A metasurface quarter-wave plate

Abstract

This invention discloses a metasurface quarter-wave plate comprising a transparent substrate and an array of nanometal structures on the transparent substrate. The nanometal structures consist of a hollow cross-shaped structure and a solid elliptical structure in the center of the hollow cross-shaped structure. The hollow cross-shaped structure and the solid elliptical structure have the same thickness. This structure provides an effective new method for exciting LSP resonance and designing wave plates, and has great potential in liquid crystal displays, optical communications, sensor detection, and imaging.

Description

Technical Field

[0001] The present invention relates to an optical structure, in particular to a quarter wave plate. Background Art

[0002] As one of the basic and inherent properties of light, the polarization state is independent of physical quantities such as amplitude, phase, and frequency. It can provide rich information about the interaction between light and matter and is widely used in many scientific research fields such as quantum information processing and biomedical sensing. Therefore, it is very important to design an optical wave plate that can flexibly control the polarization state. Traditional optical wave plates are generally made of birefringent materials. They control the polarization state by controlling the phase delay caused by the propagation of two orthogonal polarized electric fields over a certain distance. Therefore, they are large and bulky, and their functions are relatively simple, which is inconsistent with the development trend of miniaturization and integration of optical devices. In recent years, two-dimensional metasurfaces composed of subwavelength-scale structural units can realize flexible control of the amplitude, phase, and polarization state of electromagnetic waves, giving full play to their advantages of ultra-thin and compact structure, strong integration compatibility, and multiple functions, and have become a new generation of integrated light field control platform.

[0003] In early 2011, Bozhevolnyi's team designed a brick-shaped scatterer that supports vertical electric dipoles and can generate a phase difference of π / 2 in the reflection mode with a central wavelength of 770nm. By optimizing the size of the gold nanobricks on top of the substrate, detuned electric dipole resonance can be supported to achieve linear circular polarization conversion under LP incident beams with different polarization angles. Although this work provides a conceptual approach for the design of subsequent wave retarders, the polarization conversion efficiency is limited due to working in unwanted channels, and the effective working bandwidth is significantly narrow. [1] . Z Hao et al. designed a transmissive QWP, in which the unit cell is formed by two orthogonal rectangular nanoslits of optimized size in an ultrathin silver (Ag) layer. The phase difference between the two LP transmission fields can be maintained at π / 2 in a broadband wavelength range of 600 to 800 nm. In a bandwidth of 616 to 746 nm, the simulated degree of linear polarization (DoLP) is close to 1 when the QWP is illuminated with right circularly polarized (RCP) and left circularly polarized (LCP) beams. Similar to a single-layer wave plate, the conversion efficiency of this complementary QWP is limited due to unavoidable reflection losses. [2] In 2012, Roberts et al. proposed a quarter-wave plate with a periodic array of cross-shaped apertures in a silver film. The transmission efficiency and phase of the wave plate (for a fixed arm width) are sensitive to the length of the relevant arm. The conversion from linear to circular (LTC) polarization is achieved at some discrete wavelengths of visible light between 710 and 760 nm, with a silver film thickness of 140 nm. [3]In 2013, Yang et al. proposed a quarter-wave plate consisting of a periodic planar array of symmetrical L-shaped plasmonic antennas. The ellipticity of the transmitted light can reach 0.994 at 1550nm. The bandwidth with an ellipticity greater than 0.9 is 80nm, which is not ideal. [4] In 2015, by carefully designing the nanoantenna in the superantenna, Li et al. realized a quarter-wave plate composed of a 20-nanometer-thick gold nanorod array. Theoretically, the conversion and inverse conversion of CTL polarization can be achieved around 1550nm. The circular polarization rate is 0.67 and the transmission efficiency is 0.4. The ultra-thin structure can achieve CTL polarization in a broadband, but the ellipticity (amplitude ratio) of LTC polarization at 1550nm is low. [5] Song et al. proposed a universal method for full polarization generation and bandwidth-infinite polarization maintenance diffractive plane optics in the entire visible light range from 475 to 675 nm. It relies on the superposition of orthogonal polarizations generated by PB phase metasurfaces composed of unique elements to eliminate the dispersion response of the structure. By using RLL and LRR configurations, the linear superposition of amplitude-controlled polarization convenience establishes full polarization generation. [6] .

