A computed tomography spectrometer based on metasurface diffraction elements

Abstract

The present invention relates to a computed tomography spectrometer based on a metasurface diffraction element. The computed tomography spectrometer includes an optical system structure, a focal plane detector, and a data acquisition and processing system; the optical system structure includes a converging lens group, a field stop, a collimating lens group, a metasurface holographic grating, and an imaging lens group. The present invention combines a metasurface with a computed tomography spectrometer, which can avoid the manufacturing difficulties of the computed tomography spectrometer, miniaturize the computed tomography spectrometer, and produce a better diffraction effect, so that the focal plane array target surface and the overall size of the spectrometer are both in the millimeter range, and can realize an imaging spectrometer with a mobile phone terminal or even smaller structure.

Description

Technical Field

[0001] The present invention relates to the field of snapshot spectral imaging, and in particular to a computed tomography spectrometer that utilizes a two-dimensional periodic metasurface diffraction element to simultaneously capture multiple spatially-spectrally encoded diffraction orders in a single exposure. Background Art

[0002] Computed tomography spectroscopy, also known as computed tomography, reconstructs high-dimensional targets from low-dimensional projection data. Based on the principles of computed tomography, it uses optical means to detect the three-dimensional information of a target image. An imaging system first records projections of a data cube in different directions, and then uses a reconstruction algorithm to reconstruct the three-dimensional data cube. Currently, there is considerable research and significant progress in the reconstruction of spectral data cubes and target recognition using computed tomography spectrometers.

[0003] However, the holographic grating, a key component of a computed tomography spectrometer, requires electron beam lithography (EBLI), a complex process and high manufacturing cost. Furthermore, because the minimum step size of a conventional holographic grating is on the order of micrometers, the computed tomography spectrometer is relatively large. Therefore, miniaturization of the computed tomography spectrometer is also a challenge that needs to be addressed.

[0004] The manufacturing process for metasurfaces is relatively mature and compatible with traditional semiconductor processes, offering the advantage of ease of manufacture. Furthermore, the minimum step diameter of a metasurface is small, on the nanometer scale, making it lightweight and thin, making it a promising component for the manufacture of micro-optical devices. Metasurfaces composed of densely arranged nanoscale antennas can be used to manufacture gratings, lenses, prisms, and the like, thereby controlling phase, polarization, intensity, frequency, and orbital angular momentum. Through the adjustment of nanoantennas, dielectric metasurfaces exhibit excellent performance in frequency mixing, optical switching, efficiency, and high absorption. Metasurfaces are also being used to achieve structured light projection, uniformly distributing collimated light beams into arrays of uniformly intense light spots, and metasurfaces have also made progress in holographic imaging. Therefore, all-dielectric metasurface materials can address the manufacturing difficulties of the core components of computed tomography spectrometers and enable the miniaturization of computed tomography spectrometers. Summary of the Invention

[0005] The purpose of the present invention is to realize the miniaturization design of a snapshot computed tomography spectrometer without moving parts by using a two-dimensional periodic metasurface diffraction element instead of a traditional holographic grating.

[0006] To achieve the above objectives, the present invention provides a computed tomography spectrometer based on a metasurface diffraction element, the spectrometer comprising an optical system structure, a focal plane detector, and a data acquisition and processing system. The optical system structure comprises a converging lens group, a field stop, a collimating lens group, a metasurface holographic grating, and an imaging lens group, all fixedly arranged in sequence along the optical axis. The entire system has no mechanical moving parts. The metasurface holographic grating is a two-dimensional periodic structure capable of simultaneously generating multiple spatially-spectrally encoded diffraction orders in a single exposure. The operating process is as follows: the converging lens group converges the incident light carrying the target data cube information, limits the size of the beam entering the system at the field stop, collects and collimates the light, and then enters the metasurface holographic grating. After the metasurface holographic grating splits and disperses the pattern, the imaging lens group focuses the dispersed pattern to form different diffraction orders. The focal plane detector simultaneously receives each diffraction pattern, each diffraction pattern corresponding to the projection data in the diffraction direction. The data acquisition and processing system collects each diffraction pattern and restores the target data cube through an algorithm.

