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Optics Continuum

Two-level atom dynamics induced by a spin-orbit coupled optical vortex: dressed states formulation

V. E. Lembessis, A. Lyras, Omar Marzook Al-Dossary

Published May 3, 2023• 5 citations
Publisher

Abstract

We consider here the interaction of a two-level atom with a tightly focused paraxial optical vortex beam in the dressed states formalism. The interaction is characterized by a term that couples the photon spin angular momentum (SAM) with its orbital angular momentum (OAM). This term affects all the physical quantities related to the dressed states, like their energies, populations, and relaxation rates among them. We also show that the Mollow triplet associated with the resonance fluorescence spectrum of a two-level atom acquires a chiral character. We give numerical examples based on experimentally accessible values of the various parameters, and we show that the chirality can give rise to observable modifications of the various physical quantities under consideration.

DOI
10.1364/optcon.488300

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We consider here the interaction of a two-level atom with a tightly focused paraxial optical vortex beam in the dressed states formalism. The interaction is characterized by a term that couples the photon spin angular momentum (SAM) with its orbital angular momentum (OAM). This term affects all the physical quantities related to the dressed states, like their energies, populations, and relaxation rates among them. We also show that the Mollow triplet associated with the resonance fluorescence spectrum of a two-level atom acquires a chiral character. We give numerical examples based on experimentally accessible values of the various parameters, and we show that the chirality can give rise to observable modifications of the various physical quantities under consideration.
Abstract Truncated optical Bessel (TOB) beams are an orthonormal set of optical vortex beams that possess orbital angular momentum (OAM). It is characterized by a significant longitudinal electric field component at the sub-wavelength scales. The photons of a circularly polarized TOB beam also carry spin angular momentum (SAM) and can interact with a two-level atom, in the far-off resonance regime, giving rise to an optical dipole potential energy that involves a spin–orbit term that stems from the inclusion of the longitudinal field component. When a two-level atom is simultaneously irradiated by two TOB beams, the optical dipole potential energy, in some special cases, is entirely dependent on the OAM–SAM coupling. We explore the cold-atom trapping options enabled by the use of TOB beams, either single ones or combinations thereof. We put emphasis on case in which the OAM–SAM coupling is the dominant contribution. The similarities and differences between the results obtained for the TOB beams and those for the Laguerre–Gaussian beams are also discussed.
The exchange of orbital angular momentum (OAM) between paraxial optical vortex and a Bose-Einstein condensate (BEC) of atomic gases is well known. In this paper, we develop a theory for the microscopic interaction between matter and an optical vortex beyond paraxial approximation. We show how superposition of vortex states of BEC can be created with a focused optical vortex. Since the polarization or spin angular momentum (SAM) of the optical field is coupled with OAM of the field, in this case, these angular momenta can be transferred to the internal electronic and external center-of-mass motion of atoms provided both the motions are coupled. We propose a scheme for producing the superposition of matter-wave vortices using Gaussian and a focused Laguerre-Gaussian beam. We study how two-photon Rabi frequencies of stimulated Raman transitions vary with focusing angles for different combinations of OAM and SAM of optical states. We demonstrate the formation of vortex-antivortex structure and discuss interference of three vortex states in a BEC.
We investigate the propagation of an optical vector vortex weakly interacting with a coherently prepared atomic medium (phaseonium) in a three-level ΛΛΛ configuration. The vector beam consists of vortex pulse pairs with right- and left-circular polarizations, corresponding to opposite spin angular momentum (SAM), and carrying opposite orbital angular momentum (OAM) charges ±l\pm l±l. We show that during the propagation of the vortex pairs, analytically obtained in the linear regime, the medium inherits the topology of the vortex pair, mapping the OAM onto a spatially structured atomic coherence. This mapping produces 2∣l∣−2|l|-2∣l∣−fold azimuthal transparency structures that reshape the beam intensity from a ring into a petal-like pattern. The OAM-structured atomic coherence induces a corresponding optical anisotropy within the medium, which feeds back into the propagating vector beam, resulting in optical spin-orbit coupling manifested as SAM exchange, rotation, and evolution of polarization textures. Depending on the initial ground-state population of the phaseonium, the polarization state evolves between left-circular, linear, and right-circular polarizations.
