Sensing technology based on Rydberg atoms overcomes the physical limitations of traditional electromagnetic sensing systems and offers many advantages such as small size, high sensitivity, and a broad measurement frequency range. The system typically requires two beams of light, with different wavelengths (such as 510 nm and 852 nm), transmitted in opposite directions to excite the atoms. Using optical fibers, rather than space optical links, to collimate and control dual-wavelength beams for constructing optical fiber-integrated atomic antenna probes is an effective method for practical application. However, collimated coupling elements are large and prone to scattering, which causes serious dispersion effects on dual-wavelength light with significant wavelength differences. In this study, a collimation metasurface with wavelengths of 510 nm and 852 nm was designed based on the working principle of achromatic metalenses. The simulation results indicate that the structure can achieve high efficiency and confocal collimation within the bandwidth of 500?1200 nm, which can enhance the coupling efficiency and level of miniaturization, thus promoting the practical development of portable atomic sensing probes.

The proposed structure is investigated using COMSOL Multiphysics software. Perfectly matched layers (PMLs) are employed along the incident direction to eliminate boundary scattering, and periodic boundary conditions (PBCs) are applied to the lateral boundaries of the unit cell. A cross-shaped dielectric column is selected as the phase-control unit structure to ensure effective total polarization control. Compared with conventional dielectric column structures, such as circular, elliptical, and rectangular, the cross-shaped structure offers more structural parameter variables, which can achieve a wider range of phase adjustment functions, and a single structure can meet the 0-π transmission phase requirements. The substrate is SiO₂ with a refractive index of 1.47, and the dielectric column material is Si₃N₄ with a refractive index of approximately 2 at wavelengths of 510 nm and 852 nm. The period of the designed metasurface unit is 250 nm, with a height of 1300 nm, cross-arm length of 250 nm, and width of 90 nm.

The dual-wavelength collimated metasurface structure was simulated. The focus for both wavelengths is set at F=20.0 μm, and the number of unit cells is N=21 in the x-direction. The distribution of the focused electric field is shown in Fig. 5(a). Theoretically, the beams of both wavelengths should focus at F=20.0 μm after passing through the metasurface. However, in the actual structure, due to a certain deviation between the simulated and theoretical phase values, the focal points for the wavelengths are at F=20.2 μm and F=17.5 μm, respectively. The focus of the 510 nm wavelength beam is almost the same as the theoretical value, while the focus of the 852 nm wavelength beam deviates by 2.5 μm. This deviation occurs because the optical aperture for the 852 nm wavelength beam is smaller than that for the 510 nm wavelength beam under the same metasurface size; hence, the focus deviation of the focusing field is larger, and its half-height full width becomes wider. The focusing error can be reduced by increasing the size of the optical aperture. Increasing the number of x-direction elements to N=27 and N=33 showed that the focus deviations decrease with an increase in the optical aperture, as illustrated in Figs. 6(a) and 6(b).

To address the issues of low efficiency and large volume in the dual-wavelength laser collimation module of a fiber-integrated Rydberg atomic electromagnetic sensing system, a collimated metasurface suitable for wavelengths of 510 nm and 852 nm was designed. The phase conditions for dual-wavelength confocal collimation were calculated using the metalens analysis method, and the values for dual-wavelength laser dispersion compensation were determined. A cross-shaped dielectric metasurface element was designed using a dielectric waveguide structure sensitive to geometric parameters. By varying the width of the cross-shaped structure from 150 nm to 30 nm, the transmission phase could cover 0 to π, and the average transmission amplitude exceeded 90%, meeting the design requirements for dual-wavelength arbitrary focal length collimating lenses. The average phase deviation of the designed metasurface was less than 10°, and the focal length deviation was less than 6%. The metasurface structure designed in this study supports highly sensitive and miniaturized Rydberg atomic electromagnetic sensing systems.