Flat plate type super-integrated miniature light splitting imaging system based on metasurface

Abstract

The invention relates to a super-surface-based flat plate type super-integrated miniature spectral imaging system, which comprises a gold reflecting layer and a flat plate substrate reflecting layer integrated with a dispersion super-surface, a first focusing reflection super-surface, a second focusing reflection super-surface and a focusing transmission super-surface, incident light is split by the dispersion metasurface, then is reflected to the gold reflection layer, and then is sequentially focused by the two reflection metasurfaces and the transmission metasurface to be imaged on an image plane. The dispersion metasurface adopts a periodically arranged square nano-column diffraction grating structure and has stronger dispersion capability, and the other three metasurface elements are obtained by adopting phase optimization design, so that the imaging performance is excellent; the four metasurfaces are integrated on the same reflecting layer, system optics is only composed of two flat reflecting layers, wavefront regulation and control are achieved while light in a reflecting cavity is continuously reflected, the metasurface can be manufactured through the single photoetching technology, the structure is simple and compact, machining, assembling and adjusting are easy, and a new product is provided for a high-performance compact imaging spectrometer.

Description

Technical Field

[0001] The present invention relates to a spectroscopic imaging system for push-broom hyperspectral imaging, and in particular to a flat-plate ultra-integrated micro-spectroscopic imaging system based on a metasurface, belonging to the technical field of optical design. Background Art

[0002] Hyperspectral imaging is an emerging, non-destructive, advanced optical technology that has the dual functions of spectroscopy and imaging. This dual function enables hyperspectral imaging to simultaneously provide chemical and physical characteristics of the experimental object with good spatial resolution. As a special optical diagnostic technology, hyperspectral imaging can identify and quantify materials in a non-destructive and non-invasive manner. It has the characteristics of diversified imaging systems, extensive research objects, practical clinical diagnosis, and functional analysis methods. It has the potential for in-situ real-time diagnosis of living diseases, and has broad prospects for clinical application, which deserves in-depth research.

[0003] Hyperspectral imaging systems based on traditional geometric optics face some bottlenecks: traditional optical systems using catadioptric lenses are difficult to design, complex to assemble, poorly compact, and bulky, so it is urgent to develop lightweight, planar integrated hyperspectral imaging systems. Metasurfaces, as sub-wavelength planar devices, have the ability to flexibly control the phase, amplitude, and polarization of light waves, and have the potential to replace traditional catadioptric lenses to achieve miniaturization and high integration of optical systems.

[0004] The paper “Hyperspectral Imager with Folded Metasurface Optics” (ACSPhotonics, 2019, 6(8), 2161-2167) reports a folded metasurface optical hyperspectral imager. In the design of the micro-nano structure, the nanocolumn microstructure is cylindrical, and the phase is controlled only by changing the radius. As a result, the spectral line bending and color distortion reach about 300μm, making it difficult to perform subsequent spectral calibration and image processing, and the system has the disadvantage of poor imaging performance. Summary of the invention

[0005] In view of the shortcomings of existing spectral imaging technology, the present invention provides a flat-plate ultra-integrated micro-spectrographic imaging system based on a metasurface, which has excellent imaging performance, simple and compact structure, and is easy to process and assemble, and is suitable for a push-broom hyperspectral imager.

[0006] The technical solution adopted by the present invention is: to provide a flat-plate ultra-integrated micro-spectrometric imaging system based on a metasurface, wherein the optical system thereof comprises a flat-plate substrate reflective layer and a gold reflective layer arranged in parallel, and a filling medium therebetween; a dispersive metasurface, a first focusing reflective metasurface, a second focusing reflective metasurface, and a focusing transmission metasurface are integrated and etched on the flat-plate substrate; the system aperture stop is arranged at the dispersive metasurface; the incident light emitted from the object plane is split by the dispersive metasurface, reflected to the gold reflective layer, and then sequentially passes through the first focusing reflective metasurface, the gold reflective layer, and the second focusing reflective metasurface. , the gold reflective layer is folded and reflected to the focusing transmission metasurface, and the light is transmitted to the image plane; the dispersive metasurface is a periodically arranged square nanocolumn diffraction grating structure, the grating groove direction is perpendicular to the meridian plane of the optical system, and the grating line density g satisfies the condition: 950lines / mm≤g≤1050lines / mm; the first focusing reflection metasurface, the second focusing reflection metasurface and the focusing transmission metasurface are planar binary optical surfaces 1, and the unit structure and distribution of the square nanocolumns of each metasurface are obtained by phase optimization design, and its phase distribution function Φ Requirements:

