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arXiv:1803.05356 · Journal of Physics B Atomic Molecular and Optical Physics

Hyperfine Paschen–Back regime of potassium D 2 line observed by Doppler-free spectroscopy

Armen D. Sargsyan, Emmanuel Klinger, A. Tonoyan, Claude Leroy, David G. Sarkisyan

Published May 23, 2018• 19 citations
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Abstract

Abstract Selective reflection of a laser radiation from an interface formed by a dielectric window and a potassium atomic vapour confined in a nano-cell with 350 nm gap thickness is implemented for the first time to study the atomic transitions of K D 2 line in external magnetic fields. In moderate B -fields, there are 44 individual Zeeman transitions which reduce to two groups (one formed by σ + , the other one by σ − circularly-polarised light), each containing eight atomic transitions, as the magnetic field increases. Each of these groups contains one so-called ‘guiding’ transition whose particularities are to have a probability (intensity) as well as a frequency shift slope (in MHz G −1 ) that are constant in the whole range of 0–10 kG magnetic fields. In the case of π -polarised laser radiation, among eight transitions, two are forbidden at B = 0, yet their probabilities undergo a giant modification under the influence of a magnetic field. We demonstrate that for B -fields > 165 G a complete hyperfine Paschen–Back regime is observed. Other peculiarities of K D 2 line behaviour in magnetic field are also presented. We show a very good agreement between theoretical calculations and experiments. The recording of the hyperfine Paschen–Back regime of K D 2 line with high spectral resolution is demonstrated for the first time.

