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arXiv:2002.00504 · Physical Review A

Off-axis optical vortices using double-Raman singlet and doublet light-matter schemes

Hamid Reza Hamedi, Julius Ruseckas, Emmanuel Paspalakis, Gediminas Juzeliūnas

Published Jun 22, 2020• 28 citations
arXivHTMLPDF

Abstract

We study the formation of off-axis optical vortices propagating inside a double-Raman gain atomic medium. The atoms interact with two weak probe fields as well as two strong pump beams which can carry orbital angular momentum (OAM). We consider a situation when only one of the strong pump lasers carries an OAM. A particular superposition of probe fields coupled to the matter is shown to form specific optical vortices with shifted axes. Such off-axis vortices can propagate inside the medium with sub- or superluminal group velocity depending on the value of the two-photon detuning. The superluminal optical vortices are associated with the amplification as the energy of pump fields is transferred to the probe fields. The position of the peripheral vortices can be manipulated by the OAM and intensity of the pump fields. We show that the exchange of optical vortices is possible between individual probe beams and the pump fields when the amplitude of the second probe field is zero at the beginning of the atomic cloud. The model is extended to a more complex double Raman doublet interacting with four pump fields. In contrast to the double-Raman-singlet, now the generation of the off-axis sub- or superluminal optical vortices is possible even for zero two-photon detuning.

DOI
10.1103/physreva.101.063828

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We study the formation of off-axis optical vortices propagating inside a double-Raman gain atomic medium. The atoms interact with two weak probe fields as well as two strong pump beams which can carry orbital angular momentum (OAM). We consider a situation when only one of the strong pump lasers carries an OAM. A particular superposition of probe fields coupled to the matter is shown to form specific optical vortices with shifted axes. Such off-axis vortices can propagate inside the medium with sub- or superluminal group velocity depending on the value of the two-photon detuning. The superluminal optical vortices are associated with the amplification as the energy of pump fields is transferred to the probe fields. The position of the peripheral vortices can be manipulated by the OAM and intensity of the pump fields. We show that the exchange of optical vortices is possible between individual probe beams and the pump fields when the amplitude of the second probe field is zero at the beginning of the atomic cloud. The model is extended to a more complex double Raman doublet interacting with four pump fields. In contrast to the double-Raman-singlet, now the generation of the off-axis sub- or superluminal optical vortices is possible even for zero two-photon detuning.
We study the propagation of probe pulses carrying orbital angular momentum (OAM) in a crystal of molecular magnets characterized by a double-$\mathrm{\ensuremath{\Lambda}}$ light-matter coupling scheme. The model is based on the four-wave mixing (FWM) of applied fields interacting with the magnetic dipole moment of the molecular magnets. We consider the light-matter interaction under the situation where a weak probe field carries an optical vortex and investigate the exchange of optical vortices between different frequencies via the FWM in the microwave region. The propagation of OAM beams with nonzero radial indices is then explored. It is found that the conservation of both azimuthal and radial indices is satisfied over the swapping of OAM states of light. Superimposing two initially weak OAM modes with different topological charges creates specific vortex beams with a very characteristic form. The resulting beam contains a central vortex as well as several singly charged peripheral vortices distributed at the same radial distance from the center of the light beam. This complex pattern of vortices arises because the intensity radius of two interfering OAM modes is different. It is shown that the boundary region of intensity dominance of two OAM modes can be controlled by the molecular magnets parameters and strong cw electromagnetic fields. Our proposed scheme may provide a route to study the solid systems suitable for quantum technologies as well as for OAM exchange devices for quantum information processing.
We consider transfer of optical vortices between laser pulses carrying orbital angular momentum in a cloud of cold atoms characterized by the $\mathrm{\ensuremath{\Lambda}}$ configuration of the atom-light coupling. The atoms are initially prepared in a coherent superposition of the lower levels, creating a so-called phaseonium medium. If a single vortex beam initially acts on one transition of the scheme, an extra laser beam is subsequently generated with the same vorticity as that of the incident vortex beam. The absorption of the incident probe beam takes place mostly at the beginning of the atomic medium within the absorption length. The losses disappear as the probe beam propagates deeper into the medium where the atoms are transferred to their dark states. The method is extended to a tripod atom-light coupling scheme and a more general (n+1n+1n+1)-level scheme containing nnn ground states and one excited state, allowing for creation of multiple twisted light beams. We also analyze generation of composite optical vortices in the scheme using a superposition of two initial vortex beams and study lossless propagation of such composite vortices.
We study the manipulation of slow light with an orbital angular momentum propagating in a cloud of cold atoms. Atoms are affected by four co-propagating control laser beams in a double tripod configuration of the atomic energy levels involved, allowing us to minimize the losses at the vortex core of the control beams. In such a situation the atomic medium is transparent for a pair of co-propagating probe fields, leading to the creation of two-component (spinor) slow light. We study the interaction between the probe fields when two control beams carry optical vortices of opposite helicity. As a result, a transfer of the optical vortex takes place from the control to the probe fields without switching off and on the control beams. This feature is missing in a single tripod scheme where the optical vortex can be transferred from the control to the probe field only during either the storage or retrieval of light.
The transfer of optical vortices is studied based on double two-photon processes in a four-level diamond configuration system. A pair of strong fields are applied to prepare atomic coherence, while two weak probe fields are coupled with the other two transitions. When the two-photon resonances are satisfied, the analytical results for the intensities of the probe fields are calculated using perturbation theory and an adiabatic approximation approach. Our results explore whether the orbital angular momentum of an input probe beam or the second control field can be transferred to the generated probe field, and this is verified by numerical simulation. It is interesting that as the intensities of the control fields increase, the propagation of probe beams exhibits oscillation behaviors only when the one-photon detuning is nonzero. Furthermore, we show that the absorption losses are minimized, and the transfer efficiency is enhanced by appropriately modifying the one-photon detuning together with the control-field Rabi frequencies.
We propose a scheme to exchange optical vortices of slow light using the phenomenon of electromagnetically induced transparency in a four-level double-$\mathrm{\ensuremath{\Lambda}}$ atom-light coupling scheme illuminated by a pair of probe fields as well as two control fields of larger intensity. We study the light-matter interaction under the situation where one control field carries an optical vortex, and another control field has no vortex. We show that the orbital angular momentum (OAM) of the vortex control beam can be transferred to a generated probe field through a four-wave-mixing process and without switching on and off of the control fields. Such a mechanism of OAM transfer is much simpler than in a double-tripod scheme in which the exchange of vortices is possible only when two control fields carry optical vortices of opposite helicity. The losses appearing during such OAM exchange are then calculated. It is found that the one-photon detuning plays an important role in minimizing the losses. An approximate analytical expression is obtained for the optimal one-photon detuning for which the losses are minimum while the intensity of generated probe field is maximum. The influence of phase mismatch on the exchange of optical vortices is also investigated. We show that in presence of phase mismatch the exchange of optical vortices can still be efficient.
We investigate theoretically the formation of two-component light with superluminal group velocity in a medium controlled by four Raman pump fields. In such an optical scheme only a particular combination of the probe fields is coupled to the matter and exhibits superluminal propagation; the orthogonal combination is uncoupled. The individual probe fields do not have a definite group velocity in the medium. Calculations demonstrate that this superluminal component experiences an envelope advancement in the medium with respect to the propagation in vacuum.