Design method of double-cascade metalens and long-wave infrared metalens imaging system

Abstract

The invention discloses a design method of a double-cascade meta-lens and a long-wave infrared meta-lens imaging system, belonging to the field of lens and micro-nano photonics. The method comprises: using a rigorous coupled wave analysis method, calculating the phase mutation introduced by columnar structure units of different radii when incident with a long-wave infrared characteristic wavelength, and the dispersion distribution in the long-wave infrared, obtaining multiple groups of radius-phase mutation-dispersion distribution to form a columnar structure unit library; determining the target phase distribution of a metasurface hexagonal lattice periodic array with the goal of enabling the double-cascade meta-lens to perform wavefront control on the incident light and correcting the high-order aberration of the incident light; according to the target phase distribution, with the goal of achieving the highest consistency of the dispersion distribution of each hexagonal lattice in the same metasurface, determining the radius of the columnar structure unit at each hexagonal lattice from the selected results. A special dispersion design method is adopted to enable the meta-optical element to be used in the field of infrared large-aperture imaging applications.

Description

Technical Field

[0001] The present invention belongs to the field of lenses and micro-nano photonics, and more specifically, relates to a design method of a double-cascade meta-lens and a long-wave infrared meta-lens imaging system. Background Art

[0002] In order to achieve high-performance and high-quality imaging effects, traditional infrared optical imaging systems often inevitably use multiple refractive lenses, which makes the system large and heavy, which runs counter to the current development needs of flattening, miniaturization, and lightweighting of optical systems. In recent years, the highly-regarded metasurface lenses are often accompanied by huge chromatic aberrations when used for focusing imaging, and the numerical aperture, field of view, caliber and other parameters are mutually restricted. Using a metasurface lens alone still cannot achieve the same imaging quality as a traditional lens group.

[0003] The composite optical system based on metasurface lens can solve the above problems better. For example, patent CN110488394A provides a long-wave infrared composite optical system, which simplifies the structure of the optical system and optimizes the processing technology. However, the existing composite optical system based on metasurface lens ignores the problem of inconsistent dispersion performance of the metasurface under the two algorithm models when combining the metasurface element design algorithm with the traditional optical element design optimization algorithm, resulting in the actual imaging effect of the optical system failing to achieve the expected effect of simulation. Summary of the invention

[0004] In response to the defects of the prior art and the need for improvement, the present invention provides a design method for a dual-cascade metalens and a long-wave infrared metalens imaging system, which aims to solve the problem that when the existing metasurface element design algorithm is combined with the traditional optical element design optimization algorithm, the inconsistent dispersion performance of the metasurface under the two algorithm models is ignored.

[0005] To achieve the above-mentioned purpose, according to one aspect of the present invention, a design method of a double-cascade metalens is provided, wherein the double-cascade metalens is used in a long-wave infrared metalens imaging system, and comprises a dielectric substrate layer and a metasurface located on both sides of the dielectric substrate layer, wherein the metasurface is composed of a plurality of columnar structural units arranged in a hexagonal lattice periodic array, and the method comprises: S1, using a rigorous coupled wave analysis method, calculating the phase mutation introduced by columnar structural units of different radii when incident with a long-wave infrared characteristic wavelength, and the dispersion distribution in the long-wave infrared, and obtaining a plurality of mutually corresponding groups of radius-phase mutation-dispersion distribution. S2, with the goal of enabling the double-cascade metalens to perform wavefront control on the incident light and correct the higher-order aberrations of the incident light, determine the target phase distribution of the hexagonal lattice periodic array of the metasurface; S3, for any hexagonal lattice, according to the target phase of the hexagonal lattice, select multiple groups of radius-phase mutation-dispersion distributions with corresponding phase mutations from the columnar structure unit library; S4, with the goal of achieving the highest consistency in the dispersion distribution of each hexagonal lattice in the same metasurface, determine the radius of the columnar structure unit at each hexagonal lattice from the results selected in S3.

[0006] Furthermore, the radius of the columnar structural unit in S1 satisfies: the phase mutations corresponding to all radii cover 0-2π.

