Light-induced, fictitious magnetic trapping of cold alkali atoms using an optical tweezers-nanofiber hybrid platform

TL;DR

This work addresses trapping neutral atoms near optical nanofibers with tunable atom–surface distances by integrating optical tweezers (Gaussian or LG) with the evanescent ONF field in a hybrid OPTON platform. The method relies on light-induced fictitious magnetic fields from both the ONF evanescent field and the circularly polarized tweezers to create a magnetic potential, supplemented by scalar AC Stark and van der Waals terms, and stabilized by a bias field. Quantitative analysis for $^{87}$Rb shows trap minima ~100–400 nm from the fiber with depths 0.3–0.8 mK; LG tweezers can yield deeper traps than Gaussian ones at the same ONF power, while both offer μs-scale tunability of trap position. This platform enables flexible, in situ control of atom–fiber coupling for scalable quantum interfaces, with practical advantages over direct tweezers illumination of the ONF and compatibility with MOT loading and potential ring geometries around the fiber.

Abstract

We present a magnetic trapping scheme for cold 87Rb atoms based on light-induced fictitious magnetic fields generated by the evanescent field of an optical nanofiber (ONF) integrated with an optical tweezers. We calculate and compare the trapping potentials for both Gaussian and Laguerre-Gaussian modes of the tweezers beam, combined with a quasi-linearly polarized ONF-guided field. Based on the optical powers in the tweezers and ONF modes, we analyze the trap depths and the positions of the potential minima from the nanofiber surface. We show that, by varying the optical powers in the two fields, the trap position can be tuned over several hundred nanometers, while simultaneously influencing the trap depth and trap frequencies. Such control over atom-surface position is essential for studying distance-dependent effects on atoms trapped near a dielectric surface and optimizing atom-photon interfaces for quantum technology applications.

Figures (11)

• Figure 1: Vector field of the light-induced fictitious magnetic field (blue arrows) for an atom in the $\mathrm{\left|5P_{1/2}, F=2, m_F=2\right>}$ state in the $xy-$plane perpendicular to the fiber for (a) a quasi-linearly polarized along the $x$--axis fundamental ONF guided mode, (b) a circularly polarized Gaussian mode, and (c) an LG$_{01}$ mode tweezers. The fiber radius is $a=175$ nm and the free-space wavelength of the ONF guided mode is $\lambda_{\mathrm{ONF}}=787.9$ nm. The waist, $w_0$, is 500 nm, the distance between the center of the beam and the ONF surface is 825 nm (1325 nm) for the Gaussian (LG) mode, the power of the tweezers beam is 0.3 mW (regardless of mode chosen), and the free-space wavelength is $\lambda_{\mathrm{tw}}=790.2$ nm. One can see that the LG mode tweezers beam produces a larger light-induced magnetic field closer to the fiber surface. In addition, for both the LG and Gaussian modes, an $x-$component of the fictitious magnetic field is present due to the non-paraxial regime.
• Figure 2: (a) Schematic of the experimental setup. A light beam (intense red arrow) with a free-space wavelength $\lambda_{\mathrm{ONF}}$ is coupled into the optical nanofiber (ONF), exciting the fundamental, quasi-linearly polarized guided mode ($\vec{E}$). An optical tweezers beam (faint red), of either a Gaussian or Laguerre--Gaussian mode, is focused at a distance $d = 825~\mathrm{nm}$ or $d = 1325~\mathrm{nm}$ from the fiber surface, respectively. The beam waist is $w_0 = 500~\mathrm{nm}$ and the tweezers wavelength is $\lambda_{\mathrm{tw}}$. A single atom (blue sphere) is trapped between the ONF surface and the tweezers focus beam due to the combined potentials of the evanescent field and the focused beam. The propagation direction and polarization of the tweezers are indicated by $\vec{k}$ and $\sigma^+$, respectively. The inset shows the $xz$-plane at $y = 0$. (b,c) Calculated intensity profiles $I(x,z)$ of the Gaussian and Laguerre--Gaussian modes with $0.3~\mathrm{mW}$ of power in the $xz$--plane at $y = 0$.
• Figure 3: Two-dimensional plots of the amplitude of the light-induced, fictitious magnetic field generated by a Gaussian optical tweezers beam with power $P = 0.3~\mathrm{mW}$ in the nonparaxial regime for (a) the $y$--component and (b) the $x(z)$--component. The fiber center is located at $x=0~( \mathrm{or}~(x-r_f)=-175)$ .
• Figure 4: Two-dimensional plots of the light-induced, fictitious magnetic field trapping potential for a $^{87}$Rb atom in the $\mathrm{\left|5S_{1/2}, F=2, m_F=2\right>}$ ground state formed by $P_{\mathrm{ONF}}=1$ mW and $P_{\mathrm{tw}}=0.3$ mW: (a) $U_\mathrm{tot}(x,y,z=z_\mathrm{min})$ and (b) $U_\mathrm{tot}(x,\phi=\phi_\mathrm{min},z)$. The potential minimum is formed $\sim 220$ nm from the fiber surface and the trap depth is $\sim 0.3$ mK. The small tilt in the trap position from the $y=0$ line is due to the $x$-component of the fictitious magnetic field, see Fig \ref{['fig:vector field']}. Here, $\phi=\phi_\mathrm{min}$ and $z=z_\mathrm{min}$ are the coordinates of the trap minimum. The free-space wavelengths are $\lambda_\mathrm{ONF}=787.9$ nm and $\lambda_\mathrm{tw}=790.2$ nm for the fiber-guided mode and tweezers mode, respectively. The fiber radius is $r_f=175$ nm. Each plot has an offset so that the lowest point represents the trap depth, defined as the minimum potential barrier of the three-dimensional potential, $U_{\mathrm{tot}}(x,y,z)$. Unless otherwise stated, all configuration parameters, except the optical powers, are kept constant in subsequent figures to allow direct comparison between trapping configurations.
• Figure 5: Calculated one-dimensional plots of the total trapping potential for a $^{87}$Rb atom in the $\mathrm{\left|5S_{1/2}, F=2, m_F=2\right>}$ ground state formed by $P_{\mathrm{ONF}}=1$ mW and $P_{\mathrm{tw}}=0.3$ mW in the (a) radial, $U_\mathrm{tot}(x,y=0,z=0)$, (b) azimuthal, $U_\mathrm{tot}(r=r_0(\Phi'),\Phi',z=0)$, and (c) longitudinal, $\mathrm{U_{tot}(r=r_0(z),y=0,z)}$, directions . Here, $r_0(\Phi')$ and $r_0(z)$ are the distances from the fiber surface to the trap minimum for fixed values of $\Phi'$ and $z$ respectively, see Fig. \ref{['fig:2D_traps_Gauss']}(a,b). $\Phi'$ is the adjusted azimuthal component along the trapping potential in the $xy-$plane at the trap minimum.