A visible light band broadband circular dichroism chiral metasurface device

Abstract

The application discloses a visible light band broadband circular dichroism chiral metasurface device, characterized in that the device comprises a transparent substrate and an array of metasurface unit structures on the transparent substrate, wherein the metasurface unit structure has C-2 rotational symmetry. The application adopts GaN material to construct the chiral metasurface, and simultaneously adopts a four-column unit structure with C-2 rotational symmetry, thereby increasing a spatial position parameter, multiple sets of length / width and an independent rotation angle, greatly improving a design freedom, being capable of more finely manipulating near-field coupling effects and multipole interference modes in the unit, and realizing perfect consideration of a wide spectrum and high polarization selectivity in the visible light band.

Description

Technical Field

[0001] This invention relates to the field of metasurface technology, and more specifically to a broadband circular dichroic chiral metasurface device in the visible light band. Background Technology

[0002] With the continuous advancement of micro-nano optics, information technology, and advanced imaging, metasurface technology, which can overcome the limitations of traditional optical volume and flexibly control physical quantities such as the polarization state of light at the subwavelength scale, has gradually become a research hotspot. Circular dichroism devices, as one of the core applications of chiral metasurfaces, have broad application prospects in polarization imaging, optical quantum computing, and spin optical communication.

[0003] Currently, chiral metasurface devices are mainly concentrated in the infrared or near-infrared bands, with relatively few high-performance devices targeting the visible light band. Existing visible light circular dichroism devices are mainly based on two types of materials: the first type is metallic materials, which achieve chiral response through plasmon resonance; the second type is traditional all-dielectric materials, such as silicon (Si) or titanium dioxide (TiO2), which are usually designed with fixed topologies such as cross-shaped, "Z"-shaped, or double-pillar structures.

[0004] However, metallic materials suffer from severe ohmic losses in the visible light band, leading to strong absorption in devices and low overall efficiency and transmittance, making it difficult to meet the demands of high-efficiency applications. Meanwhile, among commonly used all-dielectric materials, silicon (Si) has a narrow bandgap, resulting in significant absorption losses in the visible light band; while titanium dioxide is transparent, its extremely low thermal conductivity makes it difficult to integrate into high-power systems such as lasers for long-term continuous operation. Existing simple, regular geometric configurations (such as bi-column structures) offer limited design freedom, failing to fully utilize the spatial modulation advantages of metasurfaces and making it difficult to simultaneously achieve "wide operating bandwidth," "high circular dichroism (CD)," and "high circular extinction ratio (ER)" in the visible light band. This often results in the inability to effectively suppress non-target polarized light in the transmission spectrum. Summary of the Invention

[0005] This invention addresses the shortcomings of existing technologies by providing a broadband circular dichroic chiral metasurface device in the visible light band. The chiral metasurface is constructed using GaN (gallium nitride) material, and a four-pillar unit structure with C-2 rotational symmetry is employed. This increases the spatial position parameters, multiple sets of length / width, and independent rotation angles, greatly improving the design freedom. It enables more precise manipulation of near-field coupling effects and multipole interference modes within the unit, achieving a perfect balance between a wide spectrum and high polarization selectivity in the visible light band.

[0006] To address the aforementioned technical problems, this invention provides a broadband circular dichroic chiral metasurface device in the visible light band, comprising a transparent substrate and an array of metasurface unit structures located on the transparent substrate, wherein the metasurface unit structures possess C-2 rotational symmetry. Preferably, the metasurface unit structures are made of gallium nitride.

[0007] This invention employs an asymmetric but C⁻² rotational symmetry arrangement of four nanopillars (composed of two pairs of rectangular pillars of different lengths tilted at specific angles). This complex topology with multiple independent structural parameters (length, width, and different rotation angles) breaks the symmetry constraints of traditional geometric structures, enabling more complex near-field coupling effects in space. Through the interference of these non-orthogonal modes, higher-order multipole modes (such as magnetic quadrupole modes) can be effectively excited and enhanced. In the visible light band, combined with the extremely low intrinsic absorption loss of GaN material, this higher-order interference allows circularly polarized light of a specific rotation direction (such as left-handed circularly polarized light, LCP) to be completely suppressed by strong destructive interference (i.e., transmittance is 0), while light of the other rotation direction (such as right-handed circularly polarized light, RCP) maintains high transmittance due to constructive interference. In the 500-600 nm visible light wideband, RCP maintains a high transmittance of over 0.8, while LCP is greatly suppressed, ultimately achieving an extremely high polarization extinction ratio of up to 27.5 dB.