[0007] Furthermore, for a columnar structure unit of any radius, the calculation of its dispersion distribution in the long-wave infrared in S1 includes: calculating the phase mutation introduced by the columnar structure unit of any radius when the long-wave infrared has a maximum wavelength incident, and the phase mutation introduced when the long-wave infrared has a minimum wavelength incident; calculating the difference between the phase mutation corresponding to the maximum wavelength and the phase mutation corresponding to the minimum wavelength, to obtain the dispersion distribution.

[0008] According to another aspect of the present invention, a long-wave infrared metalens imaging system is provided, comprising: a first lens group, a double-cascade metalens, a second lens group and an infrared focal plane detector arranged in sequence along the optical axis from the object side to the image side, wherein the double-cascade metalens is obtained by the design method of the double-cascade metalens as described above.

[0009] Furthermore, the focal lengths of the two metasurfaces of the double-cascade metalens and the focal length of the long-wave infrared metalens imaging system satisfy:

[0010] f 1 / f>10

[0011] f 2 / f>10

[0012] Among them, f 1 is the focal length of the double-cascaded metalens-metasurface, f 2is the focal length of the other metasurface of the double-cascade metalens, and f is the focal length of the long-wave infrared metalens imaging system.

[0013] Furthermore, the first lens group includes a first lens and a second lens, the second lens group includes a third lens and a fourth lens, and the first lens, the second lens, the double-cascade meta-lens, the third lens and the fourth lens are arranged in sequence from the object side to the image side along the optical axis; the first lens is a negative lens, and the second lens, the third lens and the fourth lens are positive lenses; the negative lens and each of the positive lenses are used to correct the primary aberration of the incident light and focus the light before emitting it.

[0014] Furthermore, the height of each columnar structure unit in the metasurface of the double-cascade metalens is the same, and the height is between the order of magnitude of the detected infrared wavelength, and the radius is between the order of magnitude of the sub-wavelength.

[0015] Furthermore, in the metasurface of the double-cascade metalens, the materials of the columnar structure units and the dielectric substrate layer are both intrinsic single crystal silicon.

[0016] Furthermore, the infrared focal plane detector includes a detector window and an infrared focal plane array arranged in sequence along the optical axis; the material of the first lens group, the second lens group and the detector window is one or any combination of germanium, barium fluoride, zinc sulfide, zinc selenide, AMTIR, IG, and IRG.

[0017] In general, the above technical solutions conceived by the present invention can achieve the following beneficial effects:

[0018] (1) A design method for a double-cascade meta-lens is provided. The dispersion distribution of columnar structural units in the long-wave infrared is solved by using the rigorous coupled wave analysis method. A columnar structural unit library with phase mutations covering the entire 2π is introduced, and columnar structural units with highly consistent dispersion distribution are selected from the library to form a double-cascade meta-lens. This effectively ensures that the actual phase distribution function of the meta-lens is well consistent with the high-order even-order polynomial function, thereby reducing the impact of the focal length drift caused by the dispersion of the meta-lens itself on the imaging quality of the system.

[0019] (2) The phase distribution function of the metasurface of the double-cascaded metalens adopts a high-order even-order polynomial form. On the one hand, it increases the degree of freedom of system optimization, which is conducive to achieving better imaging quality. On the other hand, the high-order even-order polynomial form can be used for rapid optimization using ray tracing and damped least squares method, which is compatible with the rapid optimization method of traditional lens groups and is suitable for the design and solution of composite optical systems.

[0020] (3) A long-wave infrared meta-lens imaging system is provided, which uses a traditional refractive lens to correct primary aberrations, uses a double-cascaded meta-lens to finely control the wavefront of light and correct the higher-order aberrations of the system, and uses a structure combining a refractive lens and a meta-lens to achieve integration with a small-pixel, large-array infrared focal plane array to achieve high-resolution infrared detection imaging, and the modulation transfer function reaches the diffraction limit, with good imaging quality;