[0007] Where M is the diffraction order, M=1; N is the number of polynomial coefficients in the series, To optimize the phase coefficient, and are the position coordinates of the corresponding optimized phase on the metasurface respectively.

[0008] The refractive index of the filling medium described in the present invention is n, and the Abbe number is v, which meets the following conditions: 1.46≤n≤1.51, 65≤v≤75.

[0009] The present invention describes a flat-panel ultra-integrated micro-spectrometric imaging system based on a metasurface, wherein the entrance pupil diameter is D, satisfying 0.4mm≤D≤0.6mm; the tube length is L, satisfying 5mm≤L≤6mm; the distance between the reflective surfaces of the flat substrate reflective layer and the gold reflective layer is d1, satisfying 1.51mm≤d1≤1.54mm; and the distance between the focusing transmission metasurface and the image plane is d2, satisfying 1.9mm≤d2≤2.1mm.

[0010] The height h1 of the square nanocolumn unit described in the present invention is 560nm, the surface is wrapped with a PDMS polymer with a height h2 of 1000nm, and then a gold layer is deposited to form a reflective unit structure; the material of the square nanocolumn is titanium dioxide, the material refractive index is n1, the Abbe number is v1, and the conditions are satisfied: 2.6≤n1≤2.7; 9.1≤v1≤10.1; the scanning range of the length and width of the square nanocolumn of each supersurface is 60nm~200nm.

[0011] The spectroscopic imaging system provided by the present invention adopts a folded optical path structure, so that the light is reflected and transmitted in a cavity composed of two gold reflective layers, which is beneficial to the installation and calibration of the system. At the same time, the folded structure can improve the compactness and high integration of the structure as much as possible; a dispersion metasurface with a periodic arrangement of microstructures is used instead of a diffraction grating, so that the system has a stronger dispersion ability; the wavefront is regulated by two focusing reflection metasurfaces, so as to achieve a focusing performance close to the diffraction limit; the astigmatism introduced in the folded optical path structure due to off-axis is balanced by the focusing transmission metasurface, so as to achieve high-performance imaging of the system.

[0012] The present invention provides a flat-plate ultra-integrated micro-spectrographic imaging system based on a metasurface, which adopts a square nanocolumn structure, has a higher degree of design freedom, can more accurately match the ideal phase, and further improve the imaging performance of the system. While miniaturizing the imaging spectrometer, it ensures good imaging performance to overcome the shortcomings of most current spectroscopic imaging systems, and has great practical significance for the further promotion and application of miniaturized hyperspectral imaging technology.

[0013] Compared with the prior art, the present invention has the following beneficial effects: 1. The spectroscopic imaging system provided by the present invention adopts a folded optical path structure and realizes the miniaturization of the hyperspectral imager by introducing a metasurface. The system volume is only 13.073mm 3 , achieving near-diffraction-limited focusing performance with only four metasurfaces.

[0014] 2. The present invention optimizes the phase design of the metasurface and adjusts the optical path through the metasurface phase, thereby achieving good imaging performance. The transmission focusing metasurface balances the astigmatism introduced by the off-axis in the folded optical path structure, effectively improving the spectral distortion. The spectral line bending and color distortion in the present invention are controlled within 15μm, which is conducive to subsequent spectral calibration and image processing. The results are significantly better than the currently reported micro-imaging spectrometers.