[0007] The key component of the computed tomography spectrometer, the metasurface holographic grating, consists of a substrate with a thickness of h2 and a height of h1 and a diameter of D. i The surface unit structure array is composed of a substrate having a surface extending in the x and y directions, and the surface unit structure is a cylinder perpendicular to the substrate.

[0008] In some embodiments, the surface unit structure array is arranged in a matrix and consists of one or more large periods, and the large periods are arranged in a matrix in the x and y directions of the substrate.

[0009] In some embodiments, each large period of the surface unit structure is composed of a plurality of surface unit structures having different diameters but the same height. The metasurface holographic grating controls the number of diffraction orders and the shape of the diffraction pattern on the focal plane detector by changing the diameter of the surface unit structures at different positions.

[0010] The metasurface holographic grating is a nanostructure array, and the nanostructure is a polarization-independent structure.

[0011] In some embodiments, the metasurface holographic grating substrate is silicon dioxide, and the surface unit structure is a subwavelength metal or a high refractive index medium.

[0012] In some embodiments, the imaging lens assembly is a zoom lens to change the size of the image on the focal plane detector.

[0013] In some embodiments, the converging lens group, the collimating lens group, and the imaging lens group are composed of one or more lenses.

[0014] In some embodiments, the field stop is a two-dimensional aperture.

[0015] Compared with the prior art, the present invention has the following beneficial effects: the present invention can avoid the manufacturing difficulties of the computed tomography spectrometer, miniaturize the computed tomography spectrometer and produce a better diffraction effect, so that the focal plane detector target surface and the overall size of the spectrometer are both in the millimeter order, and it is expected to realize an imaging spectrometer with a mobile phone terminal or even a smaller structure. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 It is a structural schematic diagram of the computed tomography spectrometer described in the present invention.

[0017] Figure 2 It is a schematic diagram of the structure of a large period of a metasurface holographic grating provided in an embodiment of the present invention.

[0018] Figure 3 An embodiment of the present invention provides structural parameters of a small period of a metasurface holographic grating.

[0019] Figure 4 4 is a phase distribution diagram of an embodiment of the present invention.

[0020] Figure 5 1 is a superimposed diffraction pattern of an embodiment of the present invention.

[0021] Marking explanation: 1. Optical system structure; 2. Focal plane detector; 3. Data acquisition and processing system; 11. Converging lens group; 12. Field stop; 13. Collimating lens group; 14. Metasurface holographic grating; 15. Imaging lens group. DETAILED DESCRIPTION

[0022] In order to more clearly illustrate the technical solution of the present invention, the technical solution of the present invention is further described in detail below with reference to the accompanying drawings and specific embodiments. It should be understood that the preferred embodiments described herein are only used to illustrate and explain the present invention and are not intended to limit the present invention:

[0023] The present invention provides a computed tomography spectrometer based on a metasurface diffraction element, as shown in the attached Figure 1As shown, the optical system comprises an optical system structure 1, a focal plane detector 2 and a data acquisition and processing system 3, characterized in that the optical system structure 1 comprises a converging lens group 11, a field stop 12, a collimating lens group 13, a metasurface holographic grating 14, and an imaging lens group 15; the entire system has no mechanical moving parts; wherein the metasurface holographic grating 14 is a two-dimensional periodic structure that can simultaneously generate multiple spatially-spectrally encoded diffraction orders in a single exposure; the working process is as follows: after the converging lens group 11 converges the incident light carrying the target data cube information, the size of the light beam entering the system is limited at the field stop 12, the light is collected and collimated by the collimating lens group 13 and then enters the metasurface holographic grating 14, after the metasurface holographic grating 14 splits and disperses the pattern, the imaging lens group 15 focuses the dispersed pattern to form each diffraction order, and the focal plane detector 2 simultaneously receives each diffraction pattern, each diffraction pattern corresponding to the projection data of the diffraction direction, and the data acquisition and processing system 3 collects the diffraction patterns at each level and restores the target data cube through an algorithm.