[0021] (4) Since the meta-lens element replaces multiple traditional refractive lenses, the structure of the optical system can be simplified. At the same time, the processing technology of the metasurface is compatible with the semiconductor process and adopts planar processing technology, which is suitable for large-scale batch production, allowing the meta-lens to enter the device-level application market. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 A flow chart of a design method for a double-cascaded meta-lens provided in an embodiment of the present invention;

[0023] Figure 2 A schematic diagram of the structure of a long-wave infrared metalens imaging system provided by an embodiment of the present invention;

[0024] Figure 3 A schematic diagram of ray tracing at a central wavelength of 10 μm for a long-wave infrared metalens imaging system provided by an embodiment of the present invention;

[0025] Figure 4A , Figure 4B The phase distribution functions that need to be achieved by the first metasurface and the second metasurface of the double-cascaded metalens provided in the embodiments of the present invention are respectively;

[0026] Figure 5 A diagram of light spots at a photosensitive surface of a simulated infrared focal plane array provided by an embodiment of the present invention;

[0027] Figure 6 A simulated MTF curve diagram provided by an embodiment of the present invention;

[0028] Figure 7 A schematic diagram of simulated field curvature and distortion provided by an embodiment of the present invention;

[0029] Figure 8 A simulation imaging effect diagram provided by an embodiment of the present invention;

[0030] Fig. 9A A schematic diagram of a columnar structural unit provided by an embodiment of the present invention;

[0031] Fig. 9B A top view of a columnar structural unit provided by an embodiment of the present invention;

[0032] Fig.10Schematic diagram of phase mutation and transmittance obtained by columnar structural units with different diameters provided in an embodiment of the present invention;

[0033] Fig.11 A schematic diagram of the reference phase of the columnar structure units with different diameters at the central wavelength and the relative dispersion in the working band provided by an embodiment of the present invention;

[0034] Fig.12 A schematic diagram of the structure of a double-cascaded meta-lens provided in an embodiment of the present invention;

[0035] Fig.13A Near-field phase distribution profile and phase distribution grayscale diagram of a small-aperture single-wavelength focusing meta-lens without dispersion management at different incident wavelengths provided by an embodiment of the present invention

[0036] Fig. 13B The near-field phase distribution profile and phase distribution grayscale diagram of the dispersion management and small-aperture single-wavelength focusing meta-lens provided in an embodiment of the present invention at different incident wavelengths.

[0037] In all drawings, the same reference numerals are used to denote the same elements or structures, wherein:

[0038] 1 is the first lens, 2 is the second lens, 3 is the double cascade meta-lens, 301 is the first meta-surface, 302 is the second meta-surface, 303 is the dielectric substrate layer, 4 is the third lens, 5 is the fourth lens, 6 is the infrared focal plane detector, 601 is the detector window, 602 is the infrared focal plane array, and 7 is the columnar structure unit. DETAILED DESCRIPTION

[0039] In order to make the purpose, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention. In addition, the technical features involved in the various embodiments of the present invention described below can be combined with each other as long as they do not conflict with each other.

[0040] In the present invention, the terms "first", "second", etc. (if any) in the present invention and the accompanying drawings are used to distinguish similar objects but not necessarily to describe a specific order or sequence.

[0041] Figure 1 Flow chart of the design method of the double-cascade meta-lens provided in an embodiment of the present invention. Figure 1 , in conjunction with other drawings, the design method of the double-cascade meta-lens in this embodiment is described in detail, and the method includes operations S1 to S4.

[0042] Double cascade metalens is used in long-wave infrared metalens imaging system, such as Figure 2 The double-cascade meta-lens includes a dielectric substrate layer 303 and a meta-surface located on both sides of the dielectric substrate layer. The meta-surface is composed of a plurality of columnar structural units 7 arranged in a hexagonal lattice periodic array, as shown in FIG. Fig.12 Preferably, the metasurfaces on both sides of the dielectric substrate layer are planes, and the thickness of the dielectric substrate layer is 0.6 mm. In this embodiment, the radius of each columnar structure unit in the metasurface is designed to reduce the influence of the focal length drift caused by the dispersion of the double-cascaded meta-lens itself on the imaging quality of the system.

