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Moscow University Physics Bulletin

Magnetic resonance signals from the optically oriented two-level spin system in the presence of nonresonant fields

С.Д. Петрова, R. M. Umarkhodzhaev

Published Oct 1, 2008• 0 citations
Publisher

Abstract

A theoretical and experimental investigation into the motion of the magnetization vector in the presence of two additional circularly-polarized nonresonant rf fields, when these fields rotate in the plane analogous to that of resonant rf fields with the frequency ω, has been performed. The nonresonant fields are equal in magnitude; their frequencies differ from the resonant frequency ω by the value of ±Ω, which essentially exceeds the magnetic resonance line width.

DOI
10.3103/s002713490805010x

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A theoretical and experimental investigation into the motion of the magnetization vector in the presence of two additional circularly-polarized nonresonant rf fields, when these fields rotate in the plane analogous to that of resonant rf fields with the frequency ω, has been performed. The nonresonant fields are equal in magnitude; their frequencies differ from the resonant frequency ω by the value of ±Ω, which essentially exceeds the magnetic resonance line width.
Nonlinear in laser power, near-resonant light-atom interaction leads to a number of spectacular modifications of a well-known effect -polarization plane rotation in a magnetic field applied along the direction of light propagation.In particular, the rotation can be enhanced by up to nine orders of magnitude compared to the linear case.Faraday effect [ 1 ] is the rotation of the plane of linear light polarization by a medium in a magnetic field applied in the direction of light propagation.In 1898, Macaluso and Corbino eJ, studying absorption spectra of the alkali atoms in the presence of magnetic fields, discovered that the Faraday effect in the vicinity of resonance absorption lines also had a distinct resonant character.The essence of the Macaluso-Corbino effect can be illustrated by considering a simple case of a F= 1 ~ F'=O transition (Fig. 1 ).Linearly polarized light incident on the sample can be resolved into two counter-rotating circular components ( cr~.In the absence of a magnetic field, the M=±l sublevels are degenerate and the optical resonance
Abstract We present experimental and theoretical investigations of the dependence of double Hanle resonance spectrum on the light polarization angle and the direction of the applied transverse magnetic field (TMF). The experiments are done for F g = 2 → F e = 1 transition of 87Rb D1 line using a rubidium vapor cell containing buffer gas. We show that a small light polarization component along the direction of TMF introduces asymmetry in the double Hanle resonance signal. Both the magnitude and sign of asymmetry in the signal are sensitive to the TMF orientation, suggesting a possible method for in-situ measurement of the direction of the magnetic fields generated by the coils. The physical origin of this asymmetry is explained by considering the redistribution of population among the ground-state Zeeman sublevels in the presence of TMF. In addition, we systematically vary both the polarization angle and TMF direction to study their effect on the line profile of Hanle resonances. We demonstrate that a double Hanle resonance changes to a dark Hanle resonance by rotating the light polarization vector irrespective of the TMF direction. We have developed a simple theoretical model based on a degenerate two-level system to explain our experimental observations.
In an experiment with an optical pumping of 133Cs atoms in the 62 S 1/2 ground state, the line shape of the D 2f magnetic resonance signal for the transverse alignment component oscillating at a double frequency f of a radio-frequency (RF) magnetic field is found to strongly depend on the polarization of pumping radiation. On passage from a linearly polarized pumping light to a circularly polarized (CP) light with a sufficiently strong RF field the ordinary three-peak line with the highest central peak transforms into a two-peak line with a minimum at the center, so that the D 2f signal line resembles the M f signal line of a transverse orientation oscillating at the RF field frequency f. This suggests that the orientation (the first-rank polarization moment (PM)) arising upon CP pumping affects the alignment (the second-rank PM); i.e., the PMs of Cs atoms with different parities of their ranks become coupled. No influence of the polarization of a pumping radiation on the line shape of the D 2f signal is observed in a similar experiment with the 4He atoms in the 23 S 1 metastable state.
The use of four pulsed rf fields for the excitation of multiphoton resonances in a two-level system is investigated. A general theoretical description of the experiment is provided based on the Floquet formalism and application of basic conservation laws to the overall transition process. The predictions of the theory are compared with computer simulations of the time evolution of the magnetization according to the exact time-dependent Hamiltonian. It is shown that N-photon resonances occur when the fields are applied at frequencies \ifmmode\pm\else\textpm\fi{}k\ensuremath{\omega} and \ifmmode\pm\else\textpm\fi{}l\ensuremath{\omega} from the Larmor frequency, where k and l are positive integers having no common factors and where 0\ensuremath{\le}k<l, k+l=N. For any resonance k photons are taken from the outer field pair and l are taken from the inner field pair. A total of 2[(l+k)!]/(k!l!) mutually interfering transition pathways exist for the overall process and the resultant magnitude of the effective N-photon field is very sensitive to the relative initial phases of the rf fields. The spiral motion observed for multiphoton resonances induced by double-frequency irradiation is absent here when the field intensities are symmetrical. In addition, the fraction of total applied rf intensity available for multiphoton pumping is considerably greater than for two fields. Phase cycling to enhance detection of multiphoton effects and multiphoton spin locking are considered.
The evolution of magnetization of optically pumped atoms in the presence of two rotating rf fields with different frequencies is investigated by using the phenomenological Bloch equation. It is shown that the new type resonances, which are due to the effect of the transverse pumping, together with the ordinary magnetic resonance and the multiple quantum transitions are realized under these conditions. It is also shown that the Hanle curve is broadened by a rotating rf field in the same manner as by an oscillating rf field, and is shifted towards high field region as the intensity of the rotating rf field is increased. The results obtained are, in general, applicable to the case of an elliptically rotating rf field, which is superposition of two rotating rf fields.
Coherence transfer between atomic levels requires the matching of the Larmor frequencies. For levels with different ggg factors this occurs only in zero magnetic field. By changing the ggg factors through coupling with a nonresonant rf field, one can match the Larmor frequencies and make the transfer possible in nonzero magnetic fields. Resonances in the transfer appear, depending on the rf field amplitude. Rb87{\mathrm{Rb}}^{87}Rb87-Cs133{\mathrm{Cs}}^{133}Cs133 spin-exchange experiments are reported which provide an illustration of the theory.