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arXiv:1811.10193 · Journal of the Optical Society of America B

Phase-dependent light-induced torque

Seyedeh Hamideh Kazemi, Mohammad Mahmoudi

Published Nov 8, 2019• 0 citations
arXivHTMLPDF

Abstract

We study an optical torque emerging through the interaction of orbital angular momentum with atoms/molecules in quantum systems with closed-loop interactions. First, we show how atoms with a closed-loop scheme experience a well-controlled torque whose features depend on the relative phase of applied fields. Such a controllable torque, along with simplicity of tuning the relative phase, can simplify the implementation of current flows in atomic Bose–Einstein condensates. Moreover, we calculate the optical torque exerted on chiral molecules with cyclic-transition structures and find that the enantiomers can experience different torques for proper choice of the parameters. Such features may find applications in sorting and separation of enantiomers.

DOI
10.1364/josab.36.003395

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The African review of physics•Oct 15, 2007•1 citation
We study an optical torque emerging through the interaction of orbital angular momentum with atoms/molecules in quantum systems with closed-loop interactions. First, we show how atoms with a closed-loop scheme experience a well-controlled torque whose features depend on the relative phase of applied fields. Such a controllable torque, along with simplicity of tuning the relative phase, can simplify the implementation of current flows in atomic Bose–Einstein condensates. Moreover, we calculate the optical torque exerted on chiral molecules with cyclic-transition structures and find that the enantiomers can experience different torques for proper choice of the parameters. Such features may find applications in sorting and separation of enantiomers.
We investigate a phase-controllable mechanism for generating optical torque in a five-level double-tripod (DT) atom-light coupling scheme interacting with four strong coherent control fields as well as two weak optical vortex probe beams carrying orbital angular momentum (OAM). The spatial phase gradients of the OAM-carrying probes induce a quantized torque that is transferred to the atoms, rotating them and generating a directed atomic flow within an annular geometry. Analytical solutions of the optical Bloch equations under steady-state conditions show that the induced torque and resulting rotational motion exhibit high sensitivity to phase variations. We show that the DT system coherently reconfigures into either coupled Λ or double-Λ schemes depending on the relative phases, with each configuration exhibiting distinct quantized torque characteristics. This enables precise phase control of the atomic current flow, with potential applications in quantum control, precision measurement, and quantum information processing.
The physics of optical vortices, also known as twisted light, is now a well-established and a growing branch of optical physics with a number of important applications and significant interdisciplinary connections.Optical vortex fields of widely varying forms and degrees of complexity can be realised in the laboratory by a host of different means.The interference between such beams with designated orbital angular momenta and optical spins (the latter is associated with wave polarisations) can be structured to conform to various geometrical arrangements.The focus of this review is on how such tailored forms of light can exert a controllable influence on atoms with which they interact.The main physical effects involve atoms in motion due to application of optical forces.The now mature area of atom optics has had notable successes both of fundamental nature and in applications such as atom lasers, atom guides and Bose-Einstein condensates.The concepts in atom optics encompass not only atomic beams interacting with light, but atomic motion in general as influenced by optical and other fields.Our primary concern in this review is on atoms in structured light where, in particular, the twisted nature of the light is made highly complex with additional features due to wave polarisation.These features bring to the fore a variety of physical phenomena not realisable in the context of atomic motion in more conventional forms of laser light.Atoms near resonance with such structured light fields become subject to electromagnetic fields with complex polarisation and phase distributions, as well as intricately structured intensity gradients and radiative forces.From the combined effect of optical spin and orbital angular momenta, atoms may also experience forces and torques involving an interplay between the internal and centre of mass degrees of freedom.Such interactions lead to new forms of processes including scattering, trapping and rotation and, as a result, they exhibit characteristic new features at the micro-scale and below.A number of distinctive properties involving angular momentum exchange between the light and the atoms are highlighted, and prospective applications are discussed.Comparison is made between the theoretical predictions in this area and the corresponding experiments that have been reported to date.
We investigate the generation of optical torque in coherently prepared multi-level atomic media driven by a vector vortex beam composed of two counter-rotating components carrying opposite orbital angular momenta, +lℏ+l\hbar+lℏ and −lℏ-l\hbar−lℏ. We consider a three-level ΛΛΛ configuration and a four-level tripod configuration. Using a perturbative steady-state solution of the optical Bloch equations, we obtain analytical expressions for both linear and nonlinear contributions to the optical torque. The results show that the torque is strongly controlled by atomic coherence, including the initial population imbalance and the relative phase between the vortex components. Nonvanishing torque can arise even when the two components have equal amplitudes, due to coherence-induced asymmetry in the atomic response. In the tripod configuration, the presence of a strong control field leads to electromagnetically induced transparency, which suppresses the torque near resonance and shifts the dominant response to finite detunings. These results establish a route for controlling light-induced rotational dynamics in atomic media using vector vortex fields, with potential applications in coherent optical manipulation and angular-momentum-based control in quantum systems.
Abstract In recent years, photon drag has attracted enormous attention owing to both fundamental and practical interests. In this paper, we investigate dragging effect in quantum systems with closed-loop interaction. First, we demonstrate an enhanced photon drag in a moving atomic medium with a three-level Λ-type structure. By incorporating the interference of spontaneous emissions, the properties of the atomic medium can be easily controlled by the relative phase of applied fields so that a large group index along with a transparency or even a gain can be achieved. As the photon drag is proportional to the group index, the enhancement can be also found in the dragging of light. Such enhanced dragging effect can be applied for the efficient modulators of light, position control, and the detection of slow motion. We then investigate the Fresnel’s light drag effect in another closed-loop structure, i.e. chiral molecules with a cyclic three-level Δ-type configuration, and show that left- and right-handed chiral molecules exhibit different dragging effects. This provides a possible way to probe the molecular chirality, which may find applications in the sorting and separation of chiral molecules.
We show that a persistent current flow in an atomic gas Bose-Einstein condensate could be realized when the system is subject to two counterpropagating Laguerre-Gaussian so-called doughnut beams, creating a toroidal trap. The theory is developed involving a two-photon process within three atomic levels leading to a quantized light-induced torque which rotates the atoms, generating an atomic current flow in the ring. We also show that it is possible for the torque to be controlled and even switched on and off by varying the frequencies of the incident light, thereby allowing a mechanism for the control of the current flow.
The interaction of atoms and molecules with structured light, specifically laser light endowed with the property of orbital angular momentum, such as Laguerre-Gaussian light, is discussed. The primary effects of interest here are the influence of the light on the gross motion of atoms and molecules and the possibilities this motion provides for particle manipulation in cooling, heating and trapping experiments. It turns out that, in addition to the possibility of modifying translational motion, suitably structured light can facilitate the manipulation of rotational motion. The latter possibility arises from a light-induced torque that is directly attributable to the orbital angular momentum property of the light. We outline the physics responsible for these effects and consider applications to typical cases in which atoms and ions are subject to near resonant Laguerre-Gaussian beams, leading to characteristic trajectories and eventual trapping in specific regions. Details are given for optical molasses configurations based on twisted light beams arranged in one-, two- and three-dimensional counter-propagating pairs. We extend consideration to the case of liquid crystals, subject to Laguerre-Gaussian light tuned far off-resonance, and show how this leads to the twisting of the directors in the liquid crystal, coinciding with the intensity distribution of the light.