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Torque of guided light on an atom near an optical nanofiber

Fam Le Kien, Thomas Busch

Published May 10, 2019• 2 citations
OpenAlex

Abstract

We calculate analytically and numerically the axial orbital and spin torques of guided light on a two-level atom near an optical nanofiber. We show that the generation of these torques is governed by the angular momentum conservation law in the Minkowski formulation. The orbital torque on the atom near the fiber has a contribution from the average recoil of spontaneously emitted photons. Photon angular momentum and atomic spin angular momentum can be converted into atomic orbital angular momentum. The orbital and spin angular momenta of the guided field are not transferred separately to the orbital and spin angular momenta of the atom.

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Torque of guided light on an atom near an optical nanofiber

Fam Le Kien, Thomas Busch

We calculate analytically and numerically the axial orbital and spin torques of guided light on a two-level atom near an optical nanofiber. We show that the generation of these torques is governed by the angular momentum conservation law in the Minkowski formulation. The orbital torque on the atom near the fiber has a contribution from the average recoil of spontaneously emitted photons. Photon angular momentum and atomic spin angular momentum can be converted into atomic orbital angular momentum. The orbital and spin angular momenta of the guided field are not transferred separately to the orbital and spin angular momenta of the atom.

arXiv:1809.03121•Optics Express•May 10, 2019•7 citations

Transfer of angular momentum of guided light to an atom with an electric quadrupole transition near an optical nanofiber

Fam Le Kien, Síle Nic Chormaic, Thomas Busch

We study the transfer of angular momentum of guided photons to a two-level atom with an electric quadrupole transition near an optical nanofiber. We show that the generation of the axial orbital torque of the driving guided field on the atom is governed by the internal-state selection rules for the quadrupole transition and by the angular momentum conservation law with the photon angular momentum given in the Minkowski formulation. We find that the torque depends on the photon angular momentum, the change in the angular momentum of the atomic internal state, and the quadrupole-transition Rabi frequency. We calculate numerically the torques for the quadrupole transitions between the sublevel $M=2$ of the hyperfine-structure level $5{S}_{1/2}F=2$ and the sublevels ${M}^{\ensuremath{'}}=0$, 1, 2, 3, and 4 of the hyperfine-structure level $4{D}_{5/2}{F}^{\ensuremath{'}}=4$ of a $^{87}\mathrm{Rb}$ atom. We show that the absolute value of the torque for the higher-order mode ${\mathrm{HE}}_{21}$ is larger than that of the torque for the fundamental mode ${\mathrm{HE}}_{11}$ except for the case ${M}^{\ensuremath{'}}\ensuremath{-}M=2$, where the torque for the mode ${\mathrm{HE}}_{21}$ is vanishing.

arXiv:2205.12440•Physical Review A•Jul 18, 2022•10 citations

Light-induced force and torque on an atom outside a nanofiber

Fam Le Kien, V. I. Balykin, K. Hakuta

We study the action of the force and torque induced by a guided light field on a cesium atom outside a nanofiber. We demonstrate that the evanescent light field in a circular fundamental guided mode can force the atom to rotate around the nanofiber for a macroscopic time. We find that, due to the action of the torque, the angular momentum of the atom increases with time.

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Linear and nonlinear optical torque in multi-level atomic systems driven by counter-rotating orbital angular momentum fields

Dharma P. Permana, Mažena Mackoit Sinkevičienė, Viačeslav Kudriašov +3 more

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, $+l\hbar$ and $-l\hbar$. We consider a three-level $Λ$ 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.

