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Laser Physics Letters

Effect of the phase switching of the optical FID in magnetic field

Е. Н. Чесноков, Кубарев Виталий Владимирович, P. V. Koshlyakov, Ya. I. Gorbachev

Published Mar 18, 2022• 2 citations
Publisher

Abstract

Abstract This work demonstrates the possibility of switching the phase of optical free induction decay (FID) in time-domain experiments. This possibility arises in experiments with paramagnetic molecules (free radicals) in a magnetic field. Changing the direction of the magnetic field alters the direction of rotation of the FID polarization plane and inverts the phase of one of the components of FID. This effect will be useful for heterodyne detection of the optical FID of the free radicals in the terahertz region.

DOI
10.1088/1612-202x/ac59bd

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Abstract This work demonstrates the possibility of switching the phase of optical free induction decay (FID) in time-domain experiments. This possibility arises in experiments with paramagnetic molecules (free radicals) in a magnetic field. Changing the direction of the magnetic field alters the direction of rotation of the FID polarization plane and inverts the phase of one of the components of FID. This effect will be useful for heterodyne detection of the optical FID of the free radicals in the terahertz region.
A novel form of laser frequency switching is devised which extends coherent optical transient studies to a 100-psec time scale. The technique incorporates a traveling-wave electro-optic element which imposes with unit efficiency a uniform time-varying phase and thus a frequency shift on a cw laser beam. In contrast to earlier optical traveling-wave modulators which are driven by a microwave oscillator, here the optical phase change is induced rapidly and easily by a dc electric field pulse that propagates in a microwave guide either parallel or antiparallel to the light wave. This advance enables optical free-induction decay (FID) studies on a subnanosecond time scale and reveals such new features as a rapid first-order FID that dephases with the inhomogeneous dephasing time T2∗{T}_{2}^{*}T2∗​. The well-known nonlinear FID can interfere with the first-order component at short times and decays over the much longer period $\frac{{T}_{2}}{[1+{(1+{\ensuremath{\chi}}^{2}{T}_{1}{T}_{2})}^{\frac{1}{2}}]}$, where $\ensuremath{\chi}$ is the Rabi frequency. A complete analytical expression is derived for optical FID of a transition subject to homogeneous and inhomogeneous broadening and supports detailed observations of the sodium D1{D}_{1}D1​ line.
A theory of optical free-induction decay (FID) induced by two exciting light pulses in three-level atomic systems subjected to a constant magnetic field is presented. The theory takes into account the hyperfine interaction, the level degeneracy, and the dependence of the FID signal on the polarizations of the exciting pulses. An analysis of the FID signal in a magnetic field suggests new ways to measure the g factors of the levels, the hyperfine structure constants, and the relaxation rates. The results obtained are extended to include the echelon excitation of the multilevel system by a definite set of light pulses with specially chosen frequencies and polarizations.
A new feature of optical free induction decay (FID) is observed in a coherently prepared sample of 13CH^{13}\mathrm{CH}13CH3F_{3}\mathrm{F}3​F by the method of Stark switching. The preparative phase is due to the resonant steady-state excitation by a cw laser beam, and is followed by FID upon sudden application of a dc Stark field that switches the molecular sample out of resonance. The emission is not observed as a simple decay but instead appears as a train of sharp pulses regularly spaced in time as a result of a repetitive interference. This situation arises because an entire set of infrared transitions within the Stark split manifold are initially prepared, in contrast to our previous study of a nondegenerate transition. The emission, which beats with the laser, produces a heterodyne beat spectrum, consisting of a set of regularly spaced frequencies, that is the Fourier transform of the slowly decaying pulse progression observed. We thus demonstrate what is the optical analog of the well-known NMR method of high-resolution pulse Fourier spectroscopy. The detailed behavior of the pulse train agrees well with an FID theory that assumes the transitions to be uncoupled. The subtle behavior of FID near the time origin is explored also by approximate analytic expressions that reveal either a near zero amplitude or sizable amplitude depending on the degree of saturation in the preparative stage. The experimental technique discussed offers an attractive way for obtaining high-resolution optical spectra without Doppler broadening, and for generating an optical pulse train whose time scale can be compressed by simply increasing the Stark field.
A novel form of laser frequency switching is devised that extends coherent optical transient studies to a 100-picosecond time scale, the measurements being performed in real time. Free-induction decay (FID) on a subnanosecond time scale reveals new features, such as a first-order FID that dephases with the inhomogeneous dephasing time T2* and interferes with the well-known nonlinear FID. A complete analytical expression for optical FID is derived and supports our FID observations for the sodium D1transition.
Free-induction-decay (FID) magnetometer is highly suitable for precise magnetic field sensing in unshielded environments with the benefit of exceptional accuracy and large dynamic range. The sensitivity of the FID magnetometer is directly influenced by the signal-to-noise ratio, making it critical to enhance the amplitude of the FID signal. In this study, we propose a FID magnetometer based on synchronous optical pumping and RF pulse modulation. A comprehensive theoretical description of the magnetometer is introduced, followed by simulation and experiment that compare the proposed modulation method with the synchronous optical pumping modulation method and the RF pulse modulation method. The results show that the synchronous optical pumping and RF pulse modulation achieves the enhancement of the FID signal and improves the magnetometer sensitivity. Furthermore, the dead zone of the magnetometer is reduced to the direction of the probe beam. This work is significant for further development of optically pumped magnetometers and provides a new scheme for their applications in unshielded environments.
The dead zones of atomic magnetometers bring great challenges for the mobile applications in high-sensitivity magnetic field measurement. To solve this fundamental problem, we present a dead-zone-free orientation-based atomic magnetometer employing free-induction-decay (FID) scheme. A modulation scheme combining synchronous optical pumping and radio frequency pulse manipulation is utilized to achieve the free precession of spin polarization in all magnetic field orientations, and the FID signals are detected in two directions with an orthogonal pump-probe light configuration. We perform a comprehensive theoretical analysis of spin polarization evolution for arbitrary magnetic field orientations, and both simulation and experimental results demonstrate that the fusion of information from the pump and probe beams enables the dead-zone-free measurement of the magnetometer, with the sensor exhibiting a sensitivity in the range of 0.6 – 1.4 pT / Hz 1 / 2 across all directions. The proposed scheme is of great significance for the advancement of pulsed atomic magnetometers and their various applications in mobile platforms.