Plasmon unidirectional coupling device, method and application based on metagrating

Abstract

The disclosure provides a kind of plasmon unidirectional coupling device, method and application based on superstructure grating, device includes substrate, groove and dielectric plate, groove is etched on substrate periodically, groove is covered with dielectric plate;A periodic structure includes first groove and second groove, with the axis that extends through the center of second groove and is perpendicular to the direction of groove, as the axis of symmetry, periodic structure is left-right symmetric about second groove;When structure symmetry, Bloch resonance mode is excited, when second groove is offset along the direction parallel to the direction of groove and perpendicular to the axis of symmetry, structure symmetry is destroyed, anti-symmetric non-radiation BIC mode is converted into quasi-BIC mode, after quasi-BIC mode and Bloch resonance mode are excited, by destructive interference, surface plasmon has high unidirectionality.The disclosure significantly enhances the directionality of surface plasmon propagation along the metal dielectric surface by adjusting the lateral offset of second groove, realizes efficient directional coupling, and improves propagation efficiency.

Description

Technical Field

[0001] The present disclosure relates to the field of optical technology, and in particular to a plasmon unidirectional coupling device, method and application based on a metagrating. Background Art

[0002] Surface plasmon polaritons (SPPs) provide a method for manipulating light on a subwavelength scale and are promising candidates for highly integrated photonic circuits. They have attracted widespread attention in highly integrated photonic devices such as plasmonic circuits, waveguides, and subwavelength imaging technologies. To control the directionality of SPP propagation, various plasmonic structures have been designed, such as aperture grating structures, subwavelength asymmetric grooves, non-periodic grooves, and asymmetric gratings. However, the coupling efficiency of such structures is fundamentally limited by the small electromagnetic scattering cross section, where most of the incident light is reflected by the slit instead of being coupled into the SPP mode.

[0003] The emergence of metasurfaces has opened up new possibilities for manipulating SPPs. Recent studies have demonstrated that by arbitrarily designing the real and imaginary parts of the dielectric constant using metasurfaces, it is possible to achieve unidirectional propagation of SPPs in PT-symmetric systems and unidirectional emission of unpolarized photoluminescence from light beams carrying polarization-dependent and orbital angular momentum in spatially symmetric nanostructures. However, the practical applications of these unidirectional emitters are limited by their complex geometries and require expensive fabrication processes. Furthermore, integrating such plasmonic devices into optical systems faces significant challenges, primarily generating SPP modes with high conversion efficiency and controlling the propagation direction.

[0004] Over the past decade, a variety of coupler designs have been proposed to unidirectionally convert incident light into subwavelength, nanoscale surface plasmons (SPs). These methods encompass magneto-optical effects, optical gratings, non-Hermitian systems with parity-time symmetry, topological photonic crystals, and gradient metasurfaces. While these approaches have made significant progress both theoretically and experimentally, they inherently have limitations in terms of efficiency. Meanwhile, some studies have focused on using specific light sources with spin or orbital angular momentum (OAM) to excite SPs. While these methods can achieve significant unidirectional SPs, the momentum mismatch causes most of the incident light to scatter into the background, resulting in relatively low efficiency.

[0005] Due to the inherent incompleteness of the physical mechanism used, it is fundamentally difficult to achieve perfect unidirectional coupling between light and surface plasmons using existing methods. When achieving unidirectional coupling, the momentum matching problem is inevitable, which is usually achieved through momentum compensation based on additional components, but unidirectional coupling relies on the breaking of time or space symmetry. Therefore, even if momentum matching is achieved, it is not enough to achieve perfect coupling. The coupling efficiency is largely limited by the local impedance characteristics of the incident light and the local surface plasmon polaritons (SPPs) at the interface. Because the two mode states have different topological properties, the impedance of the incident light and SPPs is naturally mismatched during the perfect unidirectional coupling process. Existing design methods do not take into account the key issue of impedance mismatch and lack an effective mechanism to overcome this limitation. Therefore, it is still extremely difficult to achieve impedance matching between incident light and surface plasmons (SPs).

[0006] In summary, it is still extremely difficult to achieve perfect unidirectional coupling of light and surface plasmons, and there is an urgent need to provide a method that can achieve perfect unidirectional coupling of surface plasmons. Summary of the Invention

[0007] In view of this, the purpose of the present disclosure is to overcome the problems existing in the prior art and provide a plasmon unidirectional coupling device, method and application based on a metagrating.

