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arXiv:1707.00688

Magnetic field-induced symmetry breaking exhibited by explicit circular dichroism for $\Delta F = \pm2$ transitions of D$_2$ lines of all alkali atoms

A. Tonoyan, Armen D. Sargsyan, Emmanuel Klinger, G. Hakhumyan, Claude Leroy, A. Papoyan, David G. Sarkisyan, Marcis P. Auzinsh

Published Jul 3, 2017• 0 citations
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Abstract

In this letter we demonstrate universal symmetry breaking by means of magnetically induced circular dichroism. Magnetic field induces forbidden at zero field atomic transitions between $\Delta F = \pm2$ hyperfine levels. In a particular range of magnetic field, intensities of these transitions experience significant enhancement. We have deduced a general rule applicable for the $D_2$ lines of all bosonic alkali atoms, that is transition intensity enhancement is larger for the case of $\sigma^+$ than for $\sigma^-$ excitation for $\Delta F = +2$, whereas it is larger (e.g. up to $10^{11}$ times for $^{85}$Rb atoms) in the case of $\sigma^-$ than for $\sigma^+$ polarization for $\Delta F = -2$. This asymmetric behaviour results in an explicit circular dichroism. For experimental verification we employed half-wavelength-thick atomic vapor nanocells using a derivative of selective reflection technique, which provides sub-Doppler spectroscopic linewidth ($\sim$50 MHz). The presented theoretical curves well describe the experimental results. This effect can find applications particularly in parity violation experiments.

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Circular dichroism of magnetically induced transitions for D 2 lines of alkali atoms

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In this letter we study magnetic circular dichroism in alkali atoms exhibiting asymmetric behaviour of magnetically induced transitions. The magnetic field B ∥ k induces transitions between Δ F = ± 2 hyperfine levels of alkali atoms and in the range of ∼ 0.1 -- 3 kG magnetic field, the intensities of these transitions experience significant enhancement. We have inferred a general rule applicable for the D 2 lines of all alkali atoms, that is the transition intensity enhancement is around four times larger for the case of σ + than for σ − excitation for Δ F = + 2 , whereas it is several hundreds of thousand times larger in the case of σ − than that for σ + polarization for Δ F = − 2 . This asymmetric behaviour results in circular dichroism. For experimental verification we employed half-wavelength-thick atomic vapor nanocells using a derivative of the selective reflection technique, which provides a sub-Doppler spectroscopic linewidth ( ∼ 50 MHz ). The presented theoretical curves well describe the experimental results. This effect can find applications particularly in parity violation experiments.

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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.

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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.

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Abstract The circular dichroism effect has been investigated for atomic transitions of the Rb D1 line in magnetic fields of up to 3 kG using circularly polarized σ+ and σ− radiation. The process of selective reflection from a 350-nm-thick nanocell has been used, which makes it possible to form narrow atomic lines and observe separately the behavior of individual transitions. Two groups consisting of six (85Rb atoms) and four (87Rb atoms) transitions are formed in magnetic fields B > 0.5 kG upon σ+ and σ− laser excitation. All transitions have been identified. It is shown that the strongest transitions for 87Rb and 85Rb atoms in magnetic fields of up to several kG are formed under σ− irradiation. A further increase in the magnetic field makes it possible to attain the Paschen–Back regime on a hyperfine structure, for which the probabilities of transitions upon σ+ and σ− excitation become identical. The theoretical model and experiment are in good agreement.

