Diffraction-angle-invariant super-structure grating suitable for double-background refractive index

Abstract

The invention discloses a diffraction angle invariable super-structure grating suitable for double background refractive indexes. The diffraction angle invariable super-structure grating comprises a nano-column unit and a substrate unit, the plurality of nano-column units are sequentially arranged on the substrate unit according to a set period length, each nano-column unit comprises a plurality of rectangular nano-columns, and gaps exist among the nano-columns. According to the scheme, on the basis of the design principle of a grating equation, the diffraction series, the period length and the incident light wavelength of the super-structure grating are optimally selected according to different background media, optimal structure parameters are designed by combining an optical simulation tool and an optimization algorithm, and finally the function of keeping the same diffraction angle in air and water environments is achieved. According to the super-structure grating, efficient diffraction and deflection of light beams in different backgrounds are achieved by adjusting the sub-wavelength periodic structure of the super-structure grating, meanwhile, the super-structure grating has the flexible design freedom degree, and the light beams can be deflected to the specified target angle.

Description

Technical Field

[0001] The invention belongs to the field of optics, and in particular relates to a diffraction angle-invariant metagrating suitable for double-background refractive indices. Background Art

[0002] Metamaterials are artificial materials with precisely designed subwavelength structures that can achieve unique optical properties that natural materials cannot have, such as negative refractive index. Metasurfaces are two-dimensional subwavelength unit structures developed on the basis of metamaterials, which have the advantages of simple structural design and excellent wavelength control capabilities. However, in some extreme cases, the light field control capabilities of metasurfaces are limited. For example, when the deflection angle of the light beam is too large, more incident light will be guided to other stray directions, resulting in a significant decrease in the deflection efficiency of the light wave. In order to solve the problems of metasurfaces in deflection efficiency and manufacturing accuracy, metagratings that combine the modulation characteristics of metamaterials and metasurfaces with the periodic coupling behavior of gratings have been proposed. With its outstanding performance in large-angle and high-efficiency beam deflection, metagratings have gradually been applied to laser radar, optical communications and other fields.

[0003] The design of metagratings can adopt traditional mode coupling theory and full-wave simulation methods. Although this method can accurately describe the physical model, the design process is relatively cumbersome. With the development of artificial intelligence technology, inverse design algorithms and deep learning algorithms based on neural networks have gradually been introduced into the research of metagratings. These methods optimize the microstructure and material properties, so that metagratings can achieve efficient deflection at specific diffraction orders, while maintaining high diffraction efficiency in a broadband range, and achieve flexible and precise control of light waves. Metagratings thus provide an effective platform for precise modulation of large-angle extreme light fields. However, current research is mostly focused on a single background refractive index environment, ignoring the effect of background refractive index changes on diffraction efficiency and diffraction angle. In fact, when the background refractive index changes, the diffraction performance of the metagrating will also change, which has important research significance in fields such as grating sensors and biomedicine. Therefore, the development of metagratings that adapt to multiple background refractive index environments is still a technical problem that needs to be overcome. Summary of the invention

[0004] In view of the above problems, the purpose of the present invention is to provide a diffraction angle invariant metagrating suitable for dual background refractive index, which can maintain the same diffraction angle in two medium environments and has a high diffraction efficiency. In addition, the metagrating can also flexibly deflect the light beam to the desired target angle to meet diverse application requirements.

[0005] The specific technical solution for achieving the purpose of the present invention is:

[0006] A diffraction angle invariant metagrating suitable for dual background refractive index, comprising a nano-pillar unit and a substrate unit;

[0007] The plurality of nanocolumn units are sequentially arranged on the substrate unit according to a set period length;

[0008] The nanocolumn unit includes a plurality of rectangular nanocolumns, and there are gaps between the nanocolumns;

[0009] Furthermore, the metagrating sets a suitable period length d, diffraction orders m1 and m2, and wavelength λ of the incident wave, so that the incident wave of the metagrating is deflected by the same diffraction angle in different background media:

[0010]

[0011] Among them, θ m1 represents the diffraction angle of the m1th diffraction order in air, θ m2 It represents the diffraction angle of the m2th diffraction order in water. The same diffraction angle is θ m1 =θ m2 ; n1 represents the refractive index of air, n2 represents the refractive index of water; θ i is the incident angle of the light source, n i is the refractive index of the medium on the incident light side, d represents the period length of the nanocolumn unit of the metagrating, and λ is the wavelength of the incident wave in vacuum.