[0043] Operation S1, using rigorous coupled wave analysis, calculates the phase mutation introduced by columnar structural units of different radii when incident with long-wave infrared characteristic wavelengths, as well as the dispersion distribution in the long-wave infrared, and obtains multiple sets of corresponding radius-phase mutation-dispersion distributions to form a columnar structural unit library.

[0044] According to an embodiment of the present invention, for a columnar structure unit of any radius, calculating its dispersion distribution in the long-wave infrared in operation S1 includes: calculating the phase mutation introduced by the columnar structure unit of any radius when the long-wave infrared has a maximum wavelength incident, and the phase mutation introduced when the long-wave infrared has a minimum wavelength incident; calculating the difference between the phase mutation corresponding to the maximum wavelength and the phase mutation corresponding to the minimum wavelength, to obtain the dispersion distribution.

[0045] According to an embodiment of the present invention, the radius of the columnar structure unit selected in operation S1 satisfies: in the columnar structure unit library, all phase mutations corresponding to the radius cover 0-2π. Phase mutation covering 0-2π means that for any phase mutation in the range of 0-2π, there are one or more corresponding to the radius.

[0046] In this embodiment, assuming that the radius of the columnar structure unit is selected within the interval (d1, d2), the interval can be divided into several sub-intervals. In each sub-interval, the phase mutation increases as the radius increases, and the phase mutation in the sub-interval covers 0-2π. Therefore, any phase mutation within the range of 0-2π has several corresponding radii corresponding to it.

[0047] Taking the radius as d, the characteristic wavelength of long-wave infrared as 10 μm, the minimum wavelength of long-wave infrared as 8 μm, and the maximum wavelength of long-wave infrared as 12 μm as an example, in operation S1, the phase mutation of the columnar structure unit with a radius of d at 8 μm, 10 μm, and 12 μm, and the phase mutation at the characteristic wavelength of 10 μm are calculated by using the rigorous coupled wave analysis method. is the reference phase. The difference between the phase mutation at the maximum wavelength of 12 μm and the phase mutation at the minimum wavelength of 10 μm is the dispersion distribution D. Based on this, a set of corresponding radius d-phase mutations are obtained. -Dispersion distribution D. Select new radius d multiple times and repeat the above operation to obtain multiple sets of radius d-phase mutations -Dispersion distribution D, forming a columnar structure unit library, such as Fig.11 shown.

[0048] See also Fig.11 Each data point in the figure corresponds to a columnar structure unit of a specific radius. The horizontal axis value of the point reflects the corresponding phase mutation, and the vertical axis value reflects the corresponding dispersion distribution. For example, the data point at the origin indicates that the phase mutation and dispersion distribution of the columnar structure unit with a radius of 0 nm are 0.

[0049] Operation S2 determines the target phase distribution of the metasurface hexagonal lattice periodic array with the goal of enabling the double-cascaded metalens to perform wavefront control on the incident light and correct the higher-order aberrations of the incident light.

[0050] Along the optical axis, the two metasurfaces of the double-cascade meta-lens are defined as a first metasurface 301 and a second metasurface 302. The first metasurface and the second metasurface are respectively composed of microstructure arrays arranged according to a certain phase distribution. The phase distribution of the two microstructure arrays is Satisfies the following expression:

[0051]

[0052]

[0053] Among them, ρ is the radial coordinate of the hypersurface, R is the normalized radius, and A n , B n , C n , D n are the polynomial coefficients of the nth hypersurface, n=1,2.

[0054] Specifically, in operation S2, the double-cascaded meta-lens is used to control the wavefront of the incident light and correct the high-order aberration of the incident light by using the ray tracing method. n , B n , C n , D n Optimize, for example, see Table 1 for specific values, so as to determine the target phase distribution of the hexagonal lattice periodic array of the first metasurface and the second metasurface. Figure 3 The phase distribution diagram of the first metasurface microstructure array obtained by optimization is shown in Figure 4A As shown, the phase distribution diagram of the second metasurface microstructure array is as follows Figure 4B shown.

