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arXiv:2309.11123 · Physical Review A

Optically driven spin-alignment precession

Anatoly A. Fomin, M. Yu. Petrov, Anatoly S. Pazgalev, G. G. Kozlov, Valerii S Zapasskii

Published Oct 16, 2023• 4 citations
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Abstract

The effect of optically driven spin precession discovered by Bell and Bloom [W. E. Bell and A. L. Bloom, Phys. Rev. Lett. 6, 280 (1961)] is widely used nowadays as a basis for numerous experiments in fundamental physics and for diverse applications. In this paper we consider a much less popular version of the light-induced spin precession that does not imply coherent precession of the spin-system magnetization and is excited by linearly (rather than circularly) polarized light. Pump-probe measurements performed on the ${D}_{2}$ line of cesium vapor show that the magnitude of the signal of the optically driven spin-alignment precession, in ``vacuum'' cells (with no buffer gas) is close to that of classical spin-orientation precession. In the presence of buffer gas, however, the signal of spin-alignment precession appears to be strongly suppressed. The discovered effect is ascribed to spin mixing of excited states of cesium atoms in the cycle of optical pumping.

DOI
10.1103/physreva.108.043109

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Optics and Spectroscopy•May 1, 2007•7 citations
The effect of optically driven spin precession discovered by Bell and Bloom [W. E. Bell and A. L. Bloom, Phys. Rev. Lett. 6, 280 (1961)] is widely used nowadays as a basis for numerous experiments in fundamental physics and for diverse applications. In this paper we consider a much less popular version of the light-induced spin precession that does not imply coherent precession of the spin-system magnetization and is excited by linearly (rather than circularly) polarized light. Pump-probe measurements performed on the D2{D}_{2}D2​ line of cesium vapor show that the magnitude of the signal of the optically driven spin-alignment precession, in ``vacuum'' cells (with no buffer gas) is close to that of classical spin-orientation precession. In the presence of buffer gas, however, the signal of spin-alignment precession appears to be strongly suppressed. The discovered effect is ascribed to spin mixing of excited states of cesium atoms in the cycle of optical pumping.
Introduction Soon after the development of optical magnetometers based on the radio-optical double resonance method (see Chapter 4), it was realized by Bell and Bloom [1] that an alternative method for optical magnetometry was to modulate the light used for optical pumping at a frequency resonant with the Larmor precession of atomic spins. In a Bell-Bloom optical magnetometer, circularly polarized light resonant with an atomic transition propagates through an atomic vapor along a direction transverse to a magnetic field B . Atomic spins immersed in B precess at the Larmor frequency Ω L , and when the light intensity is modulated at Ω m = Ω L , a resonance in the transmitted light intensity is observed. The essential ideas of the Bell–Bloom optical magnetometer are reviewed in Chapter 1 (Section 1.2), and can be summarized in terms of what Bell and Bloom termed optically driven spin precession: in analogy with a driven harmonic oscillator, in a magnetic field B atomic spins precess at a natural frequency equal to Ω L and the light acts as a driving force oscillating at the modulation frequency Ω m . From another point of view, the Bell-Bloom optical magnetometer can be described in terms of synchronous optical pumping: when Ω m = Ω L , there is a “stroboscopic” resonance in which atoms are optically pumped into a spin state stationary in the frame rotating with Ω L . Depending on the details of the atomic structure, the spin state stationary in the rotating frame can be either a dark state that does not interact with the modulated light or a bright state for which the strength of the light–atom interaction is increased.
The optical pumping of Cs atoms with light of the D 1 line under magnetic resonance conditions is numerically calculated. This calculation is done to check the suggestion that an unexpectedly strong influence of the polarization of the pumping light on the resonance signals of transverse alignment, which we observed experimentally, is caused by the coupling between orientation and alignment by means of the magnetic field and/or the pumping light. This suggestion has been confirmed: upon circularly polarized pumping, the shape of the signal line of transverse alignment proved to be similar, as well as in the experiment, to the shape of the signal line of transverse orientation and was sharply different from the shape of the line of alignment observed upon linearly polarized pumping. For metastable 23 S 1 4He atoms, in accordance with the experimental data, the calculated shape of the signal line of transverse alignment is found to be independent of the polarization of the pumping light. The calculations also confirm the possibility of a reverse effect: the influence of alignment on orientation, which manifests itself in the occurrence of orientation upon pumping with unpolarized light under magnetic resonance conditions. For Cs atoms exposed to a field of ∼0.6 Oe, the largest contribution to this effect is yielded by the field coupling of orientation and alignment, whereas, for metastable 23 S 1 4He atoms, the largest contribution is made by the light coupling of these polarization moments.