[0004] [1] A.Pors, MGNielsen, GDValle, M.Willatzen, O.Albrektsen, and S.I.Bozhevolnyi, "Plasmonic metamaterial wave retarders in reflection byorthogonally oriented detuned electrical dipoles," Opt.Lett., vol.36, pp.1626–1628, 2011.

[0005] [2]Y.Zhao and A.Alù, "Manipulating light polarization with ultrathinplasmonic metasurfaces," Phys.Rev.B, vol.84, p.205428, 2011.

[0006] [3]RobertsA,Lin L(2012)Plasmonic quarter-wave plate.Opt.Lett.37:1820–1822

[0007] [4]Yang B,Ye WM,Yuan XD,Zhu ZH,Chun Z(2013)Design of ultrathinplasmonic quarter-wave plate based on period coupling.Opt.Lett.38:679–681

[0008] [5]Li Z, Liu W, Cheng H, Chen S, Tian J (2015) Realizing broadband and invertible linear-to-circularpolarization converter with ultrathin single-layer metasurface.Sci.Rep.5:18106

[0009] [6] Q.Song, S.Khadir, S.Vezian, et al., "Bandwidth-unlimited polarization-maintaining metasurfaces," Sci.Adv., vol.7, p.eabe1112, 2021. Summary of the Invention

[0010] Purpose of the invention: In view of the above-mentioned existing technologies, a metasurface quarter-wave plate is proposed to realize a new solution for exciting LSP resonance.

[0011] Technical solution: A metasurface quarter-wave plate, comprising: a transparent substrate, and an array of nanometal structures on the transparent substrate; wherein the nanometal structure is composed of a hollow cross-shaped structure and a solid elliptical structure in the center of the hollow cross-shaped structure, and the thickness of the hollow cross-shaped structure and the solid elliptical structure are consistent.

[0012] Furthermore, the material of the substrate is SiO2, and the material of the nanometal structure is Au.

[0013] Furthermore, the hollow cross structure is an elliptical cross, the short radius of the two hollow ellipses constituting the elliptical cross is r, and the long radius is R1 and r1 respectively, and R1≠r1, and the long and short radii of the solid elliptical structure are equal and consistent with the short half r of the hollow ellipse.

[0014] Furthermore, the hollow cross structure is a rectangular cross, the width and length of the two rectangles constituting the hollow cross structure are a and b, and c and d respectively, and a≠c, b≠d, the short radius of the solid elliptical structure is r2, and the long radius is R2.

[0015] Furthermore, r is in the range of 20 to 30 nm, R1 is in the range of 97 to 107 nm, r1 is in the range of 150 to 160 nm, the period p1 of the nanometal structure array is in the range of 290 to 370 nm, and the thickness h1 of the solid elliptical structure is in the range of 8 to 12 nm.

[0016] Furthermore, the value range of a is 40~60nm, the value range of b is 160~170nm, the value range of c is 30~50nm, the value range of d is 250~290nm, the value range of r2 is 15~25nm, the value range of R2 is 20~30nm, the period p2 of the nanometal structure array is 280~360nm, and the thickness h2 of the solid elliptical structure is 8~12nm.