[0055] Table 1

[0056] R(mm) <![CDATA[A n ]]> <![CDATA[B n ]]> <![CDATA[C n ]]> <![CDATA[D n ]]> The first metasurface 100.031 -3929.457 -3.518e+5 1.505e+6 -4.972e+6 The second metasurface 100.000 2717.567 3.558e+5 -1.557e+6 5.273e+6

[0057] Operation S3, for any hexagonal lattice, according to the target phase of the hexagonal lattice, select multiple groups of radius-phase jump-dispersion distributions with corresponding phase jumps from the columnar structure unit library.

[0058] Target Phase and phase mutation There is a one-to-one correspondence between them, and a phase mutation Based on this, for any hexagonal lattice, according to its target phase, multiple sets of corresponding radius-phase mutation-dispersion distributions can be selected from the columnar structure unit library, and the phase mutations of these multiple sets of radius-phase mutation-dispersion distributions are the same and correspond to the target phase of the hexagonal lattice.

[0059] Operation S4, with the goal of achieving the highest consistency in the dispersion distribution of each hexagonal lattice in the same metasurface, determines the radius of the columnar structure unit at each hexagonal lattice from the results selected in operation S3.

[0060] In this embodiment, operation S3 determines multiple sets of alternative solutions for the radius of the columnar structure unit at each hexagonal lattice. Operation S4 comprehensively considers the dispersion distribution of all hexagonal lattices, with the goal of making the dispersion distribution at each location the most consistent, and determines the corresponding optimal solution (optimal radius-phase mutation-dispersion distribution) for the radius of the columnar structure unit at each hexagonal lattice, and determines the radius in the optimal solution as the radius of the columnar structure unit at the corresponding hexagonal lattice.

[0061] In this embodiment, the schematic diagram of the columnar structure unit is as follows Fig. 9A As shown, the top view is Fig. 9B As shown in the figure, the radius, height, array period and other parameters of the columnar structure unit are optimized by using the vector electromagnetic field numerical simulation algorithm, and the relationship between the radius and the phase is obtained. The phase mutation and transmittance of the columnar structure unit at different radii are shown in the figure. Fig.10 As shown, the height of the optimized columnar structure unit is 8 μm, and the period of the columnar structure unit is 3 μm.

[0062] The design method of the double-cascade metalens in the embodiment of the present invention solves the technical problems of complex structure of traditional optical system, short imaging distance of small-aperture metalens, poor wide-spectrum imaging effect of large-aperture metalens elements, and incompatibility of metasurface elements with traditional optical element design optimization algorithms; by adopting a special dispersion management method, a design method for large-aperture metalens is provided to ensure that the large-aperture metasurface lens performs consistently in ray tracing calculations and electromagnetic finite-difference time-domain calculations; and meta-optical elements are enabled to enter the field of infrared large-aperture imaging applications.

[0063] The embodiment of the present invention further provides a long-wave infrared meta-lens imaging system, comprising: a first lens group, a double-cascade meta-lens 3, a second lens group and an infrared focal plane detector 6 arranged in sequence along the optical axis from the object side to the image side, the double-cascade meta-lens is obtained by the design method of the double-cascade meta-lens as described above, and the structure of the system is as follows: Figure 2 shown.

[0064] The long-wave infrared meta-lens imaging system uses a traditional refractive lens to correct primary aberrations, and uses a double-cascaded meta-lens to finely control the wavefront of light, correct the system's higher-order aberrations, and simplify the structure of the optical system while ensuring imaging quality. For example, the system is aimed at imaging in the long-wave infrared band with a central wavelength of 10μm. The system's image plane diagonal height is 27.376mm, the aperture number is 0.800, and the total length of the system is less than 150mm.

[0065] According to an embodiment of the present invention, the height of each columnar structure unit in the metasurface of the double-cascade metalens is the same, and the height is between the order of magnitude of the infrared wavelength detected, and the radius is between the order of magnitude of the sub-wavelength. The double-cascade metalens is used to control the wavefront of the incident light, and realize the correction of the high-order aberration of the incident light before emission.