We uncover a highly nontrivial dependence of the spin-noise (SN) resonance broadening induced by the intense probe beam. The measurements were performed by probing the cell with cesium vapor at the wavelengths of the transition $6^{2}S_{1/2}\ensuremath{\leftrightarrow}6^{2}P_{3/2}$ (D2{D}_{2}D2​ line) with the unresolved hyperfine structure of the excited state. The light-induced broadening of the SN resonance was found to differ strongly at different slopes of the D2{D}_{2}D2​ line and, generally, varied nonmonotonically with light power. We discuss the effect in terms of the phenomenological Bloch equations for the spin fluctuations and demonstrate that the SN broadening behavior strongly depends on the relation between the pumping and excited-level decay rates, the spin precession, and decoherence rates. To reconcile the puzzling experimental results, we propose that the degree of optical perturbation of the spin system is controlled by the route of the excited-state relaxation of the atom or, in other words, that the act of optical excitation of the atom does not necessarily break down completely its ground-state coherence and continuity of the spin precession. Spectral asymmetry of the effect, in this case, is provided by the position of the ``closed'' transition $F=4\ensuremath{\leftrightarrow}{F}^{\ensuremath{'}}=5$ at the short-wavelength side of the line. This hypothesis, however, remains to be proven by microscopic calculations.
The relaxation of atomic polarization in buffer-gas-free, paraffin-coated cesium vapor cells is studied using a variation on Franzen's technique of ``relaxation in the dark'' [Franzen, Phys. Rev. 115 850 (1959)]. In the present experiment, narrow-band, circularly polarized pump light, resonant with the Cs D2 transition, orients atoms along a longitudinal magnetic field, and time-dependent optical rotation of linearly polarized probe light is measured to determine the relaxation rates of the atomic orientation of a particular hyperfine level. The change in relaxation rates during light-induced atomic desorption (LIAD) is studied. No significant change in the spin relaxation rate during LIAD is found beyond that expected from the faster rate of spin-exchange collisions due to the increase in Cs density.
Two precessing spins that are parallel (`oriented') or antiparallel (`aligned') modulate the probe light beam at the precession or double-precession frequency, respectively. Stochastic modulation of light by a spin-system at Larmor frequency is the noise of spin orientation detected in conventional spin noise spectroscopy. Polarization modulation of the probe beam at the double Larmor frequency discovered by the authors in experiments with cesium atoms is ascribed to fluctuations of spin alignment. While the spin-orientation noise is revealed as the noise of gyrotropy, the spin-alignment noise is observed in the fluctuations of linear birefringence.
The mutual coupling between the polarization moments with ranks of different parity is theoretically considered. The manifestation of this mutual coupling has been revealed previously in experiments on magnetic resonance of optically oriented cesium atoms. The two well-known types of the coupling between the polarization moments are considered: the field coupling of these moments that occur due to the breaking of the hyperfine coupling between the electronic and nuclear moments of the alkali atom by the magnetic field and the light coupling of the moments due to the absorption of the pumping light by polarized atoms. The experimentally observed similarity in the shape of resonance signals of alignment and orientation upon circularly polarized pumping can be explained by the fact that, for alkali atoms, the generation of alignment by light at the wavelength of the D 1 line is of low efficiency. Therefore, alignment arises mainly from orientation by means of either the field or the light coupling of polarization moments. For metastable 23 S 1 4He atoms, no influence of the orientation on the alignment was observed because, in these atoms, the field coupling between the polarization moments is absent and the light coupling is not displayed because the generation of alignment by the circularly polarized pumping light is more efficient than the creation of alignment from orientation by means of light coupling of polarization moments.