[0008] Furthermore, the metasurface structure comprises four centrally symmetrical rectangular nanopillars.

[0009] Furthermore, the four rectangular nanopillars include two diagonally distributed first rectangular nanopillars and two diagonally distributed second rectangular nanopillars.

[0010] Furthermore, the length of the first rectangular nanopillar is l1 = 250-400 nm, and the length of the second rectangular nanopillar is l2 = 150-280 nm, with l1 > l2.

[0011] Furthermore, the width of the first rectangular nanopillar is w1 = 80 - 150 nm, and the width of the second rectangular nanopillar is w2 = 80 - 150 nm.

[0012] Furthermore, the first rectangular nanopillar and the second rectangular nanopillar have the same height, H=500-900nm.

[0013] Furthermore, the rotation angle θ of the long axis of the first rectangular nanopillar and the long axis of the second rectangular nanopillar can be independently selected within the range of -180° to 180°.

[0014] Furthermore, the metasurface unit structure array has a period of Px = 500-700 nm in the x-axis direction and a period of Py = 600-800 nm in the y-axis direction.

[0015] Furthermore, the transparent substrate is made of aluminum oxide.

[0016] The beneficial effects of this invention are: This invention uses GaN material to construct a chiral metasurface, which utilizes its wide bandgap, high refractive index and extremely high thermal conductivity (approximately 130-200 W / m·K) in the visible light band. This completely solves the defects of high loss in traditional metal materials and poor thermal conductivity in all-dielectric materials such as TiO2, while ensuring that the device has good thermal stability and process compatibility.

[0017] This invention employs a novel four-column unit structure, which, compared to traditional simple configurations such as double-column and symmetrical four-column structures, increases spatial position parameters, multiple sets of length / width and independent rotation angles, greatly improving design freedom and enabling more precise manipulation of near-field coupling effects and multipole interference modes within the unit (such as effectively exciting higher-order modes such as magnetic quadrupoles).

[0018] The device of this invention has superior performance and supports multi-target customization. In the working wavelength range of 500 nm-600 nm, for the high extinction ratio requirement, the device can achieve an average extinction ratio of 15 dB and a maximum of 27.5 dB. For the high color reproduction requirement, it can achieve a high circular dichroism response of 0.83 on average and 0.91 on peak, achieving a perfect balance between a wide spectrum and high polarization selectivity in the visible light band. Attached Figure Description

[0019] To more clearly illustrate the technical solution of the present invention, the drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0020] Figure 1 This is a schematic diagram of the metasurface device of the present invention, wherein a is the annotation of the unit structure parameters, b is the array of chiral unit structures, c is a three-dimensional schematic diagram of the unit structure, d is a schematic diagram of the electromagnetic response of the device when right-hand circularly polarized light (RCP) is incident normally, and e is a schematic diagram of the electromagnetic response of the device when left-hand circularly polarized light (LCP) is incident normally. Figure 2 This is a flowchart illustrating the fabrication process of the broadband circular dichroic chiral metasurface device in the visible light band of this invention. Figure 3 This is a schematic diagram of the metasurface in Comparative Example 1; Figure 4These are topographic images of the device obtained in Example 1, where a is an actual photograph of the sample, b is an overall structural diagram of the sample, c is a magnified view of a part, d is a cross-sectional view of the sample, and e and f are structural diagrams at different magnifications. Figure 5 This is a schematic diagram of the optical path for sample testing; Figure 6 This describes the performance of the device in Example 1 with a high extinction ratio in the 500 nm-600 nm wavelength range. Here, a represents the transmittance of left-handed and right-handed circularly polarized light and their partial curves within their operating wavelength range; b represents the circularly polarized extinction ratio (defined as: ER = 10log(T)). RCP / T LCP ); ER (unit: dB) and its operating band partial curve; Figure 7 The performance of the device in Example 2 is that of a high circular dichroism extinction ratio device in the 500 nm-600 nm band, where a is the transmittance of left-handed and right-handed circularly polarized light and its partial curves in the working band, and b is the circular polarization extinction ratio and its partial curves in the working band. Figure 8 The transmittance of left-handed and right-handed circularly polarized light from the device in Comparative Example 1 is shown. Figure 9 This is a comparison of experimental data and simulation curves in Example 1, where ab is a comparison of experimental and theoretical values ​​of RCP and LCP transmittance with magnified curves of the target band, and cd is a comparison of experimental and simulated values ​​of circular polarization extinction ratio with magnified curves of the target band. Figure 10 The image shows a pattern with a "VIS" pattern and its electron microscope image, which is composed of a chiral structure with a high polarization extinction ratio (Example 1) and its chiral isomer. Here, a is the pattern used for preparation, and bd are electron microscope images at different magnifications. Figure 11 These are photographs of the transmitted light fields of the metasurface composed of the chiral structure (Example 1) and its isomer under RCP and LCP illumination at different wavelengths, where af represents LCP incident light at different wavelengths and gl represents RCP incident light at different wavelengths. Detailed Implementation

