Magnetic field-induced giant modification of probabilities for seven components of 6S 1/2 (F g =3)  6P 3/2 (F e =5) transition of Cs D 2 line forbidden by selection rules is observed experimentally for the first time. For the case of excitation with circularly-polarized laser radiation, the probability of F g =3,m F =-3  F e =5,m F =-2 transition becomes the largest among 25 transitions of F g =3  F e =2,3,4,5 group in a wide range of magnetic field 200 -3200 G. Moreover, the modification is the largest among D 2 lines of alkali metals. A half-wave-thick cell (length along the beam propagation axis L=426 nm) filled with Cs has been used in order to achieve subDoppler resolution which allows for separating the large number of atomic transitions that appear in the absorption spectrum when an external magnetic field is applied. For B > 3 kG the group of seven transitions Fg=3  Fe=5 is completely resolved and is located at the high frequency wing of Fg=3 Fe=2,3,4 transitions. The applied theoretical model very well describes the experimental curves.

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 ΔF = ±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 D2 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 ΔF = +2, whereas it is several hundreds of thousand times larger in the case of σ − than that for σ + polarization for ΔF = −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.

Decoupling of total electronic and nuclear spin moments of Cs atoms in external magnetic field for the case of atomic D 1 line, leading to onset of the hyperfine Paschen-Back regime has been studied theoretically and experimentally. Selective reflection of laser radiation from an interface of dielectric window and atomic vapor confined in a nanocell with 300 nm gap thickness was implemented for the experimental studies. The real time derivative of selective reflection signal with a frequency position coinciding with atomic transitions was used in measurements, providing ∼ 40 MHz spectral resolution and linearity of signal response in respect to transition probability. Behavior of 28 individual Zeeman transitions in a wide range of longitudinal magnetic field (0 -6 kG) has been tracked under excitation of Cs vapor by a low-intensity σ + -polarized cw laser radiation. For B ≥ 6 kG, only 8 transitions with nearly equal probabilities and the same frequency slope remained in the spectrum, which is a manifestation of the hyperfine Paschen-Back regime. The obtained experimental results are consistent with numerical modeling. Due to small divergence of selective reflection signal, as well as sub-wavelength thickness and sub-Doppler spectral linewidth inherent to nanocell, the employed technique can be used for distant remote sensing of magnetic field with high spatial and B-field resolution. I IntroductionOptical nanometric thin cell (nanocell) containing atomic vapor of alkali metal (Rb, Cs, K, Na) is proven to be efficient and convenient spectroscopic tool for magneto-optical studies of optical atomic transitions between the hyperfine levels in strong external magnetic fields. Two interconnected effects develop with the increase of Bfield: strong deviation of Zeeman splitting of hyperfine transitions from linear dependence, and significant change in probability of individual Zeeman transitions [1,2,3]. The efficiency of nanocell technique for quantitative spectroscopy of the Rb atomic transitions in strong magnetic field (up to 7 kG) resulting in onset of the hyperfine Paschen-Back regime was demonstrated in [4,5,6]. The presence of specific "guiding" transitions foretelling characteristics of all other transitions between magnetic sublevels of alkali atoms D 1 line excited by π-polarized radiation in a strong transversal magnetic field was recently shown in [7]. Micro-and nanocells were used in [8] to study the onset of hyperfine Paschen-Back regime for Cs D 2 line in magnetic fields up to 9 kG. Realization of 1 arXiv:1610.09807v1 [physics.atom-ph]

The existence of crossover resonances makes saturated-absorption (SA) spectra very complicated when external magnetic field B is applied. It is demonstrated for the first time, to the best of our knowledge, that the use of micrometric-thin cells (MTCs, L ≈ 40 μm) allows application of SA for quantitative studies of frequency splitting and shifts of the Rb atomic transitions in a wide range of external magnetic fields, from 0.2 up to 6 kG (20-600 mT). We compare the SA spectra obtained with the MTC with those obtained with other techniques and present applications for optical magnetometry with micrometer spatial resolution and a broadly tunable optical frequency lock.

-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 λ/2-and λ-methods for effective investigations of individual transitions of the K D1 line. The methods are based on strong narrowing of the absorption spectrum of the atomic column of thickness L equal to λ/2 and to λ (with λ = 770 nm 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 versus 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 B = 0) with the probabilities undergoing strong modification under the influence of magnetic fields. Practically the complete hyperfine Paschen-Back regime is observed at relatively low (∼ 1 kG) magnetic fields. Note that for K D2 line GT are absent. Theoretical models describe the experiment very well.Introduction. -Atomic spectroscopy with NC filled with Rb or Cs atomic vapor, with a thickness of the vapor column L which is of the order of optical radiation wavelength has been found to be very efficient to study optical atomic transitions in external magnetic fields [1]. There are two interconnected effects: splitting of the atomic energy levels to Zeeman sublevels and shifting of frequencies (deviating from the linear dependence observed in quite moderate magnetic fields), and significant change in atomic transitions probabilities as a function of the B field [2][3][4]. These studies benefit from the following features of NC: i) sub-Doppler spectral resolution for atomic vapor thickness L = λ/2 and L = λ (λ being the resonant wavelength of Rb D 1,2 or Cs D 1,2 lines) needed to resolve a huge number of Zeeman transition components in transmission or fluorescence spectra; ii) possibility to apply a strong magnetic field using permanent magnets in spite of a strong inhomogeneity of the B field (in our case it can reach 150 G/mm). Note that the variation of the B field inside the atomic vapor is negligible as the vapor column thickness is small. K vapor in magnetic fields was stud-