Meta-lens-based image inverter design method, image inverter and low-light level night vision device

Abstract

The invention discloses an image inverter design method based on a super-structure lens, an image inverter thereof and a low-light level night vision device. The method comprises the following steps: S1, structural unit parameter design; s2, optimizing an array arrangement structure; s3, calculating a phase function; s4, double-lens combination design is carried out; and step S5: process preparation. The thickness of a single lens is less than or equal to 1mm, the total thickness of two combined lenses is less than or equal to 10mm, and the weight is reduced by 30% compared with that of a traditional lens group; aberration optimization: the sub-wavelength structure unit inhibits dispersion and spherical aberration, and the central imaging resolution is improved from 68lp / mm to 80lp / mm; the integration advantage is that: the planar structure can be packaged with an image sensor wafer level, and is suitable for compact equipment such as low-light night vision devices. The super-structure lens combination designed by the method realizes 180-degree inversion of an input image, effectively reduces the size and weight of an image inverter and improves the integration degree while ensuring the imaging quality, and is suitable for equipment such as a low-light night vision device and the like.

Description

Technical Field

[0001] The present application relates to the technical field of image inverters, and in particular to a design method of an image inverter based on a meta-lens, an image inverter, and a low-light-level night vision device. Background Art

[0002] The traditional image invertor (such as prism image inverting system, lens group image inverting system) is a fiber optic image invertor, and the imaging principle is as follows Figure 1 As shown, the incident light enters the fiber optic image inverter from the input positive image side, is processed, and outputs an inverted image. The existing fiber optic image inverter obtains the incident light through the incident light surface and outputs the light through the output light surface to form an inverted image.

[0003] Existing fiber-optic image invertors have the following drawbacks: large size and weight (relying on a combination of multiple optical components, making them difficult to meet the demands of miniaturized devices), poor image quality (spherical and chromatic aberrations introduced during refraction or reflection affect image quality), and low integration (independent assembly of components, requiring high optical path alignment precision and high manufacturing costs). Metasurface lenses achieve light field manipulation through phase control of subwavelength structural units, offering advantages such as planarity, lightweight design, and multifunctional integration. However, existing single metasurface lenses cannot directly achieve 180° image inversion; instead, multiple lenses must be combined to optimize the phase distribution, resulting in a complex device structure, excessive overall mass, and low integration.

[0004] The information disclosed in the background section is only intended to enhance understanding of the overall background of the invention and should not be considered as an admission or any form of suggestion that the information constitutes the prior art already known to a person of ordinary skill in the art. Summary of the Invention

[0005] In response to the above technical problems, the present application provides a method for designing an image inverter based on a meta-lens and an image inverter thereof. Through reasonable design, two metalenses can be used to replace the fiber image inverter to achieve the function of 180° inversion. By designing the material, size and unit structure parameters of the meta-lens, the problem of large size and weight of the image inverter used in the existing image intensifier can be solved. The invention has the advantages of small size, light weight and strong designability.

[0006] The present application provides a method for designing an image inverter based on a meta-lens and an image inverter thereof, comprising the following steps:

[0007] Step S1: Structural unit parameter design: Based on the target operating wavelength, the refractive index dispersion characteristics of the metalens are calculated, and the size of the unit structure is determined based on the phase delay requirement of the incident light;

[0008] Step S2: Array arrangement structure optimization: Using a hexagonal close array model, the array period and unit spacing of each unit structure in the hexagonal close array model are optimized through finite element simulation. The optimization index is that the obtained structure can reduce light scattering loss and improve phase control accuracy;

[0009] Step S3: Phase function calculation: Based on the inverted image requirements, a hyperbolic phase function or a quadratic phase function is selected. Combined with the preset focal length and field of view of the metalens, the phase distribution matrix of the metalens in the hyperbolic phase or quadratic phase is calculated respectively.

[0010] Step S4: Dual-lens combination design: Based on the requirements that the wavefront curvature of the preceding metalens matches the wavefront compensation of the succeeding metalens and can achieve 180° inverted imaging, the spacing and relative rotation angle of the stacked preceding and succeeding metalenses are determined by ray tracing.

[0011] Step S5: Process preparation: Electron beam lithography combined with reactive ion etching is used to prepare a unit structure array on a substrate according to the size of the obtained unit structure, the array period and unit spacing of each unit structure, and the phase distribution matrix, thereby obtaining a front-stage metalens and a rear-stage metalens. According to the spacing and relative rotation angle of the front-stage metalens and the rear-stage metalens, the front-stage metalens and the rear-stage metalens are encapsulated to obtain an image inverter based on the metalens.