[0021] The technical solution of the present invention will be clearly and completely described below with reference to specific embodiments. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0022] This embodiment provides a broadband circular dichroic chiral metasurface device in the visible light band, reference... Figure 1The intermediate ac includes a transparent substrate and an array of metasurface unit structures located on the transparent substrate, wherein the metasurface unit structures possess C-2 rotational symmetry. Preferably, the metasurface unit structures are made of gallium nitride. (Reference) Figure 1 In the schematic diagram of the electromagnetic response of the device when right-hand circularly polarized light (RCP) is incident normally, due to the specific chiral resonance, most of the transmitted light remains RCP; in the schematic diagram of the electromagnetic response of the device when left-hand circularly polarized light (LCP) is incident normally, the LCP light is strongly reflected (or absorbed) by the metasurface and cannot be transmitted.

[0023] This embodiment employs an asymmetric but C⁻² rotational symmetry arrangement of four nanopillars (composed of two pairs of rectangular pillars of different lengths tilted at specific angles). This complex topology with multiple independent structural parameters (length, width, and different rotation angles) breaks the symmetry constraints of traditional geometric structures, enabling more complex near-field coupling effects in space. Through the interference of these non-orthogonal modes, higher-order multipole modes (such as magnetic quadrupole modes) can be effectively excited and enhanced. In the visible light band, combined with the extremely low intrinsic absorption loss of GaN material, this higher-order interference allows circularly polarized light of a specific rotation direction (such as LCP) to be completely suppressed by strong destructive interference (i.e., transmittance is 0), while light of another rotation direction (such as RCP) maintains high transmittance due to constructive interference. In the 500-600 nm visible light wideband, RCP maintains a high transmittance of over 0.8, while LCP is greatly suppressed, ultimately achieving an extremely high polarization extinction ratio of up to 27.5 dB.

[0024] In a preferred embodiment, the metasurface structure comprises four centrally symmetrical rectangular nanopillars; the four rectangular nanopillars include two diagonally distributed first rectangular nanopillars and two diagonally distributed second rectangular nanopillars; the length of the first rectangular nanopillars is l1 = 250-400 nm, the length of the second rectangular nanopillars is l2 = 150-280 nm, and l1 > l2; the width of the first rectangular nanopillars is w1 = 80-150 nm, and the width of the second rectangular nanopillars is w2 = 80-150 nm; the height of the first and second rectangular nanopillars is the same, H = 500-900 nm; the rotation angle θ (θ1 / θ2) of the major axis of the first and second rectangular nanopillars is independently selected from -180° to 180°; the period of the metasurface unit structure array in the x-axis direction is Px = 500-700 nm, and the period of the y-axis direction is Py = 600-800 nm.