[0017] Beneficial effects: The present invention proposes a metasurface quarter-wave plate that can be used in advanced nanophotonic devices and integrated photonic systems. For a rectangular cross-shaped hollow structure, a phase difference of 1.52 to 1.61 and a transmittance of 0.88 to 1.26 can be achieved in the range of 1500nm to 1600nm, and a phase difference of 1.57 and a transmittance of 1.07 can be achieved at 1550nm, which is exactly the window for optical fiber communication and has great potential in liquid crystal display, sensor detection and imaging. For an elliptical cross-shaped hollow structure, a phase difference of 1.53 to 1.60 and an amplitude ratio of 0.81 to 1.18 can be achieved in the range of 1672 to 1772nm, and a phase difference of 1.57 and an amplitude ratio of 0.99 can be achieved at 1723nm. Unlike traditional quarter-wave plates based on LSP (localized plasmon) resonance, this ultrathin structure has no sharp corners, so efficiency reduction can be avoided during structure manufacturing. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 Schematic diagram of the structure of a single unit of the metasurface quarter-wave plate of Example 1;

[0019] Figure 2 This is the effect of the change in the radius of the solid circle inside the structure of Example 1 on the phase difference;

[0020] Figure 3 This is an image showing the relationship between the phase difference and transmittance of the structure in Example 1 and the wavelength;

[0021] Figure 4 Schematic diagram of the structure of a single unit of the metasurface quarter-wave plate of Example 2;

[0022] Figure 5 The effect of the change in the difference between the long and short radii of the solid ellipse inside the structure of Example 2 on the phase difference;

[0023] Figure 6 This is an image showing the relationship between the phase difference and transmittance of the structure in Example 2 and the wavelength;

[0024] Figure 7 is the transmittance and phase difference of whether there is an ellipse at the center of the ellipse;

[0025] Figure 8 It is the transmittance and phase difference of whether there is an ellipse in the center of the rectangular cross. DETAILED DESCRIPTION

[0026] The present invention will be further explained below with reference to the accompanying drawings.

[0027] Example 1

[0028] like Figure 1 A metasurface quarter-wave plate is shown, comprising a transparent substrate and an array of nanometal structures on the transparent substrate. The nanometal structures consist of a hollow cross-shaped structure and a solid elliptical structure in the center of the hollow cross-shaped structure. The hollow cross-shaped structure and the solid elliptical structure have the same thickness, and the metal surface is planar. A linearly polarized plane wave is incident from below the substrate.

[0029] The substrate is made of SiO2, and the nanometal structure is made of Au. The hollow cross is an elliptical cross, with the two hollow ellipses forming the cross having a short radius of r and long radii of R1 and r1, respectively, with R1 ≠ r1. The solid elliptical structure has equal long and short radii that coincide with the short half r of the hollow ellipse, meaning it is an Au circle.

[0030] The value range of r is 20-30 nm, the value range of R1 is 97-107 nm, the value range of r1 is 150-160 nm, the period p1 of the nanometal structure array is 290-370 nm, and the thickness h1 of the solid elliptical structure is 8-12 nm.

[0031] The structure of this embodiment can be prepared through the following three steps: first, the SiO2 substrate is cleaned, a layer of Au with a thickness of h1 is deposited by vacuum coating, and then a corresponding shape is etched by focused ion beam FIB.

[0032] In conventional wave plates based on LSP resonance, incident light excites strong LSP resonances around the sharp corners within the rectangular ring. However, nanoscale structures with sharp corners present numerous challenges during fabrication. Perfect fabrication is often impossible, and rounded corners are often required, significantly weakening the LSP resonances and reducing the efficiency of these wave plates. The structure of this embodiment has no sharp corners, allowing LSP resonances to be excited within this corner-free structure, and performance is less sensitive to structural errors.