[0008] Based on the above objectives, the present disclosure provides a plasmon unidirectional coupling device based on a metagrating, including a metagrating, wherein the metagrating includes:

[0009] a substrate made of metal;

[0010] A supercell unit comprises a groove and a dielectric plate, wherein the groove is periodically etched on the substrate and the dielectric plate covers the groove;

[0011] A periodic structure of the supercell unit includes a first groove and a second groove, and an axis passing through the geometric center of the second groove and perpendicular to the extending direction of the groove is used as a symmetry axis, and the periodic structure is bilaterally symmetrical about the second groove;

[0012] The second groove can be offset in a direction parallel to the groove extension direction and perpendicular to the symmetry axis, and when the second groove is offset in this direction within a preset distance range, the symmetry of the periodic structure is destroyed;

[0013] When the periodic structure is symmetrical, the metagrating is excited into the Bloch resonance mode. When the symmetry of the periodic structure is destroyed by shifting the second groove left and right, the originally existing antisymmetric non-radiative BIC mode of the metagrating is converted into a quasi-BIC mode. After the quasi-BIC mode and the Bloch resonance mode are excited, destructive interference causes the surface plasmon to have a high degree of unidirectionality.

[0014] In one embodiment, the first trench depth is d1, the second trench depth is d2, and the depth difference |d1-d2| ≥ Δd, where Δd represents the minimum depth difference, and its value is 10nm≤Δd≤50nm.

[0015] In one embodiment, the period of a periodic structure of the supercell unit is p, the substrate thickness is t2, the dielectric plate thickness is t1, the width of the first groove and the second groove is w, and the value of p is 300-350nm, t2 is 450-550nm, t1 is 200-300nm, and w is 40-60nm.

[0016] In one embodiment, the period p=328 nm, the substrate thickness t2=500 nm, the dielectric plate thickness t1=250 nm, and the widths w of the first and second trenches are 50 nm.

[0017] In one embodiment, the offset of the second groove is Δ, which is set to |Δ|≤Δ max , where Δ max Indicates the preset maximum offset.

[0018] In one embodiment, when the second groove deflects, the moving direction of the surface plasmon is consistent with the deflection direction of the second groove.

[0019] Based on the same inventive concept, the present disclosure also provides a plasmon unidirectional coupling method based on a metagrating, which comprises the following steps:

[0020] Step S1: Select a substrate, periodically etch grooves on the substrate, and cover the grooves with a dielectric plate to construct a supercell unit to prepare a metagrating, wherein a periodic structure of the supercell unit includes a first groove and a second groove, and an axis passing through the geometric center of the second groove and perpendicular to the extension direction of the second groove is used as a symmetry axis, so that the periodic structure is bilaterally symmetrical about the second groove;

[0021] Step S2, offsetting the second groove in a direction parallel to the extending direction of the second groove and perpendicular to the symmetry axis;

[0022] Step S3, using incident light to illuminate the metagrating. When the periodic structure is symmetrical, the metagrating is excited into the Bloch resonance mode. When the symmetry of the periodic structure is destroyed by shifting the second groove left and right, the originally existing antisymmetric non-radiative BIC mode of the metagrating is converted into a quasi-BIC mode. After the quasi-BIC mode and the Bloch resonance mode are excited, destructive interference causes the surface plasmon to have a high degree of unidirectionality.

[0023] In one embodiment, step S3 further includes adjusting an offset Δ of the second groove to control the unidirectionality of the surface plasmon.

[0024] In one embodiment, the offset Δ of the second groove is set to |Δ|≤Δ max , where Δ max Indicates the preset maximum offset.

[0025] Based on the same inventive concept, the present disclosure also provides an application of a plasmon unidirectional coupling device based on a metagrating as described above, which is applied in the fields of optical communications, optical sensing, and nanophotonics imaging.

[0026] As can be seen from the above, the present disclosure provides a plasmon unidirectional coupling device, method and application based on a metagrating, which cleverly destroys the geometric symmetry of the metagrating through the lateral offset of the second groove in the supercell unit, prompting the BIC mode to transform into a quasi-BIC mode, and then interacting with the Bloch resonance mode. This interaction significantly enhances the directionality of surface plasmons (SPPs) propagating along the metal dielectric surface, while improving the propagation efficiency and successfully achieving efficient directional coupling. It is worth mentioning that its unidirectional coupling efficiency can reach 0.75, which is a relatively high efficiency level in the visible spectrum.

