Single-Atom Trapping in a Metasurface-Lens Optical Tweezer

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Single-Atom Trapping in a Metasurface-Lens Optical Tweezer

T.-W. Hsu, W. Zhu, T. Thiele, M. O. Brown, S. B. Papp, A. Agrawal, and C. A. Regal

PRX Quantum 3, 030316 – Published 1 August 2022

Abstract

Optical metasurfaces of subwavelength pillars have provided new capabilities for the versatile definition of the amplitude, phase, and polarization of light. In this work, we demonstrate that an efficient dielectric metasurface lens can be used to trap and image single neutral atoms with a long working distance from the lens of 3 mm. We characterize the high-numerical-aperture optical tweezers using the trapped atoms and compare with numerical computations of the metasurface-lens performance. We predict that future metasurfaces for atom trapping will be able to leverage multiple ongoing developments in metasurface design and enable multifunctional control in complex quantum information experiments with neutral-atom arrays.

Received 9 February 2022
Revised 6 May 2022
Accepted 23 June 2022

DOI:https://doi.org/10.1103/PRXQuantum.3.030316

Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.

Published by the American Physical Society

Physics Subject Headings (PhySH)

Metamaterials Optical lattices & traps

Atomic, Molecular & OpticalQuantum Information, Science & Technology

Authors & Affiliations

T.-W. Hsu^1,2, W. Zhu ³, T. Thiele^1,2, M. O. Brown^1,2, S. B. Papp⁴, A. Agrawal ³, and C. A. Regal ^1,2,*

¹JILA, National Institute of Standards and Technology and University of Colorado, Boulder, Colorado 80309, USA
²Department of Physics, University of Colorado, Boulder, Colorado 80309, USA
³National Institute of Standards and Technology, Gaithersburg, Maryland 20899, USA
⁴National Institute of Standards and Technology, Boulder, Colorado 80305, USA

^*regal@colorado.edu

Popular Summary

Single neutral atoms in optical tweezers are an important platform for quantum simulation, computing, and metrology. With ground-up control, individual atoms can be cooled, trapped, and entangled. Control of single neutral atoms relies heavily on optical potentials for trapping, either in lattices or arrays of tightly focused laser beams, termed optical tweezers. In optical tweezers, high-numerical-aperture (high-NA) optics are key for both creating trapping potentials and imaging the fluorescence of single atoms. However, the large conventional objective lenses are difficult to incorporate into increasingly complex vacuum chambers and multifunctional photonic systems. Recent advances in patterned low-loss dielectric metasurfaces have defined a new paradigm for optical design and offer an intriguing solution to atom-trapping challenges.

Here, we introduce the use of a transmission-mode high-NA dielectric metasurface lens to trap and image single atoms. We form an atom array by combining the metasurface lens with tunable acousto-optic deflectors and characterize the tweezer foci using the trapped atoms. We demonstrate trapping of single atoms with a measured NA of 0.55. We also carry out a full numerical simulation of the expected metalens properties using the finite-difference time-domain method and find that the result agrees with the measurement from trapped single atoms.

We expect that future optimized photonic metasurfaces that leverage ongoing advances in element design libraries and in multilayer design will be an important frontier for advancing quantum information science with neutral atoms.

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Vol. 3, Iss. 3 — August - October 2022