[0025] In a preferred embodiment, the transparent substrate is made of aluminum oxide (Al2O3). Example 1

[0026] This embodiment relates to a method for fabricating a broadband circular dichroic chiral metasurface device in the visible light band, referencing... Figure 2 It includes the following steps: (1) Thin film thinning: On the GaN epitaxial wafer on the cleaned Al2O3 substrate, the GaN layer was thinned to a height of 750 nm by inductively coupled plasma (ICP) with Cl2, BCl3 and Ar gases. (2) Electron beam lithography (EBL): Positive photoresist is spin-coated onto the GaN surface and then baked and coated with conductive adhesive. The four-column structure layout is then exposed and developed with high precision using EBL equipment. (3) Metal deposition: Electron beam evaporation deposition (EBE) is used to deposit a layer of Cr as a transition layer, and then a layer of Ni metal is deposited as an anti-etching mask. (4) Pattern transfer: After stripping off excess photoresist and metal, GaN is anisotropically etched with high aspect ratio using ICP process. Finally, residual Ni / Cr metal mask is removed with etchant to obtain the desired device.

[0027] Wherein, l1=320 nm, w1=100 nm, θ1=-132°, l2=200 nm, w2=100 nm, θ2=-86°, H=750 nm, Px=620 nm, Py=750 nm. Example 2

[0028] The preparation method of this embodiment is the same as that of embodiment 1, except that the structural parameters are as follows: l1=320 nm, w1=100 nm, θ1=-27°, l2=200 nm, w2=100 nm, θ2=20°, H=750 nm, Px=590 nm, Py=700 nm.

[0029] Comparative Example 1 The comparative preparation method is the same as in Example 1, except that a double-column structure is prepared. Figure 3 The structural parameters are: l1=320 nm; w1=100 nm; θ1=6°; l2=220 nm; w2=100 nm; θ2=52°; H=670 nm; Px=510 nm; Py=350 nm.

[0030] Test case Figure 4 Figure 'af' is a topographic image of the device obtained in Example 1. As can be seen, the sample structure and parameters conform to the design. The asymmetric four-nanopillar structure exhibits superior molding quality under actual nanofabrication technology (EBL+ICP), high sidewall perpendicularity, and strong industrial manufacturability.

[0031] refer to Figure 5 The devices obtained in the examples and comparative examples were tested, where Laser is a laser, A is an attenuator, P is a linear polarizer, QWP is a quarter-wave plate, L is a cemented doublet lens, Sample is a sample, O is a 20× objective lens, and CCD is a camera.

[0032] Figure 6 The performance of the device in Example 1 with a high extinction ratio in the 500 nm-600 nm wavelength range is shown, wherein, Figure 6 As shown in Figure a, in the 500-600 nm wavelength range, the transmittance of right-hand circularly polarized light (RCP) remains high at 0.8, while the transmittance of left-hand circularly polarized light (LCP) is suppressed to below 0.1; Figure 6 As shown in Figure b, this huge difference in transmittance enables the device to achieve a circular polarization extinction ratio (ER) of over 15 dB in this band, with a peak value of up to 27.5 dB.

[0033] Figure 7 For the performance of the device in Example 2 as a high circular dichroism device in the 500 nm-600 nm wavelength range, refer to Figure 7 As can be seen from Figure ab, by adjusting the parameters of the four-column structure, the device achieves extremely high circular dichroism (CD) in the operating band, with an average CD value of 0.83 and a peak value of 0.91, making it suitable for polarization imaging scenarios with extremely high requirements for color reproduction and high dichroism.

[0034] Figure 8 The transmittance of left-handed and right-handed circularly polarized light from the device in Comparative Example 1 is shown. Due to insufficient spatial parameters, it is impossible to simultaneously suppress LCP and maintain high RCP transmittance over a wide frequency band. The test results show that in the visible light band, the average polarization extinction ratio of the dual-pillar structure is only about 5-8 dB, and the operating bandwidth is extremely narrow (typically less than 20 nm). In contrast, the four-pillar structure of the embodiment, due to its higher degrees of freedom and stronger higher-order multipole coupling, achieves a breakthrough with a bandwidth of 100 nm and a maximum extinction ratio of 27.5 dB, significantly outperforming the traditional dual-pillar comparative example.