[0012] Preferably, the finite element simulation method is the FDTD method.

[0013] Preferably, the target operating wavelength is the near-infrared band of 850 nm;

[0014] Preferably, the hyperbolic phase function is:

[0015] φ(r)=α*ln(r / r0), where r is the radial distance, α is the phase modulation coefficient, and r0 is the reference radius;

[0016] Preferably, the quadratic phase function is: φ(r)=β*r2, where β is the quadratic phase curvature coefficient.

[0017] Preferably, the preset focal length of the meta-lens is 5 mm, and the preset field of view angle is 10°;

[0018] The unit structure is a cylinder; the determined unit structure dimensions are the diameter and height of the unit structure.

[0019] Another aspect of the present application provides an image invertor obtained by the above-mentioned design method, comprising: a front-stage meta-lens and a rear-stage meta-lens; the front-stage meta-lens and the rear-stage meta-lens are coaxially arranged along the optical axis; the front-stage meta-lens and the rear-stage meta-lens have the same focal length;

[0020] The spacing between the front-stage metalens and the rear-stage metalens is 0.5 to 2 times the focal length of the front-stage metalens and the rear-stage metalens. Through phase compensation and wavefront inversion, 180° inverted imaging of the input image is achieved; an array-arranged metastructure unit structure is set on the object-side surface of the front-stage metalens; an array-arranged metastructure unit structure is set on the image-side surface of the rear-stage metalens; the arrangement direction of the metastructure unit structure is perpendicular to the optical axis.

[0021] Preferably, the front-stage meta-lens and the rear-stage meta-lens have the same structure, both comprising: a substrate, a meta-lens unit structure array, and a phase control layer;

[0022] The meta-lens unit structure array is arranged on a substrate; the meta-lens unit structure array includes: a plurality of unit structures arranged in an array;

[0023] Phase control layer: The metalens uses hyperbolic phase function or quadratic phase function to perform phase design to achieve wavefront control of the incident light.

[0024] Preferably, the substrate sidewall of the preceding meta-lens is coupled and packaged with the substrate sidewall of the following meta-lens.

[0025] Preferably, the material used for the unit structure is any one of Si3N4, TiO2, GaN or ZnS;

[0026] Preferably, the shape of the unit structure is cylinder, rectangle, or cross;

[0027] Preferably, the substrate material is K9 glass / 7056 glass / SiO2; and the substrate thickness is 0.5-2 mm.

[0028] Preferably, the cross section of the unit structure is hexagonal or circular;

[0029] Preferably, when the unit structure is a cylinder, the diameter of the unit structure is 50-200 nm, the height is 200-800 nm, and the array period is 100-300 nm.

[0030] Another aspect of the present application provides a low-light-level night vision device, comprising: an image inverter based on a meta-lens as described above.

[0031] The beneficial effects of this application include:

[0032] 1) The present application provides a method for designing an image inverter based on a meta-lens and an image inverter thereof. The size and weight of the device designed using this method are reduced: the thickness of a single lens is ≤1mm, and the total thickness of the two lenses combined is ≤10mm, which is 30% lighter than the traditional lens group; aberration optimization: the subwavelength structural unit suppresses dispersion and spherical aberration, and the central imaging resolution is increased from 68lp / mm to 80lp / mm; integration advantage: the planar structure can be packaged with the image sensor wafer level, suitable for use in compact devices such as low-light-level night vision devices.

[0033] 2) The image inverter design method based on meta-lens and the image inverter provided in this application realize the image inversion function through the reasonable combination of two meta-lenses. The structural units are arranged on the substrate. During the design process, the hyperbolic phase or quadratic phase calculation method is used to achieve a continuous phase change of 0-2π by changing the diameter of the structural unit, while ensuring a high transmittance. The hyperbolic phase meta-lens has good focusing performance at vertical incidence, and the quadratic phase meta-lens is not very sensitive to the incident angle and can achieve better focusing within a large angle range. The present invention also includes a design method for the image inverter, which ensures that the performance of the image inverter meets the actual application requirements from a series of steps such as parameter determination, structural unit design, lens design, layout generation to experimental verification. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] Figure 1 Schematic diagram of the structure and inverted image of a traditional fiber optic image inverter;

[0035] Figure 2 The structure of the metalens image invertor and the schematic diagram of the inverted image provided by this application;

[0036] Figure 3 Schematic diagram of the unit structure provided for this application;

[0037] Figure 4 This is a graph showing the relationship between phase and unit diameter provided by this application. The graph was obtained with an incident wavelength of 850nm. The unit structure has a period of P = 150nm and a height of H = 500nm.

