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Physical Review A

Electronic spin polarization with an optical centrifuge

Johannes Floß, C. Boulet, Jean‐Michel Hartmann, A. Milner, Valery Milner

Published Oct 1, 2018• 8 citations
Publisher

Abstract

We investigate, both theoretically and experimentally, the mechanism behind the creation of a macroscopic magnetization in a gas of paramagnetic molecules with an optical centrifuge, reported in [Phys. Rev. Lett. 118, 243201 (2017)]. Our analysis shows that the centrifuged super-rotors and noncentrifuged molecules are polarized in opposite directions, while the net magnetic moment of the whole ensemble at the end of the interaction with the laser pulse remains close to zero. As the super-rotors are more stable against reorienting collisions, their spin polarization, which points along the centrifuge axis, decays more slowly than the oppositely oriented polarization of the noncentrifuged molecules. The latter lose their directional rotation much more quickly and with it the polarization of their electronic spin. We show numerically that owing to this difference in decay rates, a net magnetization in the direction of the centrifuge is generated. The proposed model is supported by experimental data.

DOI
10.1103/physreva.98.043401

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We investigate, both theoretically and experimentally, the mechanism behind the creation of a macroscopic magnetization in a gas of paramagnetic molecules with an optical centrifuge, reported in [Phys. Rev. Lett. 118, 243201 (2017)]. Our analysis shows that the centrifuged super-rotors and noncentrifuged molecules are polarized in opposite directions, while the net magnetic moment of the whole ensemble at the end of the interaction with the laser pulse remains close to zero. As the super-rotors are more stable against reorienting collisions, their spin polarization, which points along the centrifuge axis, decays more slowly than the oppositely oriented polarization of the noncentrifuged molecules. The latter lose their directional rotation much more quickly and with it the polarization of their electronic spin. We show numerically that owing to this difference in decay rates, a net magnetization in the direction of the centrifuge is generated. The proposed model is supported by experimental data.
Strong laser-induced magnetization of oxygen gas at room temperature and atmospheric pressure is achieved experimentally on the subnanosecond time scale. The method is based on controlling the electronic spin of paramagnetic molecules by means of manipulating their rotation with an optical centrifuge. Spin-rotational coupling results in a high degree of spin polarization on the order of one Bohr magneton per centrifuged molecule. Owing to the nonresonant interaction with the laser pulses, the demonstrated technique is applicable to a broad class of paramagnetic rotors. Executed in a high-density gas, it may offer an efficient way of generating macroscopic magnetic fields remotely (as shown in this work) and producing a large amount of spin-polarized electrons.
We study the dynamics of paramagnetic molecular superrotors in an external magnetic field. An optical centrifuge is used to create dense ensembles of oxygen molecules in ultrahigh rotational states. In is shown, for the first time, that the gas of rotating molecules becomes optically birefringent in the presence of a magnetic field. The discovered effect of "magneto-rotational birefringence" indicates the preferential alignment of molecular axes along the field direction. We provide an intuitive qualitative model, in which the influence of the applied magnetic field on the molecular orientation is mediated by the spin-rotation coupling. This model is supported by the direct imaging of the distribution of molecular axes, the demonstration of the magnetic reversal of the rotational Raman signal, and by numerical calculations.
We consider a diatomic molecule driven by a linearly polarized laser pulse with a polarization axis rotating with a constant acceleration. This setup is referred to as optical centrifuge, and it is known to lead to high-angular momenta for the molecule (superrotor states) and, possibly, to dissociation. Here we elucidate the dynamical mechanisms behind the creation of superrotor states and their dissociation. We unravel the role of the various parameters of the laser field in these processes by considering reduced Hamiltonian models encapsulating the different phases in the creation of superrotor states, possibly leading to dissociation.
We investigate theoretically the ability of an optical centrifuge - a laser pulse whose linear polarization is rotating at an accelerated rate, to control molecular rotation in the regime when the rigid-rotor approximation breaks down due to coupling between the vibrational and rotational degrees of freedom. Our analysis demonstrates that the centrifuge field enables controlled excitation of high rotational states while maintaining relatively low spread along the vibrational coordinate. We contrast this to the rotational excitation by a linearly polarized Gaussian pulse of equal spectral width and pulse energy which, although comparable to the centrifuge-induced rotation, is unavoidably accompanied by a substantial broadening of the vibrational wavepacket.
We report a theoretical study of the optical centrifuge acceleration of an asymmetric top molecule interacting with an electric static field by solving the time-dependent Schr\"odinger equation in the rigid rotor approximation. A detailed analysis of the mixing of the angular momentum in both the molecular and the laboratory fixed frames allows us to deepen the understanding of the main features of the acceleration process, for instance, the effective angular frequency of the molecule at the end of the pulse. For the case of the SO2{\mathrm{SO}}_{2}SO2​ molecular super-rotor, we show numerically that it rotates around one internal axis and that its dynamics is confined to the plane defined by the polarization axis of the laser, in agreement with experimental findings. Furthermore, we consider the orientation patterns induced by the dc field, showing the characteristics of their structure as a function of the strength of the static field and the initial configuration of the fields.
An optical centrifuge is a laser pulse which enables controlled rotational excitation of molecules. Centrifuged molecules rotating with well-defined angular frequencies are ideal candidates to probe many-body quantum systems at the nanoscale. Because the interaction with the quantum environment increases the effective moment of inertia of the embedded molecular probes, the required rotational acceleration of the optical centrifuge must be lowered to accommodate adiabatic spinning. We demonstrate a design of an optical centrifuge, based on the method of spectral focusing, which enables extremely low, down to zero, rotational accelerations. We discuss the potential of using such constant-frequency centrifuge to investigate molecular rotation inside helium nanodroplets.