Metasurface-enabled strontium atomic clock integrated optical architecture

Recently, researchers such as Vladimir Aksyuk of the National Institute of Standards and Technology used flip-chip bonding technology to combine integrated photonics technology and metasurface optics to demonstrate an integrated optical architecture for realizing a compact strontium atomic clock. Through the design of experiments, the researchers demonstrated that the integrated photonic stage can scale to any number of beams, each with different wavelengths, geometries, and polarizations.


Optical systems are an important part of atomic vapor, ion trap and neutral atom technology. To solve the problem of optical conversion in atomic systems, precise control of the wavelength, power, and polarization of coherent free-space light is required. This control is easy to achieve in a lab-scale setting, but becomes more challenging as optical systems shrink and commercialize. Miniaturized optical systems can be built using a combination of compact block or planar optics, while photonic integrated circuits (PICs) can provide a scalable way to fabricate atomic technology.

Photonic ICs enable production-scale integration of optical components, which can range from laser sources and modulators to on-chip detectors. Diffraction gratings can be integrated on the chip to generate a free-space beam from the guidance mode and are used to process out-of-plane atomic systems. Over the years, grating technology has evolved to enable multi-wavelength control of light, polarization control, and the generation of beams with large numerical apertures and large pattern expansion. However, there are challenges in bringing together these disparate capabilities on a single platform to achieve efficient arbitrary beam control.

For example, optical lattice clocks achieve state-of-the-art frequency instability and ultra-high accuracy, but require complex in vitro optical combinations to produce many laser beams and wavelengths for clock-referenced atomic samples.

Vladimir Aksyuk et al. of the National Institute of Standards and Technology designed and fabricated an integrated photonics package for the miniaturization of strontium atomic clocks. Based on a bonded planar stage, the optical metasurface is combined with a grating output to produce a beam with high numerical aperture, arbitrary tilt angle, defined polarization, and parallel propagation. This planar platform provides new ideas for the generation of on-chip beams and represents an important step towards the realization of atomic technology for fabricable photonic integrated circuits. The researchers demonstrated the device’s ability to simultaneously produce beams of various sizes, polarizations, and wavelengths, opening up a whole new avenue for reducing the size of atomic technology.

Innovative research

The researchers demonstrated a compact photonic chip system that produced 12 circularly polarized beams with a diameter of 10 millimeters, which were designed to form magneto-optical traps in a small volume in a vacuum chamber containing strontium vapor. In addition, the researchers demonstrated the collision combination of two separate waveguide beams to produce an optical lattice aligned with the clock-converted probe beam. The combined lattice and clock beams are pointed vertically within a range of 0.1°. Figure 1a is a schematic diagram of the magneto-optical trap installation, using an unconventional beam arrangement to obtain three-dimensional cooling and capture in a compact planar geometry. Figure 1b shows all 12 beams produced to form a magneto-optical trap device superimposed on a photonic IC chip picture, each with an optical fiber and waveguide, so that the power of each beam can be tuned independently. Figure 1c shows the use of a compact emitter combining measured lattice (green) and clock (red) beams that are generated separately on the surface of the photonic integrated circuit and overlap at the MS location.

Figure 1 Schematic diagram of magneto-optical trap device and the generated beam

Figure 2 shows a summary of the performance of blue and red magnetooptical trap beams. Figure 2a is an image of the blue beam and Figure 2b is an image of the red beam, showing the combined power curve projected horizontally and vertically along with the Gaussian fit. The beam performance characteristics after bonding the MS chip are shown in Figure 2c-2f, which is an image of a red and a blue beam taken at different heights with three visible diffraction orders of red light. Figure 2d is a map of 12 beams measured above MS, each overlaid with the polarization of the light field measured in the direction of beam propagation. Figure 2e depicts the average radial position of the red and blue beams at different heights, and Figure 2f shows the measured cross-section of the red and blue beams.

Figure 2 Summary of beam performance of blue and red magnetic traps

This work employs a scalable approach to the photonics technique for fabricating miniature strontium atomic clocks. This integrated design provides a way to implement increasingly complex optical systems. Future work focuses on combining photonics packaging with vacuum chambers for physics experiments to make photonic IC designs compatible with foundry-scale lithography techniques.

The article, published in the journal Light: Science & Applications, titled “Integrating planar photonics for multi-beam generation and atomic clock packaging on chip,” was first author by Chad Ropp and Vladimir Aksyuk as corresponding author. (Source: LightScience Applications WeChat public account)

Related paper information:‍-023-0‍1081-x

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