[0038] Figure 5 This is a graph showing the dependence of transmittance on the diameter of the structural unit provided in this application. The graph was obtained under the condition of an incident wavelength of 850nm. The period P of the unit structure is 150nm, and the height H is 500nm.

[0039] Figure 6 This is the hyperbolic phase arrangement diagram provided by this application; the conditions for obtaining this diagram are the focus contrast of the radiation angle θ = 0, 10° and 30. The first row is the electric field intensity |E| in the xz plane 2 Distribution diagram; the second row is |E| in the focal plane (xy plane) 2 Distribution diagram; the third line is the focus |E|2 With the change of position coordinate x;

[0040] Figure 7 This is the secondary phase arrangement diagram provided by this application; the conditions for obtaining this diagram are the focus contrast at incident angles θ = 0, 10°, and 30°. The first row shows the electric field intensity |E| in the xz plane. 2 Distribution diagram; the second row is |E| in the focal plane (xy plane) 2 Distribution diagram; the third line is the focus |E| 2 With the change of position coordinate x;

[0041] Figure 8 The phase distribution diagram required for achieving focusing provided in this application; where λ = 850nm, f = 10mm, n bg =1.5,NA=0.8;

[0042] Figure 9 Schematic diagram of the metalens with hexagonal unit structure provided by this application;

[0043] Figure 10 A graph showing the relationship between the numerical aperture and metalens thickness for different metalens diameters provided in this application;

[0044] Figure 11 This is an electron microscope image of the meta-lens designed and processed by the method provided in this application;

[0045] Figure 12 This is an imaging effect diagram of the meta-lens designed and processed by the method provided in this application. DETAILED DESCRIPTION

[0046] To make the objectives, technical solutions, and advantages of the embodiments of the present invention more clear, the technical solutions of the embodiments of the present invention will be clearly and completely described below in conjunction with the accompanying drawings of the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Generally, the components of the embodiments of the present invention described and shown in the drawings herein can be arranged and designed in various different configurations.

[0047] Therefore, the following detailed description of the embodiments of the present invention provided in the accompanying drawings is not intended to limit the scope of the invention as claimed, but rather merely represents selected embodiments of the present invention. All other embodiments derived by persons of ordinary skill in the art based on the embodiments of the present invention without creative effort are also within the scope of protection of the present invention.

[0048] The technical means that are not described in detail in this application and are not used to solve the technical problems of this application are all set according to the common knowledge in this field, and can be implemented in a variety of common knowledge settings.

[0049] The waveguide optical device is described in detail below with reference to specific embodiments and accompanying drawings:

[0050] Take the 850nm near-infrared band image invertor as an example:

[0051] Step S1: Structural unit parameter design: Based on the target operating wavelength (e.g., 850nm near-infrared band), the refractive index dispersion characteristics of the Si3N4 material are calculated, and the diameter and height of the cylindrical structural unit are determined so that it produces a phase delay for the incident light;

[0052] Step S2: Array arrangement optimization: Using a hexagonal tight array model, the array period and unit spacing are optimized through finite element simulation (such as FDTD method) to reduce light scattering loss and improve phase control accuracy;

[0053] (1) Structural unit parameters:

[0054] like Figure 3 As shown, the unit structure is set on the substrate; the diameter of the unit structure D = 100nm, the height H = 500nm (determined by FDTD simulation, at this time the phase delay Δφ = π

[0055] @850nm), array period P = 150nm (hexagonal center spacing), filling factor ≥ 70%. The relationship between the unit structure diameter and its corresponding phase is as follows Figure 4 As shown; Figure 4 The acquisition conditions are: incident wavelength of 850nm, unit structure period P = 150nm, height H = 500nm. The unit structure period P refers to the length of the unit structure on the substrate, which is the distance between the outer wall of the unit structure and the outer wall of the adjacent unit structure;

[0056] To ensure uniform imaging, the transmittance of each unit structure needs to be consistent. The relationship between the unit structure diameter and transmittance is as follows: Figure 5 As shown, Figure 5 The test conditions are: incident wavelength of 850nm; unit structure period P = 150nm; height H = 500nm. When the unit structure diameter D varies between 45nm and 255nm, the transmittance of each unit structure varies between 0.97 and 0.98.

