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Journal of Physics B Atomic Molecular and Optical Physics

Coherent control of magneto-optic rotation

Kanhaiya Pandey, Ajay Wasan, Vasant Natarajan A

Published Nov 10, 2008• 32 citations
Publisher

Abstract

We experimentally study the rotation of the plane of polarization of a laser beam passing through room-temperature Rb vapour. The rotation occurs because the medium behaves differently for the two orthogonally-polarized components, displaying what is known as circular birefringence or linear dichroism. The difference is induced either by a control laser applied to an auxiliary transition of a ladder-type system, or by an applied axial magnetic field. In the presence of both control laser and magnetic field, the line shape shows an interesting interplay between the two effects with regions of suppressed and enhanced rotation. The line shapes can be understood qualitatively based on a density-matrix analysis of the system.

DOI
10.1088/0953-4075/41/22/225503

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We experimentally study the rotation of the plane of polarization of a laser beam passing through room-temperature Rb vapour. The rotation occurs because the medium behaves differently for the two orthogonally-polarized components, displaying what is known as circular birefringence or linear dichroism. The difference is induced either by a control laser applied to an auxiliary transition of a ladder-type system, or by an applied axial magnetic field. In the presence of both control laser and magnetic field, the line shape shows an interesting interplay between the two effects with regions of suppressed and enhanced rotation. The line shapes can be understood qualitatively based on a density-matrix analysis of the system.
Summary form only given. Summary from only given. An isotropic medium, having atoms with m degenerate sublevels, when subjected to a magnetic field, can induce birefringence in the medium. As a result, the plane of polarization of a probe field rotates. This rotation is due to the fact that the magnetic field creates asymmetry between the susceptibilities /spl chi//spl plusmn/ of the medium corresponding to the /spl sigma//spl plusmn/ component of the probe field. We apply a coherent control field which modifies /spl chi//spl plusmn/. Therefore the rotation of probe polarization, due to magnetic field alone, can be controlled efficiently with the use of the control field. We demonstrate this enhancement of the magneto-optical rotation (MOR) of a linearly polarized light.
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
We investigate theoretically the effects of vacuum-induced coherence (VIC) on magneto-optical rotation (MOR). We carry out a model study to show that VIC in the presence of a control laser and a magnetic field can lead to large enhancement in the rotation of the plane of polarization of a linearly polarized weak laser with vanishing circular dichroism. This effect can be realized in cold molecular gases and may be used as a sensitive probe for VIC. Such a large MOR angle can also be used to detect weak magnetic field with large measurement sensitivity.
The Lamb theory of the optical maser is applied to circularly polarized atomic transitions, and used to consider the beat frequencies and the coherence properties of such orthogonal fields when axial magnetic fields are applied to the gaseous laser. The beat frequency approaches zero in near-zero magnetic fields and synchronization can then occur between the right- and left-handed circularly polarized oscillations. For a resonator with no undue polarization constraint, such a strong coupling gives rise to a linearly polarized output, and to a rotation of the plane of polarization with increasing magnetic field. A self-consistent expression is derived for this rotation under steady-state conditions, and a maximum rotation of $\ifmmode\pm\else\textpm\fi{}\frac{1}{4}\ensuremath{\pi}$ with magnetic field is indicated before the synchronization breaks down and circularly polarized beat phenomena appear. The rotation with magnetic field depends on the laser intensity, on the anisotropy in the cavity losses, and on the position of the cavity resonance within the Doppler linewidth. Also, the angle of rotation is indeterminate unless such anisotropy is present. Other regions of such coherence can occur at higher magnetic fields, where the beat frequency again approaches zero. These depend on the detailed shape of the various dispersion curves of the laser medium. The results derived from the theory used are in general agreement with experimental observations on the 1.153-\ensuremath{\mu} He-Ne laser transition.
We investigated experimentally and theoretically angular momentum alignment-to-orientation conversion created by the joint interaction of laser radiation and an external magnetic field with atomic rubidium at room temperature. In particular, we were interested in alignment-to-orientation conversion in the atomic ground state. In the experiment the laser frequency was fixed to the hyperfine transitions of the D1{D}_{1}D1​ line of rubidium. To simulate the measured signals we used a theoretical model that takes into account all neighboring hyperfine levels, the mixing of magnetic sublevels in an external magnetic field, the coherence properties of the exciting laser radiation, and the Doppler effect. The experiments were carried out by exciting the atoms with linearly polarized laser radiation. Two orthogonal circularly polarized laser induced fluorescence (LIF) components were detected and afterward their difference was taken. The combined LIF signals originating from the hyperfine magnetic sublevel transitions of 85Rb^{85}\mathrm{Rb}85Rb and 87Rb^{87}\mathrm{Rb}87Rb rubidium isotopes were included. The alignment-to-orientation conversion can be incontrovertibly identified in the difference signals for various laser frequencies, and a change in signal shapes can be observed when the laser power density is increased. We studied the formation of and the underlying physical processes behind the observed signals of the LIF components and their difference by performing an analysis of the influence of incoherent and coherent effects. We performed simulations of theoretical signals that showed the influence of ground-state coherent effects on the LIF difference signal.
Large Faraday rotations can be generated by circular birefringence of atomic samples in an axial magnetic field in the vicinity of atomic resonance lines. More generally, magneto-optical effects in atomic samples include circular dichroism in addition to birefringence, resulting in a modification of the ellipticity as well as the polarization alignment. Usually such effects are investigated for homogeneous linear polarizations, but the mechanisms apply also to polarization structures such as vector vortices. We investigate the effect of optical activity of a rubidium vapor in the hyperfine Paschen–Back regime along the D2 line, for the example of azimuthally polarized input light. We show that for low atomic densities, circular birefringence dominates over dichroism, and azimuthal polarization is rotated toward radial polarization. The rotation angle increases with increasing optical densities. At high vapor temperatures, dichroism becomes more and more relevant, leading to intricate variations of both alignment and ellipticity. Our results shine new light on the interplay between different magneto-optic effects at strong magnetic fields. Their highly sensitive response to detuning may find applications in spectroscopy and quantum sensing.