用于原子磁强计的超表面集成反射型气室设计

Design of a Metasurface⁃Integrated Reflective Vapor Cell for Atomic Magnetometers

原子磁强计以其高灵敏度和成本低等优势受到了越来越多的关注,如今,进一步提高原子磁强计的芯片集成度已成为主要趋势,因为它有利于生物磁性测量与成像。但是,目前实现原子磁强计小型化的主要障碍是微加工原子气室的光学元件分立。鉴于此,笔者提出一种基于新兴超表面的超紧凑片上原子气室方案,该方案将超表面与各向异性腐蚀的单晶硅相结合,在保证高灵敏度的同时提高了原子气室的集成度。该方案能够对圆偏振入射光束进行光路操纵,效率可达到80%。超表面采用厚度为500 nm的硅设计而成,可以通过基本的微加工工艺直接在原子气室上制造。所设计的新型原子气室具有集成度高、可大批量制造的优点,为未来生物磁性传感系统的发展提供了参考。

Objective Magnetic brain data play a crucial role in neurological analysis and monitoring human brain health.Various sensors have been developed to detect magnetic fields resulting from brain activity.These include superconducting quantum interference devices(SQUID),fluxgate magnetometers,giant magnetoresistive sensors,and atomic magnetometers.Recently,micromachined atomic magnetometers have gained significant interest due to their small size,affordability,and superior performance.Central to these magnetometers is the miniaturized alkali metal atomic gas chamber.Chip-scale alkali metal atomic gas chambers present advantages like smaller size,reduced cost,and higher yield compared to their millimeter-scale glass counterparts.However,atomic magnetometers based on chip-scale alkali metal atomic gas chambers face challenges due to the short optical range length.While many solutions have been suggested,achieving a specific light incident angle on the chip remains intricate,and the fabrication consistency is hard to maintain.Thus,integrating optical systems within alkali-metal atomic gas chambers remains a predominant challenge.However,the rise of micro-and nano-photonics,coupled with advancements in nanofabrication,has spurred interest in artificial quasitwo-dimensional electromagnetic material hypersurfaces.These are compatible with modern micro-and nano-fabrication platforms,paving the way for unprecedented miniaturization of optical systems.Since most chip-scale alkali metal atomic gas chambers manufactured using MEMS-based processes involve the triple anodic bonding of glass-silica-glass,on-chip integration of metasurfaces becomes feasible.In this study,a strategy for optical path integration of micromachined alkali metal atomic gas chambers using the supersurface method is presented.This can achieve a deflection angle of 19.48°with over 80%efficiency.The micromachined planar structure of the device allows it to bond directly to the atomic gas chamber’s transparent window.This ensures that the vertically incident light strikes the anisotropically corroded single-crystal silicon sidewall at 19.48°.Consequently,a horizontally incident beam is directed to interact with atoms along a cavity optical path.The supersurface design aligns with nanofabrication platforms,hinting at the potential for large-scale production in the future.Methods In this design,silicon,exhibiting a high refractive index and low loss in the operating band,was utilized as the material for phase gradient generation.Initially,the effect of the radius of the silicon dielectric column on transmittance and phase was analyzed using the finite-domain difference method.The super-surface unit for phase gradient generation was designed based on the established phase diameter relationship and the requirements for the transmittance of the incident light.Subsequently,the scattered light field in the x-z plane under the normal incidence of y-polarized light was examined.Results and Discussions The manipulation of the phase is primarily achieved through changing the radius of the silicon dielectric column.The layout of the anomalous refractive hypersurface is organized based on the results of the phase distribution(Fig.4)using different radii of the dielectric column at a specific column height.The refractive wavefront now propagates along a distinct angle of 19.84°(Fig.6).This indicates that the simulation results closely align with the design expectations.The requirement for the incident laser angle in the atomic gas chamber is met without introducing an additional reflector,simplifying further integration of the atomic gas chamber.Concurrently,the refractive efficiency is observed to be 85%upon normal incidence.However,when x-polarized light is incident,the efficiency drops to 65%,while the refraction angle remains unchanged.The efficiency discrepancy between the different polarizations stems from the distinct spatial alignments of the nanopillars along the two coordinate axes.Conclusions In this paper,a scheme is proposed to integrate an anomalously refractive hypersurface on the surface of an alkali metal atomic gas chamber,the sensitive core of a miniature magnetometer.This integration aims to make incident light strike an anisotropically corroded single-crystal silicon sidewall at a deflection angle of 19.48°and direct the horizontally incident beam to interact with atoms in a cavity optical path oriented along the plane of the substrate.Simulation results indicate that this method achieves a deflection of the circularly polarized pump beam of 19.48°with an efficiency exceeding 80%.The super-surface,designed in silicon with a thickness of 500 nm,is compatible with current micro-and nano-manufacturing platforms,offering potential for mass production.The evolution of high-resolution biomagnetic imaging instruments and portable atomic devices hinges on the miniaturization of magnetometers.The proposed method integrates these magnetometers using a chip approach,significantly reducing their size and setting the stage for future advancements in biomagnetic sensing systems.