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Images

Figure 1
Metasurface optics for optical-tweezer trapping. (a) A scanning electron micrograph of the fabricated metasurface lens containing a periodic array (lattice constant = 280 nm) of amorphous- $Si$ (a- $Si$ ) nanopillars (height 660 nm) of width ranging from 85 nm to 185 nm (dark blue) on top of a 500- $μ m$ -thick fused-silica substrate (light blue). The inset shows the varying nanopillar width to achieve the desired phase shift [see Fig. 4]. (b) A notional illustration of the metasurface-lens operation, showing light propagation (pink), wave fronts (dashed lines), and the secondary wavelets (black semicircles) reemitted by the nanopillars that interfere to create the focusing wave front. (c) The optical setup for trapping (pink) and fluorescence imaging (green) of single atoms in an array created with multiple input beams generated using a two-axis acousto-optic deflectors. (d) An image of a trapped $^{87}$ $Rb$ array created by averaging over multiple experiment iterations (100 in this case) with approximately $52 %$ probability of a single atom in each trap per image. The variation in the averaged intensity is caused by trap depth and shape variations that affect the relative loading probability and imaging signal in the array.
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Figure 2
Single-atom trapping in a metalens optical tweezer. (a) A camera-count histogram indicating the presence of either zero or one atoms in a tweezer trap. A threshold is chosen to determine if an atom is trapped and to calculate the loading efficiency. (b) A typical single-shot fluorescence image of a single atom imaged through the metalens with PGC imaging. (c) A typical light-shift measurement with a Gaussian fit (red line) to the shifted atomic resonance. The dashed line corresponds to the free-space $^{87}$ $Rb$ $D_{2} F = 2$ to $F^{'} = 3$ transition. (d) A typical parametric heating measurement with a Gaussian fit (red line) to extract the trap frequency ( $ν_{trap}$ ) from the modulation resonance ( $2 ν_{trap}$ ). Each point is an average of 100 trap-loading sequences. (e) The measured trap frequency versus the trap depth (light shift) obtained from multiple measurements similar to (c) and (d). The solid red line is a model fit (see main text) to extract the effective Gaussian tweezer waist seen by the trapped atom. (f) The peak-normalized cross section of the intensity transmission through a 300-nm-diameter pinhole imaged at the 852-nm trapping wavelength by the metalens. The solid blue line is an Airy function fitted to the data to extract the spot size and effective NA. [The error bars in (c) and (d) represent the standard deviation and the error bars in (e) are the standard error of the fitted Gaussian centers.]
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Figure 3
The atom array and the metalens FOV. (a) A PGC fluorescence image of an atom array trapped with metasurface optical tweezers. The image is averaged over 100 experimental cycles. The bottom-left tweezer is on the optical axis of the metalens. The off-axis tweezer sites typically have a lower loading probability and nonoptimal PGC imaging detuning, resulting in a dimmer single-atom signal. (b) An example of typical trap-frequency measurement data at approximately $13.6 μ m$ from the FOV center where asymmetric aberrations in the trap are present, along with a double Gaussian (green line) fit. (c) The extracted Gaussian waist as determined from the atom trapping as a function of the distance ( $r$ ) to the metalens optical axis (the center of the FOV). The average waist is extracted from a single Gaussian fit to the trap-frequency data (red) and the major waist (green circle) and minor waist (green diamond) are extracted from data similar to (b) when the two trap frequencies are distinguishable. We compare with the theory of the major and minor Gaussian waists fitted from FDTD simulation [see Fig. 4]. [The error bars in (b) represent the standard deviation and the error bars in (c) are the standard error.]
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Figure 4
The metalens design and simulations. (a) The transmittance $t$ and phase shift $ϕ$ as a function of the nanopillar side length $L$ . (b) The calculated deflection efficiency $η_{1}$ , the fraction of light in the undeflected zeroth-order $η_{0}$ , and the reflectance $η_{refl}$ of aperiodic metasurface beam deflectors as a function of the deflection angle $θ_{D}$ . The circles are data from RCWA simulations and the solid lines are parabolic fits. (c) The FDTD simulated beam profiles of the focal spots as a function of the angle of the incident light.
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Figure 5
The metalens in-vacuum mounting and tweezer alignment. (a) A photograph of the metalens sample optically contacted onto the wedged fused-silica sample holder that is epoxied onto the AR-coated glass cell. (b) The fabricated metalens sample with a NA of 0.55 designed for 852-nm tweezer light. (c) A schematic illustrating how the tweezer light and the science CCD camera are aligned to the metalens sample via substrate back reflection. (d) A schematic illustration of the optical-tweezer imaging path with lenses that compensate for out-of-focus imaging due to the chromatic focal shift introduced by the metalens. The insets are the ray-tracing simulation of the imaging system (object on axis and $10 μ m$ off axis) assuming that the metalens only has a chromatic focal shift and no other aberration. The result shows that L1 and L2 do not introduce additional aberrations. The black circle is the diffraction-limited Airy disk. (e) The end view of the vacuum cell, showing the probe beam (also the resonant heating beam) orientation in relation to the metalens sample and the tweezer beam. The probe beam is 1 mm in diameter, shines vertically up, and is 3 mm away from the metalens, overlapping with the optical-tweezer focus.
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Figure 6
The polarization-multiplexed metalens. (a) An a- $Si$ nanopillar (dark blue) with a rectangular cross section on a fused-silica substrate (light blue) creates a phase delay for two orthogonal polarizations independently (polarization multiplexing). (b) An illustration of the polarization-multiplexed metasurface-lens operation. The input wave front (pink dash) with orthogonal polarization for 780 nm (green) and 852 nm (blue) propagates and interacts with the metasurface. Secondary wavelets (black semicircles) reemitted by the nanopillars interfere and create identical focusing wave fronts for both 780 nm and 852 nm. (c),(d) The experimental PSF focused by the metalens for 780-nm $x$ -polarized and 852-nm $y$ -polarized input light, imaged with an 0.95-NA microscope objective without changing the focus. Two-dimensional cuts of the PSF show the fitted Airy function from which the NA is extracted.
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