cw diode lasers were used to form EIT resonance on a Λ-type system of a Cs atomic D 2 line in a strong transverse magnetic field (up to 1000 G). The resonance was formed in a Cs atomic vapor nanocell with an atomic vapor column thickness of 850 nm.