[0027] This paper, through a unique design, grants greater freedom to the operation of surface plasmons (SPPs). By leveraging the lateral offset of the second groove in the supercell, a key factor, the propagation path and characteristics of SPPs can be effectively controlled. This innovative control method significantly enhances the flexibility of unidirectional control of SPPs, enabling the introduction of new physical mechanisms in optical waveguide sensor and system construction, bringing new opportunities and possibilities for development and breakthroughs in related technologies, and helping to advance optical waveguide sensors and systems towards greater efficiency and precision.

[0028] The present invention adopts a one-dimensional complex grating composed of supercell units, which is completely different from the traditional one-dimensional grating. By customizing the supercell, it can effectively control the light coupling and the generation of SPPs, and has potential application value in optical communications, sensing and other fields. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] In order to more clearly illustrate the technical solutions in the present disclosure or related technologies, the following briefly introduces the drawings required for use in the embodiments or related technical descriptions. Obviously, the drawings described below are only embodiments of the present disclosure. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0030] Figure 1 A schematic structural diagram of a plasmon unidirectional coupling device based on a metagrating provided by the present disclosure;

[0031] Figure 2 The numerical simulation electric field variation diagram of the second groove provided by the present invention when exciting SPPs at different lateral offsets;

[0032] Figure 3 The total efficiency of excited SPPs provided by the present invention varies with the first groove depth d1 and the difference in depth between the two grooves. △d The change relationship diagram;

[0033] Figure 4 The figure shows the efficiency of unidirectionally excited SPPs by the metagrating of the present invention at an incident wavelength of 550nm, where (a) shows the relationship between the SPP efficiency on the left side of the metagrating and d1 and Δ, and (b) shows the relationship between the SPP efficiency on the right side of the metagrating and d1 and Δ;

[0034] Figure 5 Schematic diagram of the interference model provided in the present invention, where (ab) represent the effect of changes in the value of the lateral offset △ on the excitation mode, (c) represents the relationship between the amplitude coefficients of the symmetric radiation mode and the antisymmetric radiation mode and the lateral offset △, (d) represents the SPPs efficiency on the left (right) side of the metagrating excited by the incident light; and (ef) represents the interference magnetic field distribution of unidirectional SPPs at △ = ±25nm.

[0035] Figure 6 A flow chart of the plasmon unidirectional coupling method based on metagrating provided in the present disclosure.

[0036] The reference numerals are described as follows: 1, substrate; 21, groove; 22, dielectric plate. DETAILED DESCRIPTION

[0037] In order to make the objectives, technical solutions and advantages of the present disclosure more clearly understood, the present disclosure is further described in detail below in conjunction with specific embodiments and with reference to the accompanying drawings.

[0038] It should be noted that, unless otherwise defined, the technical terms or scientific terms used in the embodiments of the present disclosure should have the usual meanings understood by people with ordinary skills in the field to which the present disclosure belongs. The "first", "second" and similar words used in the embodiments of the present disclosure do not indicate any order, quantity or importance, but are only used to distinguish different components. "Include" or "comprise" and similar words mean that the elements or objects appearing before the word include the elements or objects listed after the word and their equivalents, without excluding other elements or objects. "Connect" or "connected" and similar words are not limited to physical or mechanical connections, but may include electrical connections, whether direct or indirect. "Up", "down", "left", "right" and the like are only used to indicate relative position relationships. When the absolute position of the described object changes, the relative position relationship may also change accordingly.

[0039] In order to keep the following description of the embodiments of the present disclosure clear and concise, the present disclosure omits detailed descriptions of some known functions and components.

[0040] like Figure 1 As shown, this embodiment discloses a plasmon unidirectional coupling device based on a metagrating, including a metagrating, wherein the metagrating includes a substrate 1 and a supercell unit, wherein the substrate 1 is made of metal; the supercell unit includes a groove 21 and a dielectric plate 22, wherein the groove 21 is periodically etched on the substrate 1, and the dielectric plate 22 is covered on the groove 21; wherein a periodic structure of the supercell unit includes a first groove and a second groove, and an axis passing through the geometric center of the second groove and perpendicular to the extension direction of the groove is a symmetry axis, and the periodic structure is bilaterally symmetrical about the second groove; the second groove can The second groove is offset in a direction parallel to the groove extension direction and perpendicular to the symmetry axis. When the second groove is offset along this direction by a preset distance range, the symmetry of the periodic structure is destroyed. When the periodic structure is symmetrical, the metagrating is excited into the Bloch resonance mode. When the symmetry of the periodic structure is destroyed by offsetting the second groove left and right, the originally existing antisymmetric non-radiative BIC mode of the metagrating is converted into a quasi-BIC mode. After the quasi-BIC mode and the Bloch resonance mode are excited, the surface plasmon has a high degree of unidirectionality through destructive interference.