[0012] Furthermore, the nanocolumn unit includes three rectangular nanocolumns, the widths of the nanocolumns are W1, W2, and W3, the heights are H1, H2, and H3, and the widths of the gaps between the nanocolumns are G1, G2, and G3, respectively;

[0013] The width, height and gap width of the nanorods are optimized using an optimization algorithm to achieve optimal diffraction efficiency.

[0014] Furthermore, the width, height and gap width of the nanopillars are defined as the particle parameters in the particle swarm algorithm, and the diffraction efficiency of the metagrating in air and water environments under each parameter state is determined in the iteration, which are denoted as η air and η water , the particle parameters are optimized based on the adaptive function:

[0015]

[0016] Among them, α and β are normalized proportional coefficients.

[0017] Furthermore, the substrate unit is made of SiO2, and the nanocolumn unit is made of TiO2.

[0018] Furthermore, the period length of the nanocolumn unit is set to be 2000nm.

[0019] Compared with the prior art, the present invention has the following beneficial effects:

[0020] This solution is different from the traditional metagrating that is only applicable to a single working environment (such as air). The dielectric metagrating designed by the present invention can keep the diffraction angle unchanged in a dual background refractive index environment (air and water) to avoid interference from complex environments. The design has a simple structure and a higher degree of freedom. It can not only achieve precise control of a specific diffraction angle, but also flexibly deflect the light beam to any desired target angle, thereby significantly expanding its application range and practical value.

[0021] The present invention is further described below in conjunction with specific implementation modes. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 This is a schematic diagram of the diffraction angle invariant metagrating structure suitable for dual background refractive index according to the present invention.

[0023] Figure 2 It is a schematic diagram of the nanocolumn parameter optimization process of the present invention.

[0024] Figure 3 Schematic diagram of the electric field distribution of the transmitted light of the metagrating in the embodiment of the present invention in air under the incidence of x-polarized light.

[0025] Figure 4 Schematic diagram of the electric field distribution of the transmitted light of the metagrating in water under the incident x-polarized light in an embodiment of the present invention.

[0026] Figure 5 The diffraction effect of the metagrating in the embodiment of the present invention in air and water, including all diffraction orders and corresponding diffraction angles.

[0027] Figure 6 It is a schematic diagram of the FDTD far-field projection of the metagrating in the air in an embodiment of the present invention.

[0028] Figure 7 It is a schematic diagram of the FDTD far-field projection of the metagrating in water in an embodiment of the present invention. DETAILED DESCRIPTION

[0029] Example

[0030] The technical solutions in the embodiments of the present invention will be described clearly and completely below in conjunction with the accompanying drawings in the embodiments of the present invention. The described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0031] As shown in this application and claims, unless the context clearly indicates an exception, the words "a", "an", "an kind" and / or "the" do not refer to the singular and may also include the plural. Generally speaking, the terms "include" and "comprise" only indicate the inclusion of the steps and elements that have been clearly identified, and these steps and elements do not constitute an exclusive list. The method or device may also include other steps or elements.

[0032] Unless otherwise specifically stated, the relative arrangement, numerical expressions and numerical values ​​of the parts and steps set forth in these embodiments do not limit the scope of the present application. At the same time, it should be understood that, for ease of description, the sizes of the various parts shown in the accompanying drawings are not drawn according to the actual proportional relationship. The technology, method and equipment known to those of ordinary skill in the relevant field may not be discussed in detail, but in appropriate cases, the technology, method and equipment should be considered as a part of the authorization specification. In all examples shown and discussed here, any specific value should be interpreted as being merely exemplary, rather than as a limitation. Therefore, other examples of exemplary embodiments may have different values. It should be noted that similar reference numerals and letters represent similar items in the following drawings, so that once a certain item is defined in an accompanying drawing, it does not need to be further discussed in subsequent drawings.

[0033] Combination Figure 1 , a diffraction angle invariant metagrating suitable for dual background refractive index, comprising a nano-pillar unit and a substrate unit;

[0034] The plurality of nanocolumn units are sequentially arranged on the substrate unit according to a set period length;

[0035] The nanocolumn unit includes a plurality of rectangular nanocolumns, and there are gaps between the nanocolumns;

[0036] In this embodiment, the wavelength of the incident light is 620nm, and the metagrating is composed of a SiO2 (refractive index is 1.458) substrate and three TiO2 (refractive index is 2.6) rectangular nanocolumns. To reduce the difficulty of manufacturing, the unit structure period along the x direction is set to 2000nm.