[0066] According to an embodiment of the present invention, the focal lengths of the two metasurfaces of the double-cascade metalens and the focal length of the long-wave infrared metalens imaging system satisfy:

[0067] f 1 / f>10

[0068] f 2 / f>10

[0069] Among them, f 1 is the focal length of the double-cascaded metalens-metasurface, f 2 is the focal length of the other metasurface of the double-cascaded metalens, and f is the focal length of the long-wave infrared metalens imaging system. Preferably, in this embodiment, f=25.006, f 1 =1000.001,f 2 =1000.001,f 1 / f=40,f 2 / f=40.

[0070] By f 1 、f 2, f are designed as above, so that the metasurface bears a certain optical focal length in the optical system, reduces the pressure of the refractive lens on focusing the light, and transfers the tolerance limit during the processing of the refractive lens to the metasurface, which is beneficial to reduce the tolerance sensitivity of the system. At the same time, the focal length of each metasurface is one order of magnitude larger than the focal length of the imaging system, which prevents the metasurface from bearing too much optical focal length in the optical system, and effectively ensures the smooth progress of the ray tracing algorithm and the damped least squares optimization during the simulation design.

[0071] According to an embodiment of the present invention, in the metasurface of the double-cascade metalens, the materials of the columnar structure units and the dielectric substrate layer are both intrinsic single crystal silicon.

[0072] According to an embodiment of the present invention, the infrared focal plane detector includes a detector window 601 and an infrared focal plane array 602 arranged in sequence along the optical axis. The detector window is used to filter out the stray light of the system and the light outside the detection band, and the infrared focal plane array is used to detect and image the incident infrared light after focusing.

[0073] Preferably, the pixel spacing of the infrared focal plane array is 5 μm-17 μm, and the number of pixels or the resolution is 1280 × 960. Further, the pixel spacing of the infrared focal plane array is 17 μm, and the central wavelength is 10 μm. The detector window is made of infrared germanium glass material.

[0074] The materials of the first lens group, the second lens group and the detector window are one or any combination of germanium, barium fluoride, zinc sulfide, zinc selenide, AMTIR, IG and IRG. It should be noted that the first lens group, the second lens group and the detector window may also be made of other materials that can transmit long-wave infrared.

[0075] The first lens group includes a first lens 1 and a second lens 2, the second lens group includes a third lens 4 and a fourth lens 5, and the first lens 1, the second lens 2, the double cascade meta-lens 3, the third lens 4 and the fourth lens 5, and the infrared focal plane detector 6 are arranged in sequence from the object side to the image side along the optical axis.

[0076] The first lens is a negative lens, and the second lens, the third lens and the fourth lens are positive lenses; the negative lens and each positive lens are used to correct the primary aberration of the incident light and focus the light before emitting it, and the wavelength of the incident light is in the infrared band.

[0077] Preferably, the refractive surfaces of the first lens, the second lens, the third lens and the fourth lens are even-order aspheric surfaces, the materials of the first lens and the third lens are infrared germanium glass, and the materials of the second lens and the fourth lens are IRG207 chalcogenide glass material.

[0078] Combination Figure 3As shown, the detailed structural parameters of the first lens, the second lens, the third lens, and the fourth lens are optimized and determined by using the ray tracing method and the damped least square method. For specific values, see Table 2 and Table 3.

[0079] Table 2

[0080]

[0081] Table 3

[0082]

[0083] Figure 5 The point array diagram at the photosensitive surface of the infrared focal plane array simulated by the imaging system in this embodiment shows that the imaging spot diameters of the on-axis field of view, the band field of view, and the edge field of view are all smaller than the pixel spacing of the infrared focal plane array. This means that in this embodiment, the optical imaging resolution under the full field of view is higher than the resolution of the infrared focal plane array, which verifies that the embodiment of the present invention can achieve high-resolution infrared imaging in the full field of view.

[0084] Figure 6 This is an MTF curve diagram of the imaging system simulation in this embodiment. At the cutoff frequency of the infrared focal plane detector of 30lp / mm, the modulation transfer function MTF of the full field of view is greater than 0.6, which is close to the diffraction limit. Figure 7 This is a diagram of field curvature and distortion simulated by the imaging system in this embodiment. The imaging module has barrel distortion, and the relative distortion of the entire field of view is less than 25%. Figure 8 This is a simulated imaging diagram of the imaging system in this embodiment, which shows that the imaging system can achieve high-resolution infrared imaging.