[0041] This embodiment is based on a metal substrate 1, which carries periodically etched grooves 21 and a dielectric plate 22 covering the grooves 21. This combination forms the basic structural unit of the metagrating. The periodic structure of the supercell lays the foundation for subsequent optical properties. The symmetry axis based on the geometric center of the second groove provides the structure with bilateral symmetry.

[0042] When the second groove is offset within a preset distance along a specific direction, the symmetry of the structure is broken. This process changes the electromagnetic field distribution within the metagrating, altering the electric and magnetic field distribution patterns originally based on the symmetrical structure, affecting the region and method of light-matter interaction. From the perspective of optical modes, the symmetry breakdown causes the original antisymmetric non-radiative BIC mode to transform. This is because the symmetry of the structure is one of the important factors in maintaining the stability of the BIC mode. Once it is broken, the mode characteristics also change. The Bloch resonance mode when the periodic structure is symmetrical and the quasi-BIC mode after the symmetry is broken, after being excited, produce highly unidirectional surface plasmons through destructive interference. The Bloch resonance mode involves the propagation characteristics of light in a periodic structure, while the quasi-BIC mode has new radiation and coupling characteristics after the symmetry is broken. The distribution and phase relationship of the electromagnetic fields in the two modes interact with each other, and through destructive interference, the propagation of surface plasmons in a specific direction is enhanced, while the propagation in the opposite direction is suppressed, thus achieving unidirectional propagation. This characteristic has important application value in optical communications, optical sensing and other fields. For example, in optical communications, it can achieve unidirectional and efficient transmission of signals, reducing interference and loss; in optical sensing, it can enhance the response sensitivity to changes in the substance or physical quantity to be measured in a specific direction.

[0043] The present disclosure provides a plasmon unidirectional coupling device, method, and application based on a metagrating. The device cleverly disrupts the geometric symmetry of the metagrating by lateral offsetting the second groove in the supercell unit, prompting the BIC mode to transform into a quasi-BIC mode, which then interacts with the Bloch resonance mode. This interaction significantly enhances the directionality of surface plasmons (SPPs) propagating along the metal dielectric surface, while improving propagation efficiency and successfully achieving efficient directional coupling. It is worth mentioning that its unidirectional coupling efficiency can reach 0.75, which is a relatively high efficiency level in the visible spectrum.

[0044] In this embodiment, the first groove depth is d1, the second groove depth is d2, and the depth difference between them, |d1-d2|≥Δd, where Δd represents the minimum depth difference, and its value is 10nm≤Δd≤50nm. Preferably, the value of Δd is 40nm, that is, the depth of the first groove and the depth of the second groove are not equal. By adjusting the depth difference between the first groove and the second groove, the coupling efficiency of the surface plasmon can be adjusted, providing an effective means for optimizing system performance in practical applications. High coupling efficiency means that more energy can be effectively coupled from the incident light to the surface plasmon, thereby improving the performance and efficiency of the system. For example, in optical communication systems, high coupling efficiency can reduce signal loss and increase transmission rate and distance; in optical sensing, high coupling efficiency can enhance the detection capability of weak signals.

[0045] In this embodiment, the period of a periodic structure of the supercell unit is p, the thickness of the substrate 1 is t2, the thickness of the dielectric plate 22 is t1, and the widths of the first and second grooves are w. The values ​​of p are 300-350 nm, t2 is 450-550 nm, t1 is 200-300 nm, and w is 40-60 nm. Preferably, the period p = 328 nm, the substrate thickness t2 = 500 nm, the dielectric plate thickness t1 = 250 nm, and the widths of the first and second grooves w = 50 nm. These clear numerical values ​​provide specific targets and standards for the manufacturing process of the metagrating, enabling strict quality inspection and control during the manufacturing process. These clear numerical values ​​also enable accurate theoretical analysis and calculations. By substituting these values ​​into relevant optical theoretical models, the optical properties of the metagrating, such as the excitation efficiency of surface plasmons and the one-way coupling efficiency, can be predicted. For example, based on a given period p and the thickness t1 of the dielectric plate 22, the resonant frequency and electromagnetic field distribution of the metagrating at a specific incident light wavelength can be calculated. Furthermore, based on clear numerical values, parameter optimization can be performed to improve the performance of metagratings. That is, by adjusting one or more parameters and observing their impact on performance, the optimal parameter combination can be found to achieve optimal performance. For example, the unidirectional coupling efficiency of surface plasmons can be optimized by varying the groove width w or the substrate thickness t2. Therefore, providing clear numerical values ​​is of great significance for the fabrication, performance optimization, and application of metagratings, helping to improve their performance and reliability.