[0037] The metagrating is configured with appropriate period length d, diffraction orders m1 and m2, and wavelength λ of the incident wave, so that the incident wave of the metagrating is deflected by the same diffraction angle under different background media:

[0038]

[0039] Among them, θ m1 represents the diffraction angle of the m1th diffraction order in air, θ m2 It represents the diffraction angle of the m2th diffraction order in water. The same diffraction angle is θ m1=θ m2 ; n1 represents the refractive index of air, n2 represents the refractive index of water; θ i is the incident angle of the light source, n i is the refractive index of the medium on the incident light side. In this embodiment, the incident light is directly coupled to the grating substrate, so the refractive index of the medium on the incident light side is the refractive index of the grating silicon dioxide substrate 1.458, d represents the period length of the nanocolumn unit of the metagrating, and λ is the wavelength of the incident wave in vacuum.

[0040] The metagrating of the present invention can achieve that the incident light can be deflected to the same diffraction angle regardless of how the background medium changes. In addition, the metagrating has a high degree of design flexibility and can accurately deflect the light beam to any desired target angle to meet diverse application requirements.

[0041] In actual operation, first, the diffraction angle of the metagrating in two environments is calculated according to the formula. Under the condition of determining the wavelength and period length of the incident light, in order to achieve the same diffraction angle in different media, it is found that the diffraction order of the metagrating in the air environment is -1, and in the water environment is 0. When the incident light angle is set to 58°, the diffraction angle of the metagrating in both the air and water background media is 68;

[0042] The nanocolumn unit includes three rectangular nanocolumns, the widths of the nanocolumns are W1, W2, and W3, the heights are H1, H2, and H3, and the widths of the gaps between the nanocolumns are G1, G2, and G3, respectively;

[0043] The width, height and gap width of the nanorods are optimized using an optimization algorithm to achieve optimal diffraction efficiency.

[0044] like Figure 2 As shown in the figure, by combining the particle swarm optimization algorithm with optical simulation software, the finite-difference time-domain (FDTD) method is used to calculate the diffraction efficiency of the metagrating in air and water media. The fitness function is used to evaluate the performance of the generated metagrating, and weights are assigned to each parameter according to the design goals to optimize its diffraction characteristics. In the process of multiple iterations, the efficiency of the target diffraction order is gradually improved, while other diffraction orders are effectively suppressed to ensure that the incident light beam can be accurately deflected to the specified diffraction angle. Finally, as the algorithm gradually converges, the optimal structural parameters that meet the design requirements are obtained, achieving the design goal of having a high diffraction efficiency while having the same diffraction angle in the two media, and achieving the expected effect.

[0045] Specifically, the width, height and gap width of the nanopillars are defined as the particle parameters in the particle swarm algorithm, and the diffraction efficiency of the metagrating in air and water environments under each parameter state is determined in the iteration, which are denoted as η air and η water , the particle parameters are optimized based on the adaptive function:

[0046]

[0047] In each iteration, the structural parameters are updated according to the adaptive function. Finally, as the algorithm converges, the optimal structural parameters are output to meet the design goal of high diffraction efficiency while keeping the diffraction angle of the metagrating consistent in the two media.

[0048] Where α and β are normalized proportional coefficients, which control the optimization of diffraction efficiency and the minimization of the difference between the diffraction efficiencies of the two media, respectively. The first term of the adaptive function |η air +η water | aims to maximize the overall diffraction efficiency of the metagrating in air and water and optimize its performance in the two media; while the second term |η air -η water |Aims to reduce the difference between the diffraction efficiencies in the two media, thereby achieving uniform performance of the metagrating in different environments. The proportional coefficients α and β significantly affect the convergence speed and the final diffraction efficiency value. After multiple iterations and comparisons, the optimal combination of α = 0.6 and β = 0.4 was finally determined to maximize the diffraction efficiency and minimize the difference in diffraction efficiency between the two media.

[0049] Finally, this embodiment provides a meta-grating, which achieves efficient diffraction performance in air and water under the conditions of incident light of 620nm wavelength, structural period of 2000nm, and incident angle of 58°. Specifically, the meta-grating of the present invention achieves a -1 order diffraction efficiency of 70% in the air medium and a 0th order diffraction efficiency of 61.5% in the water medium. At the same time, in both media, the grating maintains the designed main diffraction angle of 68°, showing good environmental adaptability and stability.

