基于色散型超构表面的集成光谱成像

Integrated Spectral Imaging Based on Spectroscopic Metasurface

为解决传统高光谱成像系统以光程换取分辨率而导致的光路复杂、成本高昂、体积庞大等问题,提出了基于色散型超构表面的光谱成像系统。超构表面中的每个超构色散单元,都相当于一个小型的光谱仪,按照阵列排布实现快照式光谱成像。采用遗传算法对得到的图像进行光谱反演,所得拟合结果准确。最终在400~700 nm的可见光波段实现了11个波长通道的光谱成像系统。本设计的紧凑结构、偏振不依赖特性和高光谱分辨率有利于各种小型化终端设备的集成,对光谱仪的实际应用具有重要意义。

Objective Traditional spectral imaging systems rely on bulky optical components to achieve high spectral resolution,which pose challenges for miniaturization and portability.However,as a novel subwavelength artificial optical platform,a metasurface of7fers multi-degree-of-freedom control over light,including amplitude,phase,polarization,wavelength,and orbital angular momentum.Consequently,metasurfaces have emerged as an impressive advancement in the optical field leading to the development of numerous optical systems based on metasurfaces.These systems have found applications in various optical fields,such as imaging,holography,optical encryption,and quantum information.The planar structure of the metasurface enables systems to be small and light,providing a new solution to issues commonly encountered in traditional spectral imaging systems,such as large size,complex structure,limited functionality,and high cost.Methods We utilized TiO2 to design the metasurface structure and successfully implemented a spectroscopic metasurface using the transmission phase principle.Our spectral imaging system comprised 10 channels,each offering a spectral resolution of 30 nm.Alongside the spectroscopic metasurface as the primary component,the system relied on a genetic algorithm to rebuild the spectral intensity.The chosen meta-atom structure utilized TiO2 as the medium due to its widespread availability in the visible-light band,specifically in the range of 400‒700 nm,where it exhibits high transmittance.In addition,the optical loss was minimal,and the relevant metasurface fabrication technology has been fully developed.The meta-atom structure was arranged periodically,and the transmission phase was tailored by adjusting the diameter of the meta-atoms according to the phase theory.Subsequently,the metasurface phase was carefully controlled,and phase compensation corresponding to different positions was implemented to achieve the dispersion function of the metasurface.Our design ultimately chose a square shape as the substrate for the unit structure and a cylindrical shape with spatial symmetry for the meta-atoms.During the design process,increasing the height of the meta-atoms augmented the corresponding phase change,albeit at the expense of increased fabrication complexity.To realize a sufficient phase change while simplifying processing,striking a balance between the height of the meta-atoms and the desired phase became crucial.Due to the constraints of current fabrication technology,we opted for a uniform height for all the meta-atoms within the metasurface.Utilizing the finitedifference time-domain(FDTD)simulation method,we designed the geometric parameters of the meta-atoms,allowing us to obtain the transmittances and phases of the elemental structure with varying geometric parameters.Subsequently,we compiled a database incorporating data such as phase and transmittance.Results and Discussions The fixed meta-atom structure has a height(H)of 1000 nm and a period(p)of 220 nm.By maintaining H and p constants while varying the radius of the unit cell,we obtain corresponding phase distributions and transmittances for different radii.The nanopillar radius ranges from 80 to 180 nm,ensuring comprehensive 2πphase coverage and high transmission(Fig.2).A numerical simulation method is utilized to simulate the single channel,as depicted in Fig.3.Through a ray-tracing software simulation,we project the input image onto the position of the metasurface.To replace the designed metasurface,we opt for a combination of a grating and a mask layer.The spectral system dispersion results in a 12μm length span across the bandwidth in the range of 400‒700 nm.Additionally,we obtain the full spectral images by simulation(Fig.4).In conclusion,a genetic algorithm is used to determine the light intensity coefficient of each channel to reconstruct the normalized spectrum(Fig.5).The fitting results demonstrate the practical applicability of the proposed spectral imaging system.Conclusions We designed a spectroscopic metasurface utilizing the transmission phase principle,employing TiO2 as the material.The working wavelength is selected in the range of 400‒700 nm,and a dispersion-type spectral imaging system is established.The FDTD method is utilized to optimize the meta-atom diameter and establish the parameter values along with the corresponding arrangement of the metasurface.Subsequently,11 spectral channels are selected as outputs within the visible range of 400‒700 nm.To validate the performance of the spectral imaging system,we conducted full-image simulations using ray-tracing software.During the spectral reconstruction stage,we used a genetic algorithm,optimized over 30 generations,to obtain the normalized spectra.Through inversion on multiple multispectral datasets,all results exhibit perfect alignment with real spectra,achieving a spectral resolution of 30 nm.In summary,the integration of a spectroscopic metasurface into a chargecoupled device camera directly forms a spectral imaging system immune to the polarization state of incident light.This enhancement significantly broadens the applicability of the system.Furthermore,a high spectral resolution can be achieved by introducing different metasurface parameters.While our simulation verification is limited to the visible range,the design principles and methods of this system can be extrapolated to other bands,such as the near-infrared region.