[0033] An ideal quarter-wave plate requires that the incident light's transmitted electric field components along the X and Y axes satisfy an amplitude ratio of 1 and a phase difference of π / 2. This requires separate control along both directions within the structure. Previously available nanostructures with sharp corners can be divided into horizontal and vertical sections, which can be adjusted separately to precisely control the characteristics of the X and Y components of the incident light. However, the sharp-cornered nanostructure of this embodiment is treated as a single entity, simplifying the design and reducing processing difficulty. Its physical mechanism differs significantly from that of nanostructures with sharp corners. While perspective excitation excites surface plasmon resonance, sharp-cornered structures only excite resonance at the corners, which is more dependent on the sharpness of the corners. Generally, the sharper the corner, the stronger the resonance. In contrast, the non-sharp-cornered structure of this embodiment excites resonance throughout the entire structure, relying more on the overall characteristics of the structure. Furthermore, due to the sharp-cornered structure's high sensitivity to sharp corners, it is more sensitive to parameter errors. This has the advantage of facilitating the adjustment of the operating wavelength by adjusting the structural parameters. However, its disadvantage is a lower tolerance for parameter errors during fabrication, while the non-sharp-cornered structure exhibits the opposite effect.

[0034] The finite-difference time-domain (FDTD) method was used to calculate and optimize the transmission performance and electric field inside the proposed structure. During the calculation, both the x- and y-boundaries were set to periodic boundary conditions to simulate the periodic structure, and the z-boundary was set to a fully matched layer condition to ensure that the excitation light was completely absorbed without reflection.

[0035] The inner solid Au circle plays an important role in studying the effect of its radius change on the phase difference, such as Figure 2 , it can be seen that as the radius of the solid Au circle increases, the phase difference increases.

[0036] Figure 3 The image of the relationship between the phase difference and transmittance of the proposed structure and the wavelength under the conditions of h1=10nm, r=25nm, r1=97nm, R1=156nm, and p1=330nm. The structure can achieve a phase difference from 1.53 to 1.60 and an amplitude ratio from 0.81 to 1.18 in the range of 1672-1772nm, and meet the phase difference of 1.57 and the amplitude ratio of 0.99 at 1723nm. Unlike the traditional quarter-wave plate based on LSP resonance, this ultra-thin structure has no sharp corners, so the efficiency reduction can be avoided during the manufacturing of the structure.

[0037] Example 2

[0038] like Figure 4A metasurface quarter-wave plate is shown, comprising a transparent substrate and an array of nanometal structures on the transparent substrate. The nanometal structures consist of a hollow cross-shaped structure and a solid elliptical structure in the center of the hollow cross-shaped structure. The hollow cross-shaped structure and the solid elliptical structure have the same thickness, and the metal surface is planar. A linearly polarized plane wave is incident from below the substrate.

[0039] The substrate is made of SiO2, the nanometal structure is made of Au, the hollow cross is a rectangular cross, the width and length of the two rectangles forming the hollow cross are a and b, and c and d, respectively, with a≠c and b≠d. The short radius of the solid elliptical structure is r2, and the long radius is R2.

[0040] The value range of a is 40~60nm, the value range of b is 160~170nm, the value range of c is 30~50nm, the value range of d is 250~290nm, the value range of r2 is 15~25nm, the value range of R2 is 20~30nm, the period p2 of the nanometal structure array is 280~360nm, and the thickness h2 of the solid elliptical structure is 8~12nm.

[0041] The structure of this embodiment can be prepared through the following three steps: first, the SiO2 substrate is cleaned, a layer of Au with a thickness of h1 is deposited by vacuum coating, and then a corresponding shape is etched by focused ion beam FIB.

[0042] This embodiment has a sharp-cornered structure, and strong LSP resonance is excited near the sharp corner, resulting in different phase delays of two mutually perpendicular polarization components.

[0043] The finite-difference time-domain (FDTD) method was used to calculate and optimize the transmission performance and electric field inside the proposed structure. During the calculation, both the x- and y-boundaries were set to periodic boundary conditions to simulate the periodic structure, and the z-boundary was set to a fully matched layer condition to ensure that the excitation light was completely absorbed without reflection.

[0044] The internal solid Au ellipse plays an important role in studying the effect of the difference between its long and short radii on the phase difference, such as Figure 5 As shown, it can be seen that as the difference between the long and short radii of the solid Au ellipse increases, the phase difference increases.