[0035] Figure 9 The figure below shows a comparison between experimental data and simulation curves from Example 1. It can be seen that the overall evolution trend of the transmittance and polarization extinction ratio curves measured experimentally under LCP / RCP incident light is basically consistent with the theoretical simulation results. Within the working wavelength range of 500 nm to 600 nm, the measured average CD is approximately 0.5, with a maximum value reaching 0.7. The measured average ER in the target wavelength range is approximately 8 dB, with a peak value reaching 11 dB. The errors between theory and experiment mainly originate from dimensional errors in micro / nano fabrication and testing errors.

[0036] Figure 10It is a pattern with a "VIS" pattern composed of a high polarization extinction ratio chiral structure (Example 1) and its chiral isomers (the structures of the letter area and the background area are the high extinction ratio chiral basic unit and its chiral isomers, respectively) and its electron microscope image, where a is the pattern used for preparation and bd are electron microscope images at different magnifications.

[0037] Figure 11 These are photographs of the transmitted light fields of the chiral structure (Example 1) and its isomer-based metasurface under RCP and LCP illumination at different wavelengths, where af represents LCP incident light at different wavelengths and gl represents RCP incident light at different wavelengths. It can be seen that when RCP is incident at different wavelengths, the metasurface array at the "VIS" letter strongly reflects the RCP, resulting in very low brightness in the letter pattern area of ​​the micrograph, while the background area has very high transmittance to RCP and high brightness. When the light source is changed to LCP, the brightness between the pattern and the background is reversed, with the letter appearing bright and the background dark. The experiment not only confirms the correctness of the design simulation results but also demonstrates that the designed chiral metasurface can maintain a high CD response even with a small unit array size, fully illustrating the broad application prospects of micro / nano devices in fields such as information encryption.

[0038] The present invention has been described in detail above with reference to specific embodiments and exemplary examples; however, these descriptions should not be construed as limiting the present invention. Those skilled in the art will understand that various equivalent substitutions, modifications, or improvements can be made to the technical solutions and embodiments of the present invention without departing from the spirit and scope of the invention, and all such modifications and improvements fall within the scope of the present invention. The scope of protection of the present invention is defined by the appended claims.

Claims

1. A broadband circular dichroic chiral metasurface device in the visible light band, characterized in that, It includes a transparent substrate and an array of metasurface unit structures located on the transparent substrate, wherein the metasurface unit structures have C-2 rotational symmetry.

2. The broadband circular dichroic chiral metasurface device in the visible light band as described in claim 1, characterized in that, The metasurface structure comprises four centrally symmetrical rectangular nanopillars.

3. The broadband circular dichroic chiral metasurface device in the visible light band as described in claim 2, characterized in that, The four rectangular nanopillars include two diagonally distributed first rectangular nanopillars and two diagonally distributed second rectangular nanopillars.

4. The broadband circular dichroic chiral metasurface device in the visible light band as described in claim 3, characterized in that, The length of the first rectangular nanopillar is l1 = 250-400 nm, and the length of the second rectangular nanopillar is l2 = 150-280 nm, with l1 > l2.

5. The broadband circular dichroic chiral metasurface device in the visible light band as described in claim 3, characterized in that, The width of the first rectangular nanopillar is w1 = 80 - 150 nm, and the width of the second rectangular nanopillar is w2 = 80 - 150 nm.

6. The broadband circular dichroic chiral metasurface device in the visible light band as described in claim 3, characterized in that, The first rectangular nanopillar and the second rectangular nanopillar have the same height, H=500-900 nm.

7. The broadband circular dichroic chiral metasurface device in the visible light band as described in claim 3, characterized in that, The rotation angle θ of the long axis of the first rectangular nanopillar and the long axis of the second rectangular nanopillar can be independently selected within the range of -180° to 180°.

8. The broadband circular dichroic chiral metasurface device in the visible light band as described in claim 1, characterized in that, The metasurface unit structure array has a period of Px = 500-700 nm along the x-axis and a period of Py = 600-800 nm along the y-axis.

9. The broadband circular dichroic chiral metasurface device in the visible light band as described in claim 1, characterized in that, The transparent substrate is made of aluminum oxide.

10. The broadband circular dichroic chiral metasurface device in the visible light band as described in claim 1, characterized in that, The metasurface unit structure is made of gallium nitride.

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