[0015] 3. The present invention combines the advantages of a folded optical path structure and a flat metasurface to realize a miniaturized and highly integrated spectral imaging system. The optical elements of the system include four metasurfaces, and the remaining surfaces are reflective planes. The four metasurfaces are located on the same gold reflective layer and can be manufactured through a single photolithography process. The system has a compact structure and is easy to assemble and calibrate, and has practical application value. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 is a structural schematic diagram of a spectroscopic imaging system provided by an embodiment of the present invention; Figure 2 is a schematic diagram of the structure of a metasurface unit of a spectroscopic imaging system provided by an embodiment of the present invention; Figure 3 is a phase diagram of four metasurfaces of the spectroscopic imaging system provided by an embodiment of the present invention; Figure 4 is a spectroscopic imaging flow chart of a spectroscopic imaging system provided by an embodiment of the present invention; Figure 5 is a ray tracing point diagram of a spectroscopic imaging system provided by an embodiment of the present invention; Figure 6 is a transfer function MTF curve diagram of the spectroscopic imaging system provided by an embodiment of the present invention; Figure 7 is a spectrum bending curve diagram at the image plane of the spectroscopic imaging system provided by an embodiment of the present invention; Figure 8 is a color distortion curve diagram at the image plane of the spectroscopic imaging system provided by an embodiment of the present invention; Fig. 9 is a circled energy concentration curve diagram of the spectroscopic imaging system provided by an embodiment of the present invention; Fig.10 It is an image plane point diagram of the spectroscopic imaging system provided by an embodiment of the present invention.

[0017] In the figure, 1. object plane; 2. dispersion metasurface; 3. gold reflection layer; 4. first focusing reflection metasurface; 5. second focusing reflection metasurface; 6. focusing transmission metasurface; 7. flat reflective substrate layer; 8. image plane; 9. filling medium between plates; 10. titanium dioxide nanorods; 11. PDMS polymer; 12. gold layer. DETAILED DESCRIPTION

[0018] The technical solution of the present invention is further described below in conjunction with the accompanying drawings and embodiments. Example

[0019] This embodiment provides a flat-plate ultra-integrated micro-spectrometric imaging system based on a metasurface.

[0020] See attached Figure 1 , which is a structural schematic diagram of the spectroscopic imaging system provided in this embodiment. The optical element of the spectroscopic imaging system includes two reflective layers, namely a flat reflective base layer 7 and a gold reflective layer 3, and a filling medium 9 therebetween; the incident light emitted from the object plane 1 is split by the dispersion metasurface 2, reflected to the gold reflective layer 3, and then sequentially passes through the first focusing reflective metasurface 4, the gold reflective layer 3, the second focusing reflective metasurface 5, and the gold reflective layer 3 is folded and reflected to the focusing transmission metasurface 6, and the light is transmitted to the image plane 8; except for the four metasurfaces, the other surfaces are all reflective planes; the dispersion metasurface, the first focusing reflective metasurface, the second focusing reflective metasurface and the focusing transmission metasurface are integrated and etched on the flat reflective base layer.

[0021] In this embodiment, the entrance pupil diameter D of the spectroscopic imaging system is 0.5 mm, the object imaging field of view is 10°, and the system operating band is 500-600 nm. The system tube length L is 5.8 mm, the interval d1 between the two reflective surfaces of the spectroscopic imaging system is 1.525 mm, and the interval d2 from the focusing transmission metasurface to the image plane is 2.038 mm. The medium filled between the two reflective layers is H-QK3L, with a refractive index n of 1.49 and an Abbe number v of 70.4.

[0022] As the core spectroscopic part of the system, the dispersive metasurface adopts a periodically arranged square nano-column diffraction grating structure. The grating groove direction is perpendicular to the meridian plane of the optical system, and the grating line density g is 1000 lines / mm.

[0023] See attached Figure 2 , which is a schematic diagram of the super surface unit structure of the spectroscopic imaging system provided in this embodiment. The nanocolumn is a square titanium dioxide nanocolumn 10, which is wrapped by a PDMS polymer 11 and then a gold layer 12 is deposited to form a reflective unit structure. The PDMS polymer is spin-coated and solidified as a protective layer to prevent the nanocolumn from being damaged in subsequent processes. This embodiment adopts a square nanocolumn structure, which has a higher degree of design freedom and can more accurately match the ideal phase, so that the imaging performance of the system is further improved.