[0085] Furthermore, in order to illustrate that selecting a columnar structural unit with high dispersion consistency can make the dispersion performance of the double-cascaded meta-lens consistent in the traditional optical element design optimization algorithm and in the metasurface element design algorithm, the same steps as in this embodiment are adopted to select conventional columnar structural units and high dispersion consistency structural units for periodic discretization and arrangement to construct two small-aperture single-wavelength focusing metalenses with an operating wavelength of 10 μm. Fig.13A , Fig. 13BThe near-field phase distribution profile and grayscale image of the rear surface of the metasurface focusing lens calculated by the finite difference time domain algorithm under 8μm, 10μm, and 12μm incident light for these two metalenses are shown respectively. The phase profile curve in the figure is normalized and represents the relative phase at different positions relative to the center. The metalens constructed by selecting conventional columnar structural units have poor dispersion distribution consistency due to the introduction of columnar structural units with different phase mutations, and the near-field phase distribution changes with the change of the wavelength of the incident light. The metalens constructed by selecting high dispersion consistency structural units have similar dispersion due to the introduction of columnar structures with different phase mutations, and the near-field phase distribution does not change with the change of the wavelength of the incident light. The dispersion performance is the same as the even-coefficient phase distribution polynomial obtained by optimizing the ray tracing algorithm and the damped least squares method in this embodiment, that is, the dispersion performance of the metalens is consistent in the traditional optical element design optimization algorithm and the metasurface element design algorithm.

[0086] In the embodiment of the present invention, firstly, by introducing a metasurface element (i.e., a double-cascaded meta-lens), a traditional lens structure of a large number of lenses is replaced, so that the imaging system tends to be miniaturized and lightweight. At the same time, by combining the traditional lens group, the system focal length is mainly distributed on the traditional refractive lens, which makes up for the shortcomings of the large-aperture meta-lens when used alone for imaging in a wide spectrum, so that the large-aperture meta-lens enters the infrared wide-spectrum imaging application field; secondly, the metasurface element adopts a double-cascade form, which doubles the number of optimization parameters, expands the optimization solution space to be sufficient to achieve the design goal, and strengthens the correction effect of high-order aberrations and off-axis aberrations, which is conducive to optimizing the field of view characteristics of the system. Further, the phase distribution of the metasurface microstructure array adopts the form of a high-order even-order polynomial, further expands the optimization solution space, further increases the number of optimization parameters to be sufficient to achieve the design goal, and this phase distribution form is convenient for rapid optimization using ray tracing and damped least squares method, which is compatible with the optimization algorithm of the traditional lens group and is suitable for the overall design of the composite optical system. In addition, according to the working band of the long-wave infrared meta-lens imaging module, the dispersion distribution of the periodic micro-nano structure unit in this band is solved by using the rigorous coupled wave analysis method, and the columnar structure units with phase mutations covering all 2π and dispersion distribution maintaining high consistency are selected to form a columnar structure unit library, and the periodic array of structural arrangement is selected from it, which effectively ensures that the actual phase distribution function of the meta-lens is well consistent with the high-order even-order polynomial function. Finally, the integration of the infrared imaging optical system and the infrared focal plane detector with small pixels and large arrays is realized, achieving high-resolution infrared detection imaging, and making the modulation transfer function of the optical system reach the diffraction limit.

[0087] In summary, the long-wave infrared meta-lens imaging system in the embodiment of the present invention uses a double-cascaded meta-lens to control the incident light wavefront, and achieves the effect of eliminating aberrations together with the traditional lens group. By selecting a columnar structural unit with high dispersion consistency, the meta-lens is suitable for large-aperture optical imaging systems; and a simple lens structure is used to achieve integration with a small pixel and large array infrared focal plane detector to achieve high-resolution infrared detection imaging; the modulation transfer function reaches the diffraction limit. It has the characteristics of simple structure, low cost, high resolution, and good imaging quality.