DOI
10.1088/1361-6455/aac735

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Abstract Selective reflection of a laser radiation from an interface formed by a dielectric window and a potassium atomic vapour confined in a nano-cell with 350 nm gap thickness is implemented for the first time to study the atomic transitions of K D 2 line in external magnetic fields. In moderate B -fields, there are 44 individual Zeeman transitions which reduce to two groups (one formed by σ + , the other one by σ − circularly-polarised light), each containing eight atomic transitions, as the magnetic field increases. Each of these groups contains one so-called ‘guiding’ transition whose particularities are to have a probability (intensity) as well as a frequency shift slope (in MHz G −1 ) that are constant in the whole range of 0–10 kG magnetic fields. In the case of π -polarised laser radiation, among eight transitions, two are forbidden at B = 0, yet their probabilities undergo a giant modification under the influence of a magnetic field. We demonstrate that for B -fields > 165 G a complete hyperfine Paschen–Back regime is observed. Other peculiarities of K D 2 line behaviour in magnetic field are also presented. We show a very good agreement between theoretical calculations and experiments. The recording of the hyperfine Paschen–Back regime of K D 2 line with high spectral resolution is demonstrated for the first time.
A one-dimensional nano-metric-thin cell (NC) filled with potassium metal has been built and used to study optical atomic transitions in external magnetic fields. These studies benefit from the remarkable features of the NC allowing one to use and λ methods for effective investigations of individual transitions of the K D 1 line. The methods are based on strong narrowing of the absorption spectrum of the atomic column of thickness L equal to and to λ (with being the resonant laser radiation wavelength). In particular, for a π -polarized radiation excitation the λ -method allows us to resolve eight atomic transitions (in two groups of four atomic transitions) and to reveal two remarkable transitions that we call guiding transitions (GT). The probabilities of all other transitions inside the group (as well as the frequency slope vs . magnetic field) tend to the probability and to the slope of GT. Note that for circular polarization there is one group of four transitions and GT do not exist. Among eight transitions there are also two transitions (forbidden for ) with the probabilities undergoing strong modification under the influence of magnetic fields. Practically the complete hyperfine Paschen-Back regime is observed at relatively low magnetic fields. Note that for the K D 2 line GT are absent. Theoretical models describe the experiment very well.
The selective reflection of laser radiation from the interface between a dielectric window and the atomic vapors confined in a nanocell of thickness L ≈ 350 nm is used to develop effective Doppler-broadening- free spectroscopy of potassium atoms. A small atomic line width and a relation between the signal intensity and the transition probability allowed us to resolve four lines of atomic transitions responsible for the D1 lines of the 39K and 41K isotopes. Two groups containing four atomic transitions form in an applied magnetic field upon pumping by radiation with circular polarization σ+ or σ–. Different intensities (probabilities) of transitions for the σ+ and σ– excitations are detected in magnetic field B0 ≈ A hfs /μB ≈ 165 G (A hfs is the magnetic dipole constant for the ground state and μB is the Bohr magneton). A substantially different situation is observed at B ≫ B0, since high symmetry appears for the two groups formed by radiation with circular polarization σ+ or σ–. Each group is the mirror image of the other group with respect to the frequency of the 42S1/2–42P1/2 transition, which additionally proves the occurrence of the complete Paschen–Back regime of the hyperfine structure at B ≈ 2.5 kG. A developed theoretical model well reproduces the experimental results. Possible practical applications are described. The results obtained can also be applied to the D1 lines of 87Rb and 23Na.
Decoupling of total electronic and nuclear spin moments of Cs atoms in an external magnetic field for the case of atomic D1 line, leading to onset of the hyperfine Paschen–Back (HPB) regime, is studied theoretically and experimentally. Selective reflection (SR) of laser radiation from an interface of dielectric window and atomic vapor confined in a nanocell with 300 nm gap thickness is implemented for the experimental studies. The real-time derivative of SR signal with a frequency position coinciding with atomic transitions is used in the measurements, providing ∼40 MHz spectral resolution and linearity of the signal response with respect to the transition probability. The behavior of 28 individual Zeeman transitions in a wide range of longitudinal magnetic field (0–6 kG) is tracked under the excitation of Cs vapor by a low-intensity σ+-polarized cw laser radiation. For B≥6 kG, only eight transitions with nearly equal probabilities and the same frequency slope remain in the spectrum, which is a manifestation of the HPB regime. The obtained experimental results are in very good agreement with the numerical modeling. A small divergence of the SR signal as well as subwavelength thickness and sub-Doppler spectral linewidth inherent to nanocell make it possible for the employed technique to be used for distant remote sensing of a magnetic field with high spatial and B-field resolution.
Atomic transitions of the 39 K potassium isotope in strong (up to 1 kG) longitudinal and transverse magnetic fields have been studied with a high spectral resolution. It has been shown that crossover resonances are almost absent in the saturated absorption spectrum of potassium vapors in a 30-μm-thick microcell. This, together with the small spectral width of atomic transitions (~30 MHz), allows one to use the saturated absorption spectrum for determining frequencies and probabilities of individual transitions. Among the alkali metals, potassium atoms have the smallest magnitude of the hyperfine splitting of the lower level. This allows one to observe the break of the coupling between the electronic and nuclear angular momentums at comparatively low magnetic fields B > 500 G, i.e., to implement the hyperfine Paschen–Back regime (HPB). In the HPB regime, four equidistantly positioned transitions with the same amplitude are detected in circularly polarized light (σ + ). In linearly polarized light (π) at the transverse orientation of the magnetic field, the spectrum consists of eight lines which are grouped in two groups each of which consists of four lines. Each group has a special distinguished G-transition and the transition that is forbidden in the zero magnetic field. In the HPB regime, the probabilities of transitions in a group and derivatives of their frequency shifts with respect to the magnetic field asymptotically tend to magnitudes that are typical for the aforesaid distinguished G-transition. Some practical applications for the used microcell are mentioned.
Simple and efficient λ-method and λ/2-method (λ is the resonant wavelength of laser radiation) based on a nanometric-thickness cell filled with rubidium (Rb) are implemented to study the splitting of hyperfine transitions of an Rb85 and Rb87D1 line in an external magnetic field in the range of B=0.5–0.7 T. It is experimentally demonstrated from 20 (12) Zeeman transitions allowed at low B-field in Rb85 (Rb87) spectra in the case of σ+ polarized laser radiation, only 6 (4) remain at B>0.5 T, caused by decoupling of the total electronic momentum J and the nuclear spin momentum I (hyperfine Paschen–Back regime). The expressions derived in the frame of completely uncoupled basis (J,mJ;I,mI) describe the experimental results extremely well for Rb85 transitions at B>0.6 T (that is a manifestation of hyperfine Paschen–Back regime). A remarkable result is that the calculations based on the eigenstates of the coupled (F,mF) basis, which adequately describe the system for a low magnetic field, also predict reduction of the number of transition components from 20 to 6 for Rb85 and from 12 to 4 for Rb87 spectrum at B>0.5 T. Also, the Zeeman transition frequency shifts, frequency intervals between the components and their slope versus B, are in agreement with the experiment.
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.