[0024] To further illustrate the specific situation of the metasurface holographic grating 14, a key component of the computed tomography spectrometer, the attached Figure 2 and attached Figure 3 For example:

[0025] The metasurface holographic grating 14 is a nanostructure array, and the nanostructure is a polarization-independent structure.

[0026] The metasurface holographic grating 14 is composed of a substrate with a thickness of h2 and a height of h1 and a diameter of D. i The surface unit structure array is composed of a substrate having surfaces extending in the x and y directions, which are repeated with a period d. The surface unit structures are cylindrical and perpendicular to the substrate. The propagation direction of the incident light is the z direction.

[0027] Each large period of the surface unit structure array is composed of a plurality of surface unit structures with different diameters and the same height. The metasurface holographic grating 14 controls the number of diffraction orders and the shape of the diffraction pattern on the focal plane detector 2 by changing the diameter of the surface unit structures at different positions.

[0028] The substrate of the metasurface holographic grating 14 is silicon dioxide, and the surface unit structure is subwavelength metal or high refractive index medium.

[0029] In a specific embodiment of the present invention, the surface unit structure is titanium dioxide.

[0030] When designing the diameter of the surface unit structure, the phase modulation amount of the metasurface holographic grating array 14 should satisfy a phase coverage of 2π at the designed central wavelength, and while satisfying this condition, try to achieve a larger phase coverage at each wavelength within the designed wavelength band.

[0031] In the optical system structure, the converging lens group 11, the collimating lens group 13, and the imaging lens group 15 are composed of one or more lenses, and the field stop 12 is a two-dimensional aperture.

[0032] The coordinates referred to in this article are based on the optical axis as the z coordinate, and the x and y coordinates are established perpendicular to the optical axis plane.

[0033] In this paper, after the diffraction pattern information of each level obtained on the focal plane detector 2 is acquired by the data acquisition and processing system 3, the information is restored through the image reconstruction algorithm to obtain the three-dimensional data cube again.

[0034] In a specific embodiment of the present invention, the metasurface holographic grating is a two-dimensional periodic structure, which can simultaneously generate multiple spatial-spectral encoded diffraction orders under a single exposure; within the wavelength range of 550nm-1000nm, the computed tomography spectrometer can achieve corresponding phase delay by setting the diameter of each surface unit structure in the 14 large periods of the metasurface holographic grating, disperse the incident light, realize diffraction projection of light, generate a two-dimensional diffraction pattern of 5×5 orders, and realize imaging on the focal plane detector 2.

[0035] In a specific embodiment of the present invention, through a given set of phase distributions, the metasurface holographic grating 14 can disperse the input real three-dimensional data cube sample and focus it to form a diffraction pattern. Figure 2 、 3 , 4, and 5 further describe the simulation process of the computed tomography spectrometer described in the present invention in detail.

[0036] In a specific embodiment of the present invention, when the number of small cycles in each large cycle is 15×15, the phase distribution is as shown in the attached figure. Figure 4 As shown, the large period is repeated 15 times in the x and y directions respectively. The incident wavelength range is 550nm-1000nm.

[0037] In a specific embodiment of the present invention, according to the attached Figure 4 The phase distribution of each surface unit structure is arranged to obtain a metasurface holographic grating 14. Among them, a large period structure such as the attached Figure 2 As shown. The metasurface holographic grating 14 has a substrate thickness of h2 = 150nm, a small period substrate side length of d = 250nm, a surface unit structure height of h1 = 700nm, and a diameter of 20nm≤D i The focal length of the imaging lens group 15 is 15 mm, and the target surface size of the focal plane detector 2 is 3.6 mm×3.6 mm.

