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arXiv:1312.1557 · Physical Review A

Magneto-optical spectroscopy with polarization-modulated light

Evelina Breschi, Z. D. Grujić, P. Knowles, Antoine Weis

Published Aug 16, 2013• 13 citations
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Abstract

We present a combined theoretical and experimental study of magnetic resonance transitions induced by polarization-modulated light in cesium vapor exposed to a transverse magnetic field. Signals are obtained by phase-sensitive analysis of the light power traversing the vapor cell at six harmonics of the polarization modulation frequency. Resonances appear whenever the Larmor frequency matches an integer multiple of the modulation frequency. We have further investigated the modifications of the spectra when varying the modulation duty cycle. The resonance amplitudes of both in-phase and quadrature components are well described in terms of the Fourier coefficients of the modulation function. The background-free signals generated by the polarization modulation scheme have a high application potential in atomic magnetometry.

DOI
10.1103/physreva.88.022506

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We present a combined theoretical and experimental study of magnetic resonance transitions induced by polarization-modulated light in cesium vapor exposed to a transverse magnetic field. Signals are obtained by phase-sensitive analysis of the light power traversing the vapor cell at six harmonics of the polarization modulation frequency. Resonances appear whenever the Larmor frequency matches an integer multiple of the modulation frequency. We have further investigated the modifications of the spectra when varying the modulation duty cycle. The resonance amplitudes of both in-phase and quadrature components are well described in terms of the Fourier coefficients of the modulation function. The background-free signals generated by the polarization modulation scheme have a high application potential in atomic magnetometry.
We report on a study of polarization-modulation experiments on the 4 → 3 hyperfine component of the D1 transition in Cs vapor contained in a paraffin-coated cell. The laser beam's polarization was switched between left- and right-circular polarization at a rate of 200 Hz. Variations of the transmitted light power were recorded while varying the amplitude of a transverse magnetic field. The power shows electromagnetically induced transparency (EIT) resonances when the atomic Larmor frequency matches a harmonic of the modulation frequency. We made a quantitative study of the resonance amplitudes with square-wave modulations of various duty cycles, and find an excellent agreement with recent algebraic model predictions.
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We investigate the polarization modulation between two linear orthogonal states of the laser beam that synchronously pumps time-dependent atomic alignment in caesium atoms exposed to a static magnetic field. Because of the atomic alignment symmetry two independent groups of resonances can be distinguished in the transmitted light: when modulation frequency matches either the Larmor frequency or its second harmonics, ${\ensuremath{\omega}}_{L}$ and $2{\ensuremath{\omega}}_{L}$, respectively. We report on our experiments, and discuss a model that perfectly reproduces the observed spectra. We have observed that the amplitudes and linewidths of resonances at ${\ensuremath{\omega}}_{L}$ and $2{\ensuremath{\omega}}_{L}$ show magnetic-field direction dependence. This peculiar behavior makes our approach interesting for future application to atomic magnetometry, in view of a dead-zone free high-sensitivity magnetometer.
We report on experimental and theoretical investigations of the polarization dependence of magnetic resonance generated by synchronous optical pumping. Magnetic resonances with narrow linewidth are generated experimentally using a rubidium vapor cell with octadecyltrichlorosilane antirelaxation coating on inner walls. We studied the effect of light ellipticity on the amplitudes and widths of magnetic resonances by matching the light modulation frequency with $2{\mathrm{\ensuremath{\Omega}}}_{L}$ (alignment) and ${\mathrm{\ensuremath{\Omega}}}_{L}$ (orientation) in a Bell-Bloom interaction geometry, where ${\mathrm{\ensuremath{\Omega}}}_{L}$ corresponds to the Larmor frequency. Both $2{\mathrm{\ensuremath{\Omega}}}_{L}$ and ${\mathrm{\ensuremath{\Omega}}}_{L}$ resonance amplitudes showed a strong dependence on the light ellipticity. In addition, we showed that the duty cycle of light modulation changes the slope of amplitude variations in $2{\mathrm{\ensuremath{\Omega}}}_{L}$ and ${\mathrm{\ensuremath{\Omega}}}_{L}$ resonances with light ellipticity. As a potential application, we showed that the difference between $2{\mathrm{\ensuremath{\Omega}}}_{L}$ and ${\mathrm{\ensuremath{\Omega}}}_{L}$ resonance amplitudes can be used for in situ measurement of light ellipticity. We also studied the dependence of $2{\mathrm{\ensuremath{\Omega}}}_{L}$ and ${\mathrm{\ensuremath{\Omega}}}_{L}$ resonance amplitudes on the polarization angle of linearly polarized light. These amplitudes oscillate periodically with the polarization angle. We found this oscillatory behavior to be sensitive to the tilt in magnetic field direction from the polarization plane. Such a property could be used to realize a vector magnetometer. A density matrix based theoretical model is developed to simulate the magnetic resonance spectrum for different light polarizations. Our theoretical model accurately reproduces the above-mentioned experimental observations.
Larmor precession of laser-polarized atoms contained in antirelaxation-coated cells, detected via nonlinear magneto-optical rotation (NMOR), is a promising technique for a new generation of ultrasensitive atomic magnetometers. For magnetic fields directed along the light propagation direction, resonances in NMOR appear when linearly polarized light is frequency or amplitude modulated at twice the Larmor frequency. Because the frequency of these resonances depends on the magnitude but not the direction of the field, they are useful for scalar magnetometry. Additional NMOR resonances at the Larmor frequency appear when the magnetic field is tilted away from the light propagation direction in the plane defined by the light propagation and polarization vectors. These resonances, studied both experimentally and with a density matrix calculation in the present work, offer a convenient method of achieving additional information about a direction of the magnetic field.
Nonlinear magneto-optical resonances occurring for near-zero magnetic field are studied in Rb vapor using light-noise spectroscopy. With a balanced detection polarimeter, we observe high contrast variations of the noise power (at fixed analysis frequency) carried by diode laser light resonant with the $5{\mathrm{S}}_{1∕2}\phantom{\rule{0.2em}{0ex}}(F=2)\ensuremath{\rightarrow}5{\mathrm{P}}_{1∕2}\phantom{\rule{0.2em}{0ex}}(F=1)$ transition 87Rb^{87}\mathrm{Rb}87Rb and transmitted through a rubidium vapor cell as a function of magnetic field BBB. A symmetric resonance doublet of anticorrelated noise is observed for orthogonal polarizations around B=0B=0B=0 as a manifestation of ground-state coherence. We also observe sideband noise resonances when the magnetic field produces an atomic Larmor precession at a frequency corresponding to one-half of the analysis frequency. The resonances on the light fluctuations are the consequence of phase to amplitude noise conversion owing to nonlinear coherence effects in the response of the atomic medium to the fluctuating field. A theoretical model (derived from linearized Bloch equations) is presented that reproduces the main qualitative features of the experimental signals under simple assumptions.