Optics and Spectroscopy•Dec 1, 2018•2 citations
Magnetic field-induced symmetry breaking exhibited by explicit circular dichroism for ΔF=±2\Delta F = \pm2ΔF=±2 transitions of D2_22​ lines of all alkali atoms
In this letter we demonstrate universal symmetry breaking by means of magnetically induced circular dichroism. Magnetic field induces forbidden at zero field atomic transitions between ΔF=±2\Delta F = \pm2ΔF=±2 hyperfine levels. In a particular range of magnetic field, intensities of these transitions experience significant enhancement. We have deduced a general rule applicable for the D2D_2D2​ lines of all bosonic alkali atoms, that is transition intensity enhancement is larger for the case of σ+\sigma^+σ+ than for σ−\sigma^-σ− excitation for ΔF=+2\Delta F = +2ΔF=+2, whereas it is larger (e.g. up to 101110^{11}1011 times for 85^{85}85Rb atoms) in the case of σ−\sigma^-σ− than for σ+\sigma^+σ+ polarization for ΔF=−2\Delta F = -2ΔF=−2. This asymmetric behaviour results in an explicit circular dichroism. For experimental verification we employed half-wavelength-thick atomic vapor nanocells using a derivative of selective reflection technique, which provides sub-Doppler spectroscopic linewidth (∼\sim∼50 MHz). The presented theoretical curves well describe the experimental results. This effect can find applications particularly in parity violation experiments.
In this letter we study magnetic circular dichroism in alkali atoms exhibiting asymmetric behaviour of magnetically induced transitions. The magnetic field B ∥ k induces transitions between Δ F = ± 2 hyperfine levels of alkali atoms and in the range of ∼ 0.1 -- 3 kG magnetic field, the intensities of these transitions experience significant enhancement. We have inferred a general rule applicable for the D 2 lines of all alkali atoms, that is the transition intensity enhancement is around four times larger for the case of σ + than for σ − excitation for Δ F = + 2 , whereas it is several hundreds of thousand times larger in the case of σ − than that for σ + polarization for Δ F = − 2 . This asymmetric behaviour results in circular dichroism. For experimental verification we employed half-wavelength-thick atomic vapor nanocells using a derivative of the selective reflection technique, which provides a sub-Doppler spectroscopic linewidth ( ∼ 50 MHz ). The presented theoretical curves well describe the experimental results. This effect can find applications particularly in parity violation experiments.
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.
Atomic transitions in alkali metals that have zero probability in the absence of a magnetic field but have large probabilities in the presence of a magnetic field are called magnetically induced (MI). They are of interest because of their large probabilities, which exceed the probabilities of usual transitions in a wide magnetic field range. Magnetically induced transitions are classified as type-1 (MI1) and type-2 (MI2) and their total number is about 100. In this work, MI2 transitions are examined between ground Fg and excited levels Fe of the hyperfine structure satisfying the condition Fe−Fg=ΔF=±2{{F}_{{\text{e}}}} - {{F}_{{\text{g}}}} = \Delta F = \pm 2Fe​−Fg​=ΔF=±2 , which are forbidden in zero magnetic field but have large probabilities in the presence of a magnetic field. The probabilities of the MI2 transitions with ΔF=+2\Delta F = + 2ΔF=+2 and the MI transitions with ΔF=−2\Delta F = - 2ΔF=−2 are maximal in the case of optical radiation with the σ+ and σ– polarizations, respectively. This difference is called type-1 magnetically induced circular dichroism (MICD1). It has been shown for the first time that the probability of the strongest MI2 transition in the 85Rb atom corresponding to the D2 line in magnetic fields >100 G in the case of σ+{{\sigma }^{ + }}σ+ radiation is larger than the probability of the strongest MI2 transition in the case of σ– radiation by a factor of 2.5. This difference is called type-2 magnetically induced circular dichroism (MICD2). It has been shown how to determine the strongest MI transition for any alkali metal atom, which is important for its application in magneto-optical processes. Theoretical curves reproduce well experimental results.
Abstract Magnetically induced (MI) transitions of the D2 line of 39K atom in an external magnetic field of 10–600 G have been experimentally and theoretically investigated for the first time using circularly polarized σ+ and σ− radiations. According to the selection rules, the transitions between the sublevels of the ground and excited levels of a hyperfine structure with Fe – Fg = ΔF = ±2 are forbidden in the zero magnetic field, whereas their probabilities increase radically in a magnetic field. For the Fg = 1 → Fe = 3 (ΔF = +2) and Fg = 2 → Fe = 0 MI transitions, the highest probabilities are achieved using the σ+ and σ– radiations, respectively. The atomic transitions have been spectrally resolved using selective reflection of laser radiation from a nanocell filled with potassium atomic vapor, which makes it possible to investigate the behavior of the MI transitions. The experimental and theoretical data are shown to be in good agreement.
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.
Abstract The circular dichroism effect has been investigated for atomic transitions of the Rb D1 line in magnetic fields of up to 3 kG using circularly polarized σ+ and σ− radiation. The process of selective reflection from a 350-nm-thick nanocell has been used, which makes it possible to form narrow atomic lines and observe separately the behavior of individual transitions. Two groups consisting of six (85Rb atoms) and four (87Rb atoms) transitions are formed in magnetic fields B > 0.5 kG upon σ+ and σ− laser excitation. All transitions have been identified. It is shown that the strongest transitions for 87Rb and 85Rb atoms in magnetic fields of up to several kG are formed under σ− irradiation. A further increase in the magnetic field makes it possible to attain the Paschen–Back regime on a hyperfine structure, for which the probabilities of transitions upon σ+ and σ− excitation become identical. The theoretical model and experiment are in good agreement.