[0024] In the optical system, the first focusing reflective metasurface, the second focusing reflective metasurface and the focusing transmissive metasurface are planar binary optical surfaces 1. The unit structure and distribution of the square nanocolumns of each metasurface are obtained by phase optimization design, and the phase distribution function is Φ Requirements: Where M is the diffraction order, M=1; N is the number of polynomial coefficients in the series, is the phase coefficient to be optimized, and are the position coordinates of the corresponding phase on the metasurface.

[0025] In this embodiment, the phase coefficients of each metasurface are shown in Table 1. The other phase coefficients not given are all taken as 0. The phase distribution of the optimized focusing metasurface is shown in the attached Figure 3 .

[0026] Table 1:

[0027] In this embodiment, the metasurface adopts the transmission phase regulation method to achieve compensation of optical path difference by changing the length and width of the nanocolumns. In this embodiment, the period of the titanium dioxide square nanocolumns is 250nm, the height h1 is 500nm, and the scanning range of the length and width of the nanocolumns is from 60nm to 200nm. The refractive index n1 of the titanium dioxide nanocolumns is 2.64, the Abbe number v1 is 9.87, and the height h2 of the PDMS polymer is 1000nm.

[0028] This embodiment uses a dispersive metasurface with stronger dispersion capability than a diffraction grating; the system has a simple optical configuration, consisting only of a single flat metasurface and a gold reflective layer, and achieves wavefront control while continuously reflecting light in the reflection cavity; the four metasurfaces are all located on the same gold reflective layer and can be manufactured using a single-shot photolithography technique, with a simple structure and easy processing.

[0029] See attached Figure 3 , which is the phase image color of each metasurface in the spectroscopic imaging system provided in this embodiment; among them, (a) is a scattering metasurface, (b) is the first focusing reflection metasurface, (c) is the second focusing reflection metasurface, and (d) is the focusing transmission metasurface. There are four metasurfaces, consisting of Figure 1 It can be seen that the largest surface area is 3.86mm 2 , and is processable.

[0030] See attached Figure 4 , which is a spectroscopic imaging flow chart of the spectroscopic imaging system provided in this embodiment; when the spectroscopic imaging system provided in this embodiment is used for imaging, first, the light enters the reflection cavity composed of two gold reflection layers, and is reflected at the dispersion metasurface to achieve the effect of dispersion spectroscopic analysis; the light beam after dispersion spectroscopic analysis is reflected to the gold reflection layer, and then reflected to the first focusing metasurface again, the metasurface regulates the wavefront, and then reflects again; after being reflected again to the second focusing metasurface through the gold reflection layer, the metasurface further regulates the wavefront, and then reflects again; after being reflected again to the focusing transmission metasurface through the gold reflection layer, the incident light of different wavelengths is focused by the two reflection metasurfaces and one transmission metasurface and converges at the image plane, completing the spectral imaging process.

[0031] See attached Figure 5 , which is a ray tracing point diagram of five wavelengths of light passing through the spectroscopic imaging system provided by this embodiment. In the figure, (a) is 500nm, (b) is 525nm, (c) is 550nm, (d) is 575nm, and (e) is 600nm. Figure 5 It can be seen that the root mean square radius of the spot diagram of each field of view corresponding to the five wavelengths is less than 3.37μm, and the geometric radius of the spot diagram is less than 8.02μm, and the imaging quality is good.

[0032] See attached Figure 6 , which is the MTF curve of the transfer function on the image plane corresponding to each field of view of the three wavelengths of the spectroscopic imaging system provided in this embodiment; at 77Lp / mm, 500nm is (a), 550nm is (b) and 600nm is (c), from Figure 6 It can be seen that the MTF values ​​of each field of view at the three wavelengths are all greater than 0.3, close to the diffraction limit, and the curves are relatively smooth, indicating that the lens imaging is clear and uniform, and the system has good imaging quality in the full band and full field of view.

[0033] See attached Figure 7 , which is the spectrum bending curve at each imaging field of view of the spectroscopic imaging system provided in this embodiment. Figure 7 It can be seen that the spectral line bending corresponding to different wavelengths in the full field of view of the spectroscopic imaging system is less than 2.1μm, that is, the spectral line bending of the spectroscopic imaging system is well corrected.