[0038] The diffraction pattern obtained by the metasurface holographic grating 14 is shown in the attached figure. Figure 5As shown, there are five diffraction orders (0, ±1, ±2). The superimposed spectral image has 11 bands with a wavelength interval of 35 nm. The expectation-maximization algorithm is used to restore the dispersion map of the multi-spectral superposition obtained on focal plane detector 2, resulting in a restored three-dimensional data cube.

[0039] The principles and specific implementations of this structure are described in detail with reference to the accompanying drawings and examples. Furthermore, those skilled in the art will appreciate that variations in the specific implementations, i.e., the scope of application, may occur based on the principles of this invention. In summary, the contents of this specification should not be construed as limiting the invention; all applications and inventions utilizing the principles of this invention are protected.

Claims

1. A computed tomography spectrometer based on a metasurface diffraction element, comprising an optical system structure (1), a focal plane detector (2) and a data acquisition and processing system (3), characterized in that: The optical system structure is composed of a converging lens group (11), a field stop (12), a collimating lens group (13), a metasurface holographic grating (14), and an imaging lens group (15) which are fixedly arranged in sequence along the optical axis. The entire system has no mechanical moving parts. The metasurface holographic grating (14) is a two-dimensional periodic structure and can simultaneously generate multiple spatial-spectral encoded diffraction orders under a single exposure. The working process is as follows: after the converging lens group (11) converges the incident light carrying the target data cube information, The size of the light beam entering the system is limited at the field of view aperture (12). The light is collected and collimated by the collimating lens group (13) and then enters the metasurface holographic grating (14). After the metasurface holographic grating (14) splits the light and disperses the pattern, the imaging lens group (15) focuses the dispersion pattern to form each diffraction order. The focal plane detector (2) simultaneously receives the diffraction pattern of each level. Each diffraction pattern corresponds to the projection data of the diffraction direction. The data acquisition and processing system (3) collects the diffraction pattern of each level and restores the target data cube through the algorithm.

2. The computed tomography spectrometer based on a metasurface diffraction element according to claim 1, wherein: The super surface holographic grating (14) is composed of a substrate with a thickness of h2 and a height of h1 and a diameter of D. i The substrate is composed of an array of surface unit structures, wherein the substrate has a surface extending in the x and y directions, and the surface unit structure is a cylinder perpendicular to the substrate.

3. The computed tomography spectrometer based on a metasurface diffraction element according to claim 2, wherein: The surface unit structure array is arranged in a matrix and consists of one or more large periods, and the large periods are arranged in a matrix in the x and y directions of the substrate.

4. The computed tomography spectrometer based on a metasurface diffraction element according to claim 3, wherein: Each large period of the surface unit structure is composed of a plurality of surface unit structures having different diameters and the same height. The metasurface holographic grating (14) regulates the number of diffraction orders and the shape of the diffraction pattern on the focal plane detector (2) by changing the diameters of the surface unit structures at different positions.

5. The computed tomography spectrometer based on a metasurface diffraction element according to claim 1, wherein: The supersurface holographic grating (14) is a nanostructure array, and the nanostructure is a polarization-independent structure.

6. The computed tomography spectrometer based on a metasurface diffraction element according to any one of claims 3 to 5, characterized in that: The substrate is silicon dioxide, and the surface unit structure is subwavelength metal or high refractive index medium.

7. The computed tomography spectrometer based on a metasurface diffraction element according to claim 1, wherein: The imaging lens group (15) is a zoom lens for changing the size of the image on the focal plane detector.

8. The computed tomography spectrometer based on a metasurface diffraction element according to claim 1, wherein: The converging lens group (11), the collimating lens group (13), and the imaging lens group (15) are composed of one or more lenses.

9. The computed tomography spectrometer based on a metasurface diffraction element according to claim 1, wherein: The field stop (12) is a two-dimensional aperture.

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