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Journal of Physics B Atomic Molecular and Optical Physics

Sign reversal of Hanle electromagnetically induced absorption with orthogonal circularly polarized optical fields

Nibedita Ram, M. Pattabiraman

Published Nov 29, 2010• 17 citations
Publisher

Abstract

We study by computation and experiment an electromagnetically induced absorption resonance in the Hanle configuration with a transverse magnetic field on a closed F g → F e = F g +1 transition with co-propagating orthogonal circularly polarized probe and coupling optical fields. At high coupling field intensities, the Hanle resonance changes sign due to a shift in atomic population from Zeeman sublevels associated with a probe field cyclic transition to sublevels associated with a coupling field cyclic transition at zero magnetic field. We also show that a similar sign reversal does not occur for π-polarized and σ-polarized coupling fields.

DOI
10.1088/0953-4075/43/24/245503

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We study by computation and experiment an electromagnetically induced absorption resonance in the Hanle configuration with a transverse magnetic field on a closed F g → F e = F g +1 transition with co-propagating orthogonal circularly polarized probe and coupling optical fields. At high coupling field intensities, the Hanle resonance changes sign due to a shift in atomic population from Zeeman sublevels associated with a probe field cyclic transition to sublevels associated with a coupling field cyclic transition at zero magnetic field. We also show that a similar sign reversal does not occur for π-polarized and σ-polarized coupling fields.
Abstract The influence of the closed transition on the neighboring and open transitions of the 87 Rb D 2 line, partially resolved under Doppler broadening, is studied at different ellipticities in the Hanle configuration with longitudinal and transverse magnetic field scans. We show that a sign reversal from an electromagnetically induced transparency (EIT) resonance to an electromagnetically induced absorption (EIA) resonance occurs with an increase in ellipticity for the transition in a transverse field scan. This transformation is not observed for a longitudinal field scan. These observations are attributed to the higher amplitude of the EIA resonance for the transition in transverse field scans for elliptical polarized light. The EIA character in the Hanle transmission profile of and transitions is enhanced (weakened) when the laser beam frequency is detuned towards (away from) the transition. We show by computation that at higher ellipticities, a transformation from an EIT to an EIA resonance for the transition occurs at low frequency shifts (towards the transition) in a transverse field scan.
The enhanced absorption Hanle effect has been studied for a closed transition Jg = 2 → Je = 3 with a transverse magnetic field in the presence of a coupling optical field. From an analysis of the individual probe and coupling field absorption profiles, it is shown that the Hanle electromagnetically induced absorption is governed by the transfer of Δm = ±2 and Δm = ±1 Zeeman coherences from the excited state to the ground state via spontaneous emission. The individual coherence contributions are governed by the intensity ratio of the optical fields. We show by computation and experiment that the magnetic field dependence of the forward scattered intensity can be used to distinguish the transfer of coherence contributions to the Hanle profile.
We analyze the Hanle effect on the closed Fg equals 3 yields Fe equals 4 D2-line transition of 85Rb, by both numerical solutions of the optical Bloch equations and by experimental investigations on cell containing rubidium vapor. For laser radiation with linear polarization orthogonal to the applied magnetic field, the Hanle signal is characterized by narrow bright resonances at low laser intensities. A bright resonance Hanle signal is obtained also exciting the rubidium atoms by circularly polarized laser light and scanning a transversely applied magnetic field, at different values of an applied longitudinal magnetic field. We report experimental and numerical evidence of this new bright resonance.
Using the closed ${F}_{g}\ensuremath{\rightarrow}{F}_{e}={F}_{g}+1$ D2D2D2 line transition of Rb atoms, we report experimental and numerical evidence of influence of additional transverse magnetic field when examining electromagnetically induced absorption (EIA) in the Hanle configuration. The effect was analyzed with two directions of additional magnetic field and in the cases of excitation with linear, elliptical, and circular laser light polarization. The transverse magnetic field brings substantial differences in the resonance line shape, amplitude, and width. Our theoretical model includes Doppler broadening. Numerical solutions of our theoretical model are in good agreement with experimental results. This analysis points out the importance of the presence of stray magnetic fields when studying EIA.
The method for observing the high-contrast and narrow-width resonances of electromagnetically induced absorption (EIA) in the Hanle configuration under counterpropagating light waves is proposed. We theoretically analyze the absorption of a probe light wave in presence of counterpropagating one with the same frequency as the function of a static magnetic field applied along the vectors of light waves, propagating in a vapour cell. Here, as an example, we study a "dark" type of atomic dipole transition Fg=1-->Fe=1 in D1 line of 87Rb, where usually the electromagnetically induced transparency (EIT) can be observed. To obtain the EIA signal one should proper chose the polarizations of light waves and intensities. In contrast of regular schemes for observing EIA signals (in a single travelling light wave in the Hanle configuration or in a bichromatic light field consisted of two travelling waves), the proposed scheme allows one to use buffer gas to significantly enhance properties of the resonance. Also the dramatic influence of atomic transition openness on contrast of the resonance is revealed, that gives great advantage in comparison with cyclic atomic transitions. The obtained results can be interesting in high-resolution spectroscopy, nonlinear and magneto-optics.
We computationally compare Hanle-type resonances for a transition of the line for magnetic field scans parallel (longitudinal scan) and perpendicular (transverse scan) to the direction of propagation of the optical field in the presence of an additional transverse magnetic field (TMF). For a linearly polarized light, the coherent population trapping (CPT) resonances split at line centre and are identical for both longitudinal and transverse scans. When the probe beam ellipticity is varied, the effect of the TMF is found to be opposite for longitudinal and transverse scans. For a longitudinal scan, the splitting observed in the CPT resonance evolves into an enhanced absorption resonance with an increase in ellipticity. For a transverse scan, the splitting vanishes at higher ellipticities. This can be understood in terms of population redistribution in the ground state sublevels and near-neighbor ground state coherences created by the TMF. We also show that the enhanced absorption signal that splits the CPT resonance strongly depends on transit time, and the CPT resonance strength depends on the excited state dephasing rate.