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Direction-dependent coupling between a nanofiber-guided light field and a two-level atom with an electric quadrupole transition

Fam Le Kien, Síle Nic Chormaic, Thomas Busch

We study the directional dependence of the coupling between a nanofiber-guided light field and a two-level atom with an electric quadrupole transition. We examine the situation where the nanofiber is aligned along the $z$ axis, the atom lies on the fiber transverse $x$ axis, the quantization axis for the atomic internal states is the other orthogonal transverse $y$ axis, the atomic upper and lower levels are the magnetic sublevels ${M}^{\ensuremath{'}}$ and $M$ of hyperfine-structure levels of an alkali-metal atom, and the field is in a quasilinearly polarized fundamental guided mode ${\mathrm{HE}}_{11}$ with the polarization $\ensuremath{\xi}=x$ or $y$. We find that the absolute value of the quadrupole Rabi frequency depends on the propagation direction of the light field in the cases of (${M}^{\ensuremath{'}}\ensuremath{-}M=\ifmmode\pm\else\textpm\fi{}1, \ensuremath{\xi}=y$) and (${M}^{\ensuremath{'}}\ensuremath{-}M=\ifmmode\pm\else\textpm\fi{}2, \ensuremath{\xi}=x$). We show that the directional dependence of the coupling leads to the directional dependence of spontaneous emission into guided modes. We find that the directional dependence of the atom-field coupling in the case of quadrupole transitions is not entirely due to spin-orbit coupling of light: there are some other contributions resulting from the gradient of the spatial phase factor of the field.

arXiv:2211.06000•Physical Review A•Jan 19, 2023•4 citations

Directional nanophotonic atom--waveguide interface based on spin--orbit interaction of light

R. Mitsch, C. Sayrin, B. Albrecht +2 more

Optical waveguides in the form of glass fibers are the backbone of global telecommunication networks. In such optical fibers, the light is guided over long distances by continuous total internal reflection which occurs at the interface between the fiber core with a higher refractive index and the lower index cladding. Although this mechanism ensures that no light escapes from the waveguide, it gives rise to an evanescent field in the cladding. While this field is protected from interacting with the environment in standard optical fibers, it is routinely employed in air- or vacuum-clad fibers in order to efficiently couple light fields to optical components or emitters using, e.g., tapered optical fiber couplers. Remarkably, the strong confinement imposed by the latter can lead to significant coupling of the light's spin and orbital angular momentum. Taking advantage of this effect, we demonstrate the controlled directional spontaneous emission of light by quantum emitters into a sub-wavelength-diameter waveguide. The effect is investigated in a paradigmatic setting, comprising cesium atoms which are located in the vicinity of a vacuum-clad silica nanofiber. We experimentally observe an asymmetry higher than 10:1 in the emission rates into the counterpropagating fundamental guided modes of the nanofiber. Moreover, we demonstrate that this asymmetry can be tailored by state preparation and suitable excitation of the quantum emitters. We expect our results to have important implications for research in nanophotonics and quantum optics and for implementations of integrated optical signal processing in the classical as well as in the quantum regime.