[0057] Step S3: Phase function calculation: According to the inverted image requirements, select the hyperbolic phase or quadratic phase function, combine the lens focal length (such as 5mm) and the field of view angle (such as 10°), and calculate the phase distribution matrix;

[0058] (2) Phase function calculation:

[0059] Calculate the phase distribution according to the hyperbolic phase calculation formula φ(r)=α*ln(r / r0), and select the appropriate structural unit arrangement based on the calculation results. Figure 6 As shown in the figure, simulation analysis shows that the focal position is essentially consistent with the design, with a good focal spot shape, approximately 40% focusing efficiency, and a focal peak width at half maximum of 356nm. However, focusing performance degrades at oblique incidence, such as at angles of 10° and 30°. The focal length also varies with wavelength, with a change of approximately 1.6λ for a 20nm wavelength change, and the dispersion is minimal.

[0060] According to the quadratic phase calculation formula φ(r)=β*r 2 Calculate the phase distribution and select the structural unit. Figure 7 As shown in the figure, at normal incidence, the focal spot width at half maximum is 356nm, the same as the hyperbolic phase focal spot size, but with a focusing efficiency of 20%. When the incident light angle reaches 30°, the focusing performance remains excellent, with the focal spot size remaining essentially unchanged, while the focal position increases with increasing incident angle. Its dispersion is comparable to that of the hyperbolic phase for different wavelengths, and its modulation transfer function is high, enabling higher imaging resolution.

[0061] In summary, the quadratic phase function is selected because it has a higher modulation transfer function than the hyperbolic phase function and can achieve higher imaging resolution, so β = 2π × 10 6 m -2 , corresponding to the focal length f = 5mm, which satisfies the phase distribution required for focusing. Figure 8 As shown;

[0062] like Figure 9 As shown, a phase distribution matrix of 500×500 pixels is generated, and each pixel corresponds to the phase control value of a structural unit.

[0063] Step S4: Dual-lens combination design: Use ray tracing to determine the spacing and relative rotation angle between the two meta-lenses, ensuring that the wavefront curvature of the preceding lens matches the wavefront compensation of the succeeding lens, achieving 180° inverted imaging.

[0064] (3) Double lens combination:

[0065] According to the actual use scenario requirements, the diameter of the image invertor is determined to be 18mm. Considering the processing difficulty and imaging quality factors, the numerical aperture NA of the meta-lens is initially selected to be 0.8. Figure 10 For example, when NA = 0.8, the thickness of the image invertor is about 13.5 mm, from which the focal length of the lens can be determined.

[0066] In this embodiment, the phase distribution of the front lens is positive quadratic phase (converging wavefront), and the rear lens is negative quadratic phase (diverging wavefront), and the spacing is set to 10mm (twice the focal length);

[0067] Through Zemax optical simulation verification, the input is an upright "F" image, and the output is an inverted "F" image. The edge distortion is less than 5%, and the imaging effect is as follows Figure 2 shown.

[0068] Step S5: Process preparation: Electron beam lithography combined with reactive ion etching technology is used to prepare a Si3N4 structural unit array on the substrate to complete the processing and packaging of the meta-lens.

[0069] (4) Preparation process:

[0070] Electron beam lithography was used to define the cell array pattern on the K9 substrate;

[0071] Reactive ion etching (RIE) was used to etch Si3N4 film (thickness 500nm) at an etching rate of 50nm / min. The prepared superlens was as follows Figure 11 As shown in Figure 2, multiple unit structures are arranged on the surface of the super lens.

[0072] After plasma debonding, the double-sided anti-reflection film (reflectivity <1%@850nm) is evaporated and cut into samples with the same diameter as the fiber optic image inverter.

[0073] The device designed using this method has reduced size and weight: the thickness of a single lens is ≤1mm, and the total thickness of the two lenses combined is ≤10mm, which is 30% lighter than the traditional lens group; aberration optimization: the subwavelength structural unit suppresses dispersion and spherical aberration, and the central imaging resolution is increased from 68lp / mm to 80lp / mm; integration advantage: the planar structure can be packaged with image sensors at the wafer level, suitable for use in compact devices such as low-light-level night vision devices.

[0074] Another aspect of the present application provides a low-light-level night vision device, including: an image invertor based on the metalens as described above. The metalens obtained by the above method is encapsulated to form an image invertor, which is then incorporated into a low-light-level night vision device with an existing structure for imaging. The resulting imaging effect is shown in Figure 12 . As can be seen from the figure, the metalens designed by the method provided by the present application, combined with the image invertor, can achieve inverted imaging.

