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Physical Review A

Charge-current generation in atomic systems induced by optical vortices

Koray Köksal, Jamal Berakdar

Published Dec 11, 2012• 42 citations
Publisher

Abstract

We envisage the possibility of producing and tuning electronic orbital currents in atoms by weak, spatially inhomogeneous laser pulses with optical vortices. As already known, when interacting with such pulses an electronic system attains a definite amount of angular momentum. Here we explore this effect in the case when the system size is atomically small, i.e., much smaller than the scale of the inhomogeneity of the light pulses. We show that, nonetheless, angular momentum is transferred to the atomic system. The amount of the attained angular momentum depends on the position of the atom inside the field. Our numerical calculations and analytical analysis show that, indeed, light-induced orbital magnetism emerges in a dilute atomic gas when it is exposed to resonant or broadband optical vortices. This effect can be assessed, for instance, in a Faraday rotation experiment.

DOI
10.1103/physreva.86.063812

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We envisage the possibility of producing and tuning electronic orbital currents in atoms by weak, spatially inhomogeneous laser pulses with optical vortices. As already known, when interacting with such pulses an electronic system attains a definite amount of angular momentum. Here we explore this effect in the case when the system size is atomically small, i.e., much smaller than the scale of the inhomogeneity of the light pulses. We show that, nonetheless, angular momentum is transferred to the atomic system. The amount of the attained angular momentum depends on the position of the atom inside the field. Our numerical calculations and analytical analysis show that, indeed, light-induced orbital magnetism emerges in a dilute atomic gas when it is exposed to resonant or broadband optical vortices. This effect can be assessed, for instance, in a Faraday rotation experiment.
The noise-induced coherence created via the quantum interference of incoherent radiation in atomic three-level systems of V and \ensuremath{\Lambda} types driven by a pair of weak laser pulses is shown to result in exchange of optical vortices. In the three- level V-type atom-light coupling the system is populated in its ground level, while in the \ensuremath{\Lambda} model the system is initially prepared in an electromagnetically induced transparency state. By solving the quantum optical Maxwell-Bloch equations and with quantum interference of incoherent radiation present, we show that the orbital angular momentum (OAM) of the vortex probe beam can be transferred to a generated signal field. While the exchange efficiency in the V configuration is higher, the losses are less in the \ensuremath{\Lambda} scheme when we consider such a noise-induced coherence. We further discuss the effects of phase mismatching and inhomogeneous broadening on the energy conversion between light beams carrying OAM.
Photons can carry angular momentum, not only due to their spin, but also due to their spatial structure. This extra twist has been used, for example, to drive circular motion of microscopic particles in optical tweezers as well as to create vortices in quantum gases. Here we excite an atomic transition with a vortex laser beam and demonstrate the transfer of optical orbital angular momentum to the valence electron of a single trapped ion. We observe strongly modified selection rules showing that an atom can absorb two quanta of angular momentum from a single photon: one from the spin and another from the spatial structure of the beam. Furthermore, we show that parasitic ac-Stark shifts from off-resonant transitions are suppressed in the dark centre of vortex beams. These results show how light's spatial structure can determine the characteristics of light-matter interaction and pave the way for its application and observation in other systems.
Photons carry one unit of angular momentum associated with their spin~\cite{Beth1936}. Structured vortex beams carry additional orbital angular momentum which can also be transferred to matter~\cite{Allen1992}. This extra twist has been used for example to drive motion of microscopic particles in optical tweezers as well as to create vortices in degenerate quantum gases~\cite{He1995,Andersen2006}. Here we demonstrate the transfer of optical orbital angular momentum from the transverse spatial structure of the beam to the internal (electronic) degrees of freedom of an atom. Probing a quadrupole transition of a single trapped 40^{40}40Ca+^++ ion localized at the center of the vortex, we observe strongly modified selection rules, accounting for both the photon spin and the vorticity of the field. In particular, we show that an atom can absorb two quanta of angular momentum from a single photon even when rotational symmetry is conserved. In contrast to previous findings~\cite{Araoka2005,Loeffler2011a,Mathevet2013}, our experiment allows for conditions where the vorticity of the laser beam determines the optical excitation, contributing to the long-standing discussion on whether the orbital angular momentum of photons can be transferred to atomic internal degrees of freedom~\cite{VanEnk1994,Babiker2002,Jauregui2004, Schmiegelow2012, Mondal2014, Scholz-Marggraf2014} and paves the way for its use to tailor light-matter interactions.
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 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.