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Scientific Reports

Laguerre-Gaussian modes generated vector beam via nonlinear magneto-optical rotation

Mohsen Ghaderi Goran Abad, Mohammad Mahmoudi

Published Mar 16, 2021• 34 citations
Publisher

Abstract

Laguerre-Gaussian (LG) beams contain a helical phase front with a doughnut-like intensity profile. We use the LG beam to introduce a rather simple method for generation of a vector beam (VB), a beam with spatially-dependent polarization in the beam cross section, via the nonlinear magneto-optical rotation (NMOR). We consider the NMOR of the polarization of a linearly polarized probe field passing through an inverted Y-type four-level quantum system interacting with a LG control field and a static magnetic field. It is shown that the polarization of the transmitted field is spatially distributed by the orbital angular momentum (OAM) of the LG control field, leading to generation of the VB with azimuthally symmetric polarization distribution. We show that the polarization and intensity distributions of the VB spatially vary by changing the OAMs of the LG control field. Moreover, the radial index of the LG control field has a major role in more spatially polarization distributing of the VB. It is shown that the intensity of the generated VBs in different points of the beam cross section can be controlled by the OAM as well as the radial index of the LG control field. However, the VB with highly spatially distributed can be generated for higher values of the radial index of LG control field. The analytical calculations determine the contribution of the different nonlinear (cross-Kerr effect) phenomena on the generation of the VB. We show that the VB is mainly generated via birefringence induced by the applied fields. Finally, we use asymmetric LG (aLG) beams for making the VBs with asymmetric polarization distribution. It is shown that by applying aLG beams, the azimuthal symmetry of the polarization distribution breaks and the asymmetric polarization distribution can be controlled by OAM and radial index of the aLG control field. The obtained results may find more interesting applications in fiber/free space optical communication to enhance the capacity of the information transmission.