[0050] Figure 3 and Figure 4 The electric field distribution of the transmitted light of the metagrating of this embodiment in air and water medium under the incident x-polarized light is shown respectively. The left column of each sub-figure represents the component of the electric field in the x direction, and the right column represents the component of the electric field in the z direction. The lines inserted in the figure mark the deflection direction of the light beam, which is completely consistent with the target diffraction angle of 68°, further verifying the accuracy of the design.

[0051] Figure 5The efficiency of each diffraction order of the metagrating in air and water media is compared in detail. The " / / / / " bar graph represents the diffraction efficiency in air, and the "\\\\" bar graph corresponds to the diffraction efficiency in water. According to the statistical results, the -1 order diffraction efficiency of the present invention in air reaches 70%, while the 0th order diffraction efficiency in water is 61.5%. In addition, the transmission efficiency of other diffraction orders is less than 10%, and the efficiency of some diffraction orders is even close to zero, which further demonstrates the high selective diffraction performance of the present invention.

[0052] Figure 6 and Figure 7 The numerical simulation results of the present invention based on the finite time-domain difference method (FDTD) are shown respectively. The color depth in the illustration reflects the diffraction efficiency distribution of the far-field diffraction pattern, verifying the diffraction order in the two media of air and water. Despite the significant difference in the refractive index of air and water, the metagrating of the present invention still maintains a consistent main diffraction angle of 68° in the two media, fully demonstrating its superior optical properties and design stability.

[0053] The above-mentioned embodiments only express several implementation methods of the present application, and the descriptions thereof are relatively specific and detailed, but they cannot be understood as limiting the scope of the invention patent. It should be pointed out that, for a person of ordinary skill in the art, several variations and improvements can be made without departing from the concept of the present application, and these all belong to the protection scope of the present application. Therefore, the protection scope of the patent of the present application shall be subject to the attached claims.

Claims

1. A diffraction angle invariant metagrating suitable for dual background refractive index, characterized in that: including a nano-pillar unit and a substrate unit; The plurality of nanocolumn units are sequentially arranged on the substrate unit according to a set period length; The nanocolumn unit includes a plurality of rectangular nanocolumns, and gaps exist between the nanocolumns.

2. The diffraction angle invariant metagrating suitable for dual background refractive index according to claim 1, characterized in that: The metagrating is configured with appropriate period length d, diffraction orders m1 and m2, and wavelength λ of the incident wave, so that the incident wave of the metagrating is deflected by the same diffraction angle under different background media: Among them, θ m1 represents the diffraction angle of the m1th diffraction order in air, θ m2 It represents the diffraction angle of the m2th diffraction order in water. The same diffraction angle is θ m1 =θ m2 ; n1 represents the refractive index of air, n2 represents the refractive index of water; θ i is the incident angle of the light source, n i is the refractive index of the medium on the incident light side, d represents the period length of the nanocolumn unit of the metagrating, and λ is the wavelength of the incident wave in vacuum.

3. The diffraction angle invariant metagrating suitable for dual background refractive index according to claim 1, characterized in that: The nanocolumn unit includes three rectangular nanocolumns, the widths of the nanocolumns are W1, W2, and W3, the heights are H1, H2, and H3, and the widths of the gaps between the nanocolumns are G1, G2, and G3, respectively; The width, height and gap width of the nanorods are optimized using an optimization algorithm to achieve optimal diffraction efficiency.

4. The diffraction angle invariant metagrating suitable for dual background refractive index according to claim 3, characterized in that: The width, height and gap width of the nanopillars are defined as the particle parameters in the particle swarm algorithm. The diffraction efficiency of the metagrating in air and water environments under each parameter state is determined in the iteration, which are denoted as η air and η water , the particle parameters are optimized based on the adaptive function: Among them, α and β are normalized proportional coefficients.

5. The diffraction angle invariant metagrating suitable for dual background refractive index according to claim 1, characterized in that: The substrate unit is made of SiO2, and the nanocolumn unit is made of TiO2.

6. The diffraction angle invariant metagrating suitable for dual background refractive index according to claim 4, characterized in that: The period length of the nanocolumn unit is 2000 nm.

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