arXiv:1406.0896•Jun 3, 2014•1 citation
We calculate analytically and numerically the axial orbital and spin torques of guided light on a two-level atom near an optical nanofiber. We show that the generation of these torques is governed by the angular momentum conservation law in the Minkowski formulation. The orbital torque on the atom near the fiber has a contribution from the average recoil of spontaneously emitted photons. Photon angular momentum and atomic spin angular momentum can be converted into atomic orbital angular momentum. The orbital and spin angular momenta of the guided field are not transferred separately to the orbital and spin angular momenta of the atom.
We calculate analytically and numerically the axial orbital and spin torques of guided light on a two-level atom near an optical nanofiber. We show that the generation of these torques is governed by the angular momentum conservation law in the Minkowski formulation. The orbital torque on the atom near the fiber has a contribution from the average recoil of spontaneously emitted photons. Photon angular momentum and atomic spin angular momentum can be converted into atomic orbital angular momentum. The orbital and spin angular momenta of the guided field are not transferred separately to the orbital and spin angular momenta of the atom.
We study the transfer of angular momentum of guided photons to a two-level atom with an electric quadrupole transition near an optical nanofiber. We show that the generation of the axial orbital torque of the driving guided field on the atom is governed by the internal-state selection rules for the quadrupole transition and by the angular momentum conservation law with the photon angular momentum given in the Minkowski formulation. We find that the torque depends on the photon angular momentum, the change in the angular momentum of the atomic internal state, and the quadrupole-transition Rabi frequency. We calculate numerically the torques for the quadrupole transitions between the sublevel M=2M=2M=2 of the hyperfine-structure level 5S1/2F=25{S}_{1/2}F=25S1/2​F=2 and the sublevels ${M}^{\ensuremath{'}}=0$, 1, 2, 3, and 4 of the hyperfine-structure level $4{D}_{5/2}{F}^{\ensuremath{'}}=4$ of a 87Rb^{87}\mathrm{Rb}87Rb atom. We show that the absolute value of the torque for the higher-order mode HE21{\mathrm{HE}}_{21}HE21​ is larger than that of the torque for the fundamental mode HE11{\mathrm{HE}}_{11}HE11​ except for the case ${M}^{\ensuremath{'}}\ensuremath{-}M=2$, where the torque for the mode HE21{\mathrm{HE}}_{21}HE21​ is vanishing.
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.
We study the directional dependence of the coupling between a nanofiber-guided light field and a two-level atom with an electric quadrupole transition. We examine the situation where the nanofiber is aligned along the zzz axis, the atom lies on the fiber transverse xxx axis, the quantization axis for the atomic internal states is the other orthogonal transverse yyy axis, the atomic upper and lower levels are the magnetic sublevels ${M}^{\ensuremath{'}}$ and MMM of hyperfine-structure levels of an alkali-metal atom, and the field is in a quasilinearly polarized fundamental guided mode HE11{\mathrm{HE}}_{11}HE11​ with the polarization $\ensuremath{\xi}=x$ or yyy. We find that the absolute value of the quadrupole Rabi frequency depends on the propagation direction of the light field in the cases of (${M}^{\ensuremath{'}}\ensuremath{-}M=\ifmmode\pm\else\textpm\fi{}1, \ensuremath{\xi}=y$) and (${M}^{\ensuremath{'}}\ensuremath{-}M=\ifmmode\pm\else\textpm\fi{}2, \ensuremath{\xi}=x$). We show that the directional dependence of the coupling leads to the directional dependence of spontaneous emission into guided modes. We find that the directional dependence of the atom-field coupling in the case of quadrupole transitions is not entirely due to spin-orbit coupling of light: there are some other contributions resulting from the gradient of the spatial phase factor of the field.
Optical waveguides in the form of glass fibers are the backbone of global telecommunication networks. In such optical fibers, the light is guided over long distances by continuous total internal reflection which occurs at the interface between the fiber core with a higher refractive index and the lower index cladding. Although this mechanism ensures that no light escapes from the waveguide, it gives rise to an evanescent field in the cladding. While this field is protected from interacting with the environment in standard optical fibers, it is routinely employed in air- or vacuum-clad fibers in order to efficiently couple light fields to optical components or emitters using, e.g., tapered optical fiber couplers. Remarkably, the strong confinement imposed by the latter can lead to significant coupling of the light's spin and orbital angular momentum. Taking advantage of this effect, we demonstrate the controlled directional spontaneous emission of light by quantum emitters into a sub-wavelength-diameter waveguide. The effect is investigated in a paradigmatic setting, comprising cesium atoms which are located in the vicinity of a vacuum-clad silica nanofiber. We experimentally observe an asymmetry higher than 10:1 in the emission rates into the counterpropagating fundamental guided modes of the nanofiber. Moreover, we demonstrate that this asymmetry can be tailored by state preparation and suitable excitation of the quantum emitters. We expect our results to have important implications for research in nanophotonics and quantum optics and for implementations of integrated optical signal processing in the classical as well as in the quantum regime.