DOI
10.1038/s41598-021-85249-8

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•Mar 23, 2021•0 citations
Laguerre-Gaussian (LG) beams contain a helical phase front with a doughnut-like intensity profile. We use the LG beam to introduce a rather simple method for generation of a vector beam (VB), a beam with spatially-dependent polarization in the beam cross section, via the nonlinear magneto-optical rotation (NMOR). We consider the NMOR of the polarization of a linearly polarized probe field passing through an inverted Y-type four-level quantum system interacting with a LG control field and a static magnetic field. It is shown that the polarization of the transmitted field is spatially distributed by the orbital angular momentum (OAM) of the LG control field, leading to generation of the VB with azimuthally symmetric polarization distribution. We show that the polarization and intensity distributions of the VB spatially vary by changing the OAMs of the LG control field. Moreover, the radial index of the LG control field has a major role in more spatially polarization distributing of the VB. It is shown that the intensity of the generated VBs in different points of the beam cross section can be controlled by the OAM as well as the radial index of the LG control field. However, the VB with highly spatially distributed can be generated for higher values of the radial index of LG control field. The analytical calculations determine the contribution of the different nonlinear (cross-Kerr effect) phenomena on the generation of the VB. We show that the VB is mainly generated via birefringence induced by the applied fields. Finally, we use asymmetric LG (aLG) beams for making the VBs with asymmetric polarization distribution. It is shown that by applying aLG beams, the azimuthal symmetry of the polarization distribution breaks and the asymmetric polarization distribution can be controlled by OAM and radial index of the aLG control field. The obtained results may find more interesting applications in fiber/free space optical communication to enhance the capacity of the information transmission.
Recently, the generation and detection of structured light field have drawn a great deal of attention, due to their importance in high-capacity optical data storage and quantum technology. In this letter, we explore the azimuthal modulation of optical transparency in a four-level double-V type quantum system near a plasmonic nanostructure. A Laguerre-Gaussian beam and the interaction of the system with free-space vacuum modes have been employed to create the phase-dependent absorption of a non-vortex probe field. First, we demonstrate how to identify the azimuthal index associated with the conventional LG beam via measuring the probe absorption so that the phase information of such a beam gets encoded on the spatially-dependent absorption profile with angularly-distributed lobes. Also, a spatially-varying optical transparency can be formed, due to the periodic variation of the absorption spectrum. Then, asymmetrical Laguerre-Gaussian beams are used to extend the selective spatial transparency mechanism to asymmetric spatially-structured windows, allowing for optical manipulation of spatial modes at arbitrary positions. Moreover, we investigate the influence of the asymmetric parameter on the features of the spatial inhomogeneities and show how the beams enable us to imprint the phase information of the orbital angular momentum at a desired position.
We consider the optical rotation of the polarization of a linearly polarized probe field passing through an M-type atomic system by using the interaction between two vortex control fields and optical transitions. We investigate theoretically to generate the spatially dependent structured light with the atoms acting as a spatially varying circular birefringent medium. We show that the polarization and intensity distributions of the vector beam spatially vary by changing the orbital angular momentum (OAM) of the vortex control field.
Abstract We analyze azimuthal modulation of probe field absorption profile in a closed four-level inverted Y-type atomic system. We employ a Laguerre–Gaussian beam and a microwave field to realize spatially varying atomic susceptibility, which is responsible for the generation of structured light. An extra nonvortex control beam is used to ensure electromagnetically induced transparency at the core of Laguerre–Gaussian beam. Our proposed model exhibits both absorption and gain at different azimuthal angles, and orbital angular momentum of light can be identified via absorption profile of the probe field. We further discuss the transfer of optical vortices from Laguerre–Gaussian beam to the transmitted probe field.
The effect of the orbital angular momentum (OAM) of light beams on the optical bistability (OB) phenomenon has been investigated in a three-level quantum system. In a first step, we consider a Λ -type configuration in which the coupling field is assumed as a Laguerre–Gaussian (LG) beam. It is shown that the OB strength spatially depends on the intensity profile as well as the topological charge (TC) of the light beam. In a second scheme, we apply a strong microwave field to the lower level transition and obtain an analytical expression for the OB behavior of the probe field in the presence of an LG field in the multi-photon resonance condition. We analytically obtain a generalized input–output bistability relation for the systems containing the LG beam. More complicated phase-dependent OB profiles can be obtained for higher order LG modes due to the OAM transfer and classical interference between the generated field and applied fields. The results of this paper may provide a potential application of OB in generating simultaneously reconfigurable sets of various optical logic gates in the same structure, as well as in enhancing the capacity of fast volatile bit-level optical memories.
We investigate a scheme to guide a weak vector beam using optically written waveguides inside atomic vapor, while controlling its polarization rotation. The atoms are prepared in the four-level tripod configuration in the presence of an external magnetic field. The three transitions of the tripod system are driven by a strong control beam and the two orthogonal polarization components of a weak vector beam (VB). A suitable spatial intensity profile and the detuning of the control field facilitate the generation of an optical fiberlike refractive index gradient across the atomic vapor. This enables waveguiding of the weak VB for a couple of Rayleigh lengths. Further, the magnetic-field-induced anisotropy of the tripod system creates a difference in the refractive indices of its two components. This is the reason behind the polarization shaping of VB. The mechanism of efficient guiding of VB with controllable polarization may have important applications in high-density optical communication and quantum information.
We present a theoretical analysis of the spatial shape (transverse and longitudinal) of two degenerate four-wave mixing (FWM) signals induced by fields Ea and Eb, with nonuniform transverse profiles, and wave-vectors ka and kb. The two signals are generated in a cold sample of two-level atoms, in the forward directions 2ka − kb and 2kb − ka. In all cases, the transverse shapes of the generated signals (both near- and far-field) are dictated by the projection of the nonlinear coherence onto the Laguerre-Gaussian (LG) function space. Our calculations show that the longitudinal profile of each generated beam is affected by the spectral characteristics of the medium, described by the third order nonlinear susceptibility. For Gaussian incident fields with equal detunings from resonance, the FWM beam shapes are completely symmetric, with an asymmetry induced by different detunings of incident fields. By considering incident beams with LG distributions, the topological charge conservation determines the orbital angular momentum of each signal and the longitudinal profiles show similar behaviors to the case with Gaussian beams. The phase discontinuities of LG modes reveal a rotation and distortion of the phase distribution of the FWM beams caused by the detunings. We discuss for selected configurations, the relations between both generated beams and their propagation properties. We also present preliminary experimental results of the power spectra of the two FWM signals generated in our magneto-optical trap of rubidium atoms.