[0034] See attached Figure 8 , which is the color distortion curve at each imaging field of view of the spectroscopic imaging system provided in this embodiment. Figure 8 It can be seen that the color distortion corresponding to different wavelengths in each spectral band of the spectroscopic imaging system is less than 15μm, that is, the color distortion of the spectroscopic imaging system has been well corrected.

[0035] See attached Fig. 9 , which is the circled energy concentration curve of three wavelengths of the spectroscopic imaging system provided in this embodiment, 500nm is (a), 575nm is (b) and 600nm is (c); Fig. 9 It can be seen that more than 80% of the energy is concentrated within the range of the Airy spot, and the energy is relatively concentrated.

[0036] See attached Fig.10 , which is the image plane point arrangement diagram of five wavelengths and intervals of 0.6nm of the spectroscopic imaging system provided in this embodiment, 500nm is (a), 525nm is (b), 550nm is (c), 575nm is (d) and 600nm is (e); Fig.10 It can be seen that the spectral resolution of each spectral band of the system is better than 0.6nm.

Claims

1. A flat-plate ultra-integrated micro-spectroscopic imaging system based on a metasurface, characterized in that: Its optical system includes a parallel flat substrate reflection layer (7) and a gold reflection layer (3), and a filling medium (9) therebetween; a dispersion metasurface (2), a first focusing reflection metasurface (4), a second focusing reflection metasurface (5), and a focusing transmission metasurface (6) are integrated and etched on the flat substrate (7); a system aperture stop is arranged at the dispersion metasurface; incident light emitted from the object plane (1) is split by the dispersion metasurface (2), reflected to the gold reflection layer (3), and then sequentially passes through the first focusing reflection metasurface (4), the gold reflection layer (3), the second focusing reflection metasurface (5), the gold The reflective layer (3) is folded and reflected to the focusing transmission metasurface (6), and the light is transmitted to the image plane (8); the dispersive metasurface is a periodically arranged square nanocolumn diffraction grating structure, the grating groove direction is perpendicular to the meridian plane of the optical system, and the grating line density g satisfies the condition: 950 lines / mm≤g≤1050 lines / mm; the first focusing reflection metasurface, the second focusing reflection metasurface and the focusing transmission metasurface are planar binary optical surfaces 1, and the unit structure and distribution of the square nanocolumns of each metasurface are obtained by phase optimization design, and the phase distribution function Φ Requirements: ； Where M is the diffraction order, M=1; N is the number of polynomial coefficients in the series, To optimize the phase coefficient, and are the position coordinates of the corresponding optimized phase on the metasurface respectively.

2. The flat-plate ultra-integrated micro-spectroscopic imaging system based on a metasurface according to claim 1, characterized in that: The refractive index of the filling medium is n, and the Abbe number is v, which satisfies the following conditions: 1.46≤n≤1.51, 65≤v≤75.

3. The flat-plate ultra-integrated micro-spectroscopic imaging system based on a metasurface according to claim 1, characterized in that: Its entrance pupil diameter is D, satisfying 0.4mm≤D≤0.6mm; the tube length is L, satisfying 5mm≤L≤6mm; the distance between the reflective surfaces of the flat substrate reflective layer and the gold reflective layer is d1, satisfying 1.51mm≤d1≤1.54mm; the distance between the focusing transmission metasurface and the image plane is d2, satisfying 1.9mm≤d2≤2.1mm.

4. The flat-plate ultra-integrated micro-spectroscopic imaging system based on a metasurface according to claim 1, characterized in that: The height h1 of the square nanocolumn unit is 560nm, the surface is coated with a PDMS polymer with a height h2 of 1000nm, and then a gold layer is deposited to form a reflective unit structure; the material of the square nanocolumn is titanium dioxide, the material refractive index is n1, and the Abbe number is v1, which satisfies the conditions: 2.6≤n1≤2.7; 9.1≤v1≤10.1; the scanning range of the length and width of the square nanocolumns of each metasurface is 60nm~200nm.

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