Aram Papoyan

Hyperfine Paschen–Back regime realized in Rb nanocell

A simple and efficient scheme based on a one-dimensional nanometric-thin cell filled with Rb and strong permanent ring magnets allows direct observation of the hyperfine Paschen-Back regime on the D(1) line in the 0.5-0.7 T magnetic field. Experimental results are perfectly consistent with the theory. In particular, with σ(+) laser excitation, the slopes of the B-field dependence of frequency shifts for all 10 individual transitions of (85,87)Rb are the same and equal to 18.6 MHz/mT. Possible applications for magnetometry with submicron spatial resolution and tunable atomic frequency references are discussed.

Saturated absorption spectroscopy: Elimination of crossover resonances with the use of a nanocell

It is demonstrated that the velocity-selective optical pumping/saturation resonances of the reduced absorption in a Rb vapor nanocell with thickness L = λ, 2λ, and 3λ (resonant wavelength λ = 780 nm) allow for the complete elimination of crossover (CO) resonances. We observe well-pronounced resonances corresponding to the Fg = 3 → Fe = 2, 3, and 4 hyperfine transitions of the 85Rb D2 line with line widths close to the natural width. A small CO resonance located midway between Fg = 3 → Fe = 3 and Fg = 3 → Fe = 4 transitions appears only for L ≥ 4λ. The D2 line (λ = 852 nm) in a Cs nanocell exhibits a similar behavior. From the amplitude ratio of the CO and VSOP resonances, it is possible to determine the thickness of the column of alkali vapor in the range of 1–1000 μm. The absence of the CO resonances for nanocells with L ∼ λ is attractive for the frequency reference application and for studying the transitions between the Zeeman sublevels in external magnetic fields.

Hyperfine Paschen–Back regime in alkali metal atoms: consistency of two theoretical considerations and experiment

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.

Giant modification of atomic transition probabilities induced by a magnetic field: forbidden transitions become predominant

The magnetic field-induced giant modification of probabilities for seven components of 6S1/2, Fg = 3 → 6P3/2, Fe = 5 transition of the Cs D2 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 a Fg = 3, mF = −3 → Fe = 5, mF = −2 transition becomes the largest of 25 transitions of the Fg = 3 → Fe = 2,3,4,5 group in a wide-range magnetic field of 200–3200 G. Moreover, the modification is the largest among D2 lines of alkali metals. A half-wave-thick cell (the length along the beam propagation axis L = 426 nm) filled with Cs has been used in order to achieve sub-Doppler resolution, which allows the large number of atomic transitions that appear in the absorption spectrum to be separated when an external magnetic field is applied. For B > 3000 G the group of seven transitions Fg = 3 → Fe = 5 is completely resolved and is located at the high frequency level of Fg= 3 → Fe = 2,3,4 transitions. The applied theoretical model describes very well the experimental curves.

N-type resonances in a buffered micrometric Rb cell: splitting in a strong magnetic field.

N-type resonances excited in rubidium atoms confined in micrometric-thin cells with variable thickness from 1 μm to 2 mm are studied experimentally for the cases of a pure Rb atomic vapor and of a vapor with neon buffer gas. Good contrast and narrow linewidth were obtained for thicknesses as low as 30 μm. The higher amplitude and sharper profile of N-type resonances in the case of a buffered cell was exploited to study the splitting of the 85Rb D1 N-resonance in a magnetic field of up to 2200 G. The results are fully consistent with the theory. The mechanism responsible for forming N-resonances is discussed. Possible applications are addressed.

Sub-Doppler spectroscopy of Rb atoms in a sub-micron vapour cell in the presence of a magnetic field

We report the first sub-Doppler study of the magnetic field dependence of laser-induced fluorescence excitation spectra of alkali atoms making use of an extremely thin vapour cell (thickness ∼ 400 nm). This thin cell allows fo rs ub-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 o ft he 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 differ 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.

Selective reflection from an Rb layer with a thickness below λ/12 and applications.

We have studied the peculiarities of selective reflection from an Rb vapor cell with a thickness L<70  nm, which is smaller than the length scale of evanescent fields λ/2π and more than an order of magnitude smaller than the optical wavelength. A 240 MHz redshift due to the atom-surface interaction is observed for a cell thickness of L=40  nm. In addition, complete frequency-resolved hyperfine Paschen-Back splitting of atomic transitions to four components for Rb87 and six components for Rb87 is recorded in a strong magnetic field (B>2  kG).

Essential features of optical processes in neon-buffered submicron-thin Rb vapor cell

A new submicron thin cell (STC) filled with Rb and neon gas is developed and comparison of resonant absorption with STC containing pure Rb is provided. The effect of collapse and revival of Dicke-type narrowing is still observable for the thickness L = lambda /2 and L = lambda , where lambda is a resonant laser wavelength 794 nm (D(1) line). For an ordinary Rb cm-size cell with addition of buffer gas, the velocity selective optical pumping/saturation (VSOP) resonances in saturated absorption spectra are fully suppressed if neon pressure > 0.5 Torr. A spectacular difference is that for L = lambda , VSOP resonances are still observable even when neon pressure is > or = 6 Torr. Narrow fluorescence spectra at L = lambda /2 allow one to realize online buffer gas pressure monitoring. A good agreement with theoretical model is observed.

Selective reflection from Rb vapor in half- and quarter-wave cells: Features and possible applications

The features of the effect of selective reflection from rubidium vapor in a nanocell with the thickness L ≈ λ/2 and L ≈ λ/4, where λ = 795 nm is the wavelength of laser radiation resonant with the Rb D1 line, are studied. It is shown that, because of the behavior of the nanocell as a low-Q-factor Fabry–Pérot etalon, the sign of the derivative of the selective reflection spectra changes near L ≈ λ/2 from negative at L > λ/2 to positive at L < λ/2. The simplicity of the experimental implementation, large amplitude, and sub-Doppler width (40MHz) of a detected signal at an atomic transition frequency are appropriate for applications in metrology and magnetometry. In particular, selective reflection from the nanocell is a convenient frequency marker of atomic transitions; in this case, the amplitudes of peaks are proportional to the transition probabilities. The remote optical monitoring of a magnetic field with a spatial resolution L = λ/4 ≈ 199 of nm is possible on the basis of the splitting of selective reflection peaks in a strong magnetic field (up to 3 kG). A theoretical model describes well the experimental results.

Alkali metal atoms in strong magnetic fields: “Guiding” atomic transitions foretell the characteristics of all transitions of the D1 line

It has been shown that the D1 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 85Rb and two groups of four transitions for 87Rb 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.