[0088] It will be easily understood by those skilled in the art that the above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims

1. A design method for a double-cascaded meta-lens, characterized in that: The double-cascade metalens is used in a long-wave infrared metalens imaging system, comprising a dielectric substrate layer and a metasurface located on both sides of the dielectric substrate layer, wherein the metasurface is composed of a plurality of columnar structural units arranged in a hexagonal lattice periodic array, and the method comprises: S1, using the rigorous coupled wave analysis method, calculate the phase mutation introduced by columnar structural units of different radii when incident with the characteristic wavelength of long-wave infrared, as well as the dispersion distribution in the long-wave infrared, and obtain multiple sets of corresponding radius-phase mutation-dispersion distribution to form a columnar structural unit library; S2, determining a target phase distribution of the metasurface hexagonal lattice periodic array with the goal of enabling the double-cascaded metalens to perform wavefront control on incident light and correct high-order aberrations of the incident light; S3, for any hexagonal lattice, selecting multiple groups of radius-phase mutation-dispersion distributions of corresponding phase mutations from the columnar structure unit library according to the target phase of the hexagonal lattice; S4, with the goal of achieving the highest consistency in the dispersion distribution of each hexagonal lattice in the same metasurface, determine the radius of the columnar structure unit at each hexagonal lattice from the results selected in S3; For a columnar structure unit of any radius, the calculation of its dispersion distribution in the long-wave infrared in S1 includes: Calculate the phase mutation introduced by the columnar structure unit of any radius when the maximum wavelength of the long-wave infrared is incident, and the phase mutation introduced when the minimum wavelength of the long-wave infrared is incident; The difference between the phase mutation corresponding to the maximum wavelength and the phase mutation corresponding to the minimum wavelength is calculated to obtain the dispersion distribution.

2. The design method of the double-cascaded meta-lens according to claim 1, characterized in that: The radius of the columnar structural unit in S1 satisfies: the phase mutations corresponding to all radii cover 0-2π.

3. A long-wave infrared metalens imaging system, characterized in that: include: A first lens group, a double cascade meta-lens, a second lens group and an infrared focal plane detector are arranged in sequence from the object side to the image side along the optical axis, wherein the double cascade meta-lens is obtained by the design method of the double cascade meta-lens as claimed in claim 1 or 2.

4. The long-wave infrared metalens imaging system according to claim 3, characterized in that: The focal lengths of the two metasurfaces of the double-cascade metalens and the focal length of the long-wave infrared metalens imaging system satisfy: f1 / f>10 f2 / f>10 Among them, f1 is the focal length of one metasurface of the double cascade metalens, f2 is the focal length of the other metasurface of the double cascade metalens, and f is the focal length of the long-wave infrared metalens imaging system.

5. The long-wave infrared metalens imaging system according to claim 3, characterized in that: The first lens group includes a first lens and a second lens, the second lens group includes a third lens and a fourth lens, and the first lens, the second lens, the double-cascade meta-lens, the third lens and the fourth lens are arranged in sequence from the object side to the image side along the optical axis; The first lens is a negative lens, and the second lens, the third lens and the fourth lens are positive lenses; the negative lens and each of the positive lenses are used to correct the primary aberration of the incident light and focus the light before emitting it.

6. The long-wave infrared metalens imaging system according to claim 3, characterized in that: The height of each columnar structure unit in the metasurface of the double-cascade metalens is the same, and the height is between the order of magnitude of the infrared wavelength detected, and the radius is between the order of magnitude of the sub-wavelength.

7. The long-wave infrared metalens imaging system according to claim 3, characterized in that: In the metasurface of the double-cascade metalens, the materials of the columnar structure units and the dielectric substrate layer are both intrinsic single crystal silicon.

8. The long-wave infrared metalens imaging system according to any one of claims 3 to 7, characterized in that: The infrared focal plane detector comprises a detector window and an infrared focal plane array arranged in sequence along the optical axis; The materials of the first lens group, the second lens group and the detector window are one or any combination of germanium, barium fluoride, zinc sulfide, zinc selenide, AMTIR, IG and IRG.

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