[0045] Figure 6The image of the relationship between phase difference and transmittance and wavelength of the proposed structure under the conditions of h2=10nm, a=50nm, b=160nm, c=40nm, d=290nm, R2=25nm, r2=20nm, and p2=320nm is shown. The structure can achieve a phase difference from 1.52 to 1.61 and a transmittance from 0.88 to 1.26 in the range of 1500nm to 1600nm, and the phase difference is 1.57 and the transmittance is 1.07 at 1550nm, which is exactly the window for optical fiber communication. The quarter-wave plate can be used in communication systems and near-infrared band systems, and can be integrated with other nanoscale optical devices to realize polarization manipulation, detection and sensing, and has great potential in liquid crystal display and imaging.

[0046] Figure 7 and Figure 8 The figures are the transmittance and phase difference of the elliptical cross and rectangular cross with and without an ellipse in the center, respectively, where N is without a central elliptical structure and Y is with a central elliptical structure. It can be seen that in the absence of a central elliptical structure, the phase difference is higher than that with an elliptical structure in the center. If you want to obtain a phase difference of π / 2, you have to reduce the operating wavelength, which in turn will cause the transmittance to change, that is, it will no longer be close to 1. Even through parameter optimization, it is difficult to simultaneously achieve the required transmittance ratio and phase difference. By adding an elliptical structure in the center, the phase difference can be effectively adjusted when the transmittance ratio does not change much over the wavelength range. This is equivalent to adding an additional parameter adjustment dimension, and this dimension has greater advantages than other dimensions because this dimension has very different sensitivities to phase difference and transmittance. It is more conducive to designing the structure according to the target wavelength and achieving a phase difference of π / 2 and a transmittance of 1 in the same band.

[0047] The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principles of the present invention. These improvements and modifications should also be regarded as within the scope of protection of the present invention.

Claims

1. A metasurface quarter-wave plate, characterized in that: include: A transparent substrate, and a nanometal structure array on the transparent substrate; wherein the nanometal structure is composed of a hollow cross-shaped structure and a solid elliptical structure in the center of the hollow cross-shaped structure, and the hollow cross-shaped structure and the solid elliptical structure have the same thickness; The hollow cross-shaped structure is an elliptical cross-shaped structure, the short radius of the two hollow ellipses constituting the elliptical cross is r, and the long radius is R1 and r1 respectively, and R1≠r1, and the long and short radii of the solid elliptical structure are equal and consistent with the short half r of the hollow ellipse; The value range of r is 20~30nm, the value range of R1 is 97~107nm, the value range of r1 is 150~160nm, the period p1 of the nanometal structure array is 290~370nm, and the thickness h1 of the solid elliptical structure is 8~12nm.

2. A metasurface quarter-wave plate, characterized in that: include: A transparent substrate, and a nanometal structure array on the transparent substrate; wherein the nanometal structure is composed of a hollow cross-shaped structure and a solid elliptical structure in the center of the hollow cross-shaped structure, and the hollow cross-shaped structure and the solid elliptical structure have the same thickness; The hollow cross-shaped structure is a rectangular cross, the width and length of the two rectangles constituting the hollow cross-shaped structure are a and b, and c and d, respectively, and a≠c, b≠d, the short radius of the solid elliptical structure is r2, and the long radius is R2; The value range of a is 40~60nm, the value range of b is 160~170nm, the value range of c is 30~50nm, the value range of d is 250~290nm, the value range of r2 is 15~25nm, the value range of R2 is 20~30nm, the period p2 of the nanometal structure array is 280~360nm, and the thickness h2 of the solid elliptical structure is 8~12nm.

3. The metasurface quarter-wave plate according to claim 1 or 2, characterized in that: The material of the substrate is SiO2, and the material of the nano-metal structure is Au.

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