[0075] Although the present invention has been described in detail with reference to the aforementioned embodiments, it is still possible for those skilled in the art to modify the technical solutions described in the aforementioned embodiments, or to make equivalent substitutions for some of the technical features therein. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A method for designing an image invertor based on a meta-lens, characterized in that: The following steps are involved: Step S1: Structural unit parameter design: Based on the target operating wavelength, the refractive index dispersion characteristics of the metalens are calculated, and the size of the unit structure is determined based on the phase delay requirement of the incident light; Step S2: Array arrangement structure optimization: Using a hexagonal close array model, the array period and unit spacing of each unit structure in the hexagonal close array model are optimized through finite element simulation. The optimization index is that the obtained structure can reduce light scattering loss and improve phase control accuracy; Step S3: Phase function calculation: Based on the inverted image requirements, a hyperbolic phase function or a quadratic phase function is selected. Combined with the preset focal length and field of view of the metalens, the phase distribution matrix of the metalens in the hyperbolic phase or quadratic phase is calculated respectively. Step S4: Dual-lens combination design: Based on the requirements that the wavefront curvature of the preceding metalens matches the wavefront compensation of the succeeding metalens and can achieve 180° inverted imaging, the spacing and relative rotation angle of the stacked preceding and succeeding metalenses are determined by ray tracing. Step S5: Process preparation: Electron beam lithography combined with reactive ion etching is used to prepare a unit structure array on a substrate according to the size of the obtained unit structure, the array period and unit spacing of each unit structure, and the phase distribution matrix, thereby obtaining a front-stage metalens and a rear-stage metalens. According to the spacing and relative rotation angle of the front-stage metalens and the rear-stage metalens, the front-stage metalens and the rear-stage metalens are encapsulated to obtain an image inverter based on the metalens.

2. The method for designing an image invertor based on a meta-lens and the image invertor thereof according to claim 1, characterized in that: The finite element simulation method is the FDTD method.

3. The method for designing an image inverter based on a meta-lens and the image inverter thereof according to claim 1, characterized in that: The target operating wavelength is the near-infrared band of 850nm; Preferably, the hyperbolic phase function is: φ(r)=α*ln(r / r0), where r is the radial distance, α is the phase modulation coefficient, and r0 is the reference radius; Preferably, the quadratic phase function is: φ(r)=β*r2, where β is the quadratic phase curvature coefficient.

4. The method for designing an image inverter based on a meta-lens and the image inverter thereof according to claim 1, characterized in that: The preset focal length of the meta-lens is 5mm, and the preset field of view angle is 10°; The unit structure is a cylinder; the determined unit structure dimensions are the diameter and height of the unit structure.

5. An image invertor obtained by the design method according to any one of claims 1 to 4, characterized in that: include: Front-stage meta-lens, rear-stage meta-lens; The front meta-lens and the rear meta-lens are coaxially arranged along the optical axis; the front meta-lens and the rear meta-lens have the same focal length; The distance between the front meta-lens and the back meta-lens is 0.5 to 2 times the focal length of the front meta-lens and the back meta-lens. Through phase compensation and wavefront inversion, 180° inverted imaging of the input image is achieved. An array of meta-unit structures is provided on the object side of the front-stage meta-lens; an array of meta-unit structures is provided on the image side of the rear-stage meta-lens; and the arrangement direction of the meta-unit structures is perpendicular to the optical axis.

6. The image invertor based on a meta-lens according to claim 5, characterized in that: The front-stage meta-lens and the rear-stage meta-lens have the same structure, both including: a substrate, a meta-lens unit structure array, and a phase control layer; The meta-lens unit structure array is arranged on a substrate; the meta-lens unit structure array includes: a plurality of unit structures arranged in an array; Phase control layer: The metalens uses hyperbolic phase function or quadratic phase function to perform phase design to achieve wavefront control of the incident light.

7. The image invertor based on a meta-lens according to claim 6, characterized in that: The substrate sidewall of the front-stage meta-lens is coupled and packaged with the substrate sidewall of the rear-stage meta-lens.

8. The image invertor based on a meta-lens according to claim 6, characterized in that: The material used for the unit structure is any one of Si3N4, TiO2, GaN or ZnS; Preferably, the shape of the unit structure is cylinder, rectangle, or cross; Preferably, the substrate material is K9 glass / 7056 glass / SiO2; and the substrate thickness is 0.5-2 mm.

9. The image invertor based on a meta-lens according to claim 8, characterized in that: The cross section of the unit structure is hexagonal or circular; Preferably, when the unit structure is a cylinder, the diameter of the unit structure is 50-200 nm, the height is 200-800 nm, and the array period is 100-300 nm.

10. A low-light-level night vision device, characterized in that: include: The image invertor based on a meta-lens according to any one of claims 5 to 9.

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