We demonstrate coupling between the atomic spin- and orbital-angular momentum (OAM) of the atom's center-of-mass motion in a Bose-Einstein condensate (BEC). The coupling is induced by Raman-dressing lasers with a Laguerre-Gaussian beam and creates coreless vortices in an F=1 ^{87}Rb spinor BEC. We observe correlations between spin and OAM in the dressed state and characterize the spin texture; the result is in good agreement with the theory. In the presence of the Raman field, our dressed state is stable for 0.1 s or longer, and it decays due to collision-induced relaxation. As we turn off the Raman beams, the vortex cores in the bare spin |m_{F}=1⟩ and |-1⟩ split. These spin-OAM coupled systems with the Raman-dressing approach have great potential for exploring new topological textures and quantum states.
We propose a scheme aimed at achieving spatiospectral control over spontaneous emission within a four-level atom-light coupling system interacting with optical vortices carrying orbital angular momentum (OAM). The atom comprises a ground level and two excited states coupled with two laser fields, forming a V subsystem where the upper states exclusively decay to a common fourth state via two channels. By investigating various initial states of the atom and considering the presence or absence of quantum interference in spontaneous emission channels, we analyze how the characteristics of the OAM-carrying vortex beam imprint onto the emission spectrum. The interplay between the optical vortex and the quantum system, including its environment modes, induces a wide variety of spatiospectral behavior, including two-dimensional spectral-peak narrowing, spectral-peak enhancement, spectral-peak suppression, and spontaneous emission reduction or quenching in the spatial azimuthal plane. Our findings shed light on the dynamics of atom--vortex-beam light interactions and offer insights into the manipulation of emission properties at the quantum level.
A cavity-modified master equation is derived for a coherently driven two-level atom coupled to a single-mode cavity in the bad cavity limit, in which the cavity frequency is tuned to either the center or one of the sidebands of the Mollow triplet. The atomic populations in both the bare- and dressed-state representations are analyzed in terms of the cavity-modified transition rates. In the bare-state basis, the role of the cavity may be interpreted as enhancing the stimulated absorption of the atom while suppressing the stimulated emission. The bare-state population may thus be inverted under appropriate conditions. The dressed-state inversion, however, originates from the enhancement of the atom-cavity interaction when the cavity is resonant with the atomic dressed-state transition. We show that two-phase quadratures of the atomic polarization decay at different rates. The decay of the in-phase (or out-of-phase) quadrature may be greatly inhibited as the driving intensity increases, depending on the cavity resonant frequency. The spectrum of the atomic fluorescence emitted out the side of the cavity is also studied. The spectral profiles are sensitive to the cavity frequency. When the cavity frequency is tuned to the center of the Mollow resonances, the fluorescence spectrum is symmetrical with three peaks whose linewidths and heights are intensity dependent. When the cavity frequency is tuned to one of the Mollow sidebands, however, it is asymmetric, and the central peak and the sideband on resonance with the cavity can be significantly suppressed for strong driving fields. All three spectral lines can be narrowed by increasing the Rabi frequency. The physics of these striking spectral features is explored in the dressed-state basis. We also investigate the probe absorption spectrum. When the cavity frequency is tuned to the center of the Mollow fluorescence triplet, the central component exhibits a Lorentzian line shape, while the side bands show the Rayleigh-wing line shape. Probe gain may occur at line center due to cavity-induced, bare-state population inversion. When the cavity is tuned to resonance with one sideband, only two sidebands, with Lorentzian profiles, dominate. Gain can occur at the sideband far off resonant with the cavity. The sideband gain is a consequence of an unbalanced dressed-state population distribution.