[0046] In this embodiment, the offset of the second groove is Δ, which is set to |Δ|≤Δ max , where Δ max Represents the preset maximum offset. Clarifying the range of the offset value helps to make reasonable parameter selection when designing and applying metagratings. On the one hand, limiting the offset can avoid instability of the metagrating structure or a sharp drop in performance due to excessive offset; on the other hand, by presetting the maximum offset, the impact of different offset degrees on the surface plasmon characteristics can be explored within a certain range, thereby finding the optimal offset parameter. When the second groove is offset, the movement direction of the surface plasmon is consistent with the offset direction of the second groove, that is, the offset of the second groove can effectively guide the propagation direction of the surface plasmon. Therefore, by controlling the offset of the second groove, the propagation path of the surface plasmon can be precisely controlled. For example, in optical communication systems, the offset of the second groove can be adjusted to guide the optical signal to transmit along a specific direction, thereby improving communication efficiency and stability. At the same time, this controllability also provides a guarantee for the flexibility and adaptability of metagratings in different application scenarios.

[0047] The embodiment gives greater freedom to the operation of surface plasmons (SPPs) through a unique design, and effectively controls the propagation path and characteristics of SPPs by using the key factor of the lateral offset of the second groove in the supercell unit. This innovative control method significantly improves the flexibility of one-way control of SPPs, enabling the introduction of new physical mechanisms in the construction of optical waveguide sensors and systems, and bringing new opportunities and possibilities for the development and breakthrough of related technical fields, and helps to promote optical waveguide sensors and systems towards more efficient and more accurate directions.

[0048] The embodiment adopts a one-dimensional complex grating composed of supercell units, which is completely different from traditional one-dimensional gratings. By customizing the supercell, effective control of light coupling and SPPs generation can be implemented, which has potential application value in the fields of optical communication, sensing, etc.

[0049] The performance of the plasmonic one-way coupling device based on the superstructure grating provided by the disclosure is verified by experiment simulation.

[0050] Figure 1 (a) shows the principle diagram of the plasmonic one-way coupling device based on the superstructure grating of the embodiment, in which the substrate (preferably silver sheet) is covered by a dielectric plate (preferably SiO2 plate). At the working frequency of 545THz (i.e. the incident wavelength is 550nm), the wave vector of SPPs at the SiO2-silver interface is where k0=2π / λ represents the incident wave vector, ε m and ε d represent the dielectric constants of the metal and the dielectric, respectively. For convenience, Figure 1 The area between the two dashed lines in the above formula is designed with a groove structure, as shown in Figure 1 (b). The supercell unit contains two grooves with the same width w but different depths, denoted as d1 and d2, respectively. In the experiment, t1=250nm, t2=500nm, w=50nm, d2=d1+Δd, and d1 is a variable. To satisfy the momentum matching of the transverse magnetic polarized light incident only along the y direction, the wave vector is β spp =k0sinθ in +mG, where θ in is the angle between the incident light beam and the z direction, G=2π / p represents the inverse lattice vector of the periodic unit structure, and it is assumed that the period p=328nm, so m=±1 represents the diffraction order of the grating structure. Due to the mirror symmetry of the structure, and this symmetric structure can excite symmetric radiation Bloch resonance mode, so the typical SPPs bidirectional excitation can be seen as Figure 2(b) shows that the coupling efficiency can be controlled by the depth difference. Numerical calculations show that when Δd = 39.5 nm, the total efficiency can reach up to 90%. In addition, the unidirectional characteristic is achieved by breaking the mirror symmetry by laterally moving the second groove while maintaining p, that is, introducing a lateral offset Δ, which defines the displacement of the second groove along the x direction, as shown in Figure 1 As shown in (c), when Δ>0, it means that the second groove is offset along the +x direction; conversely, when Δ<0, it means that the second groove is offset along the -x direction. Once the mirror symmetry of the metagrating is broken, the adjacent coupling of adjacent atoms will be changed, resulting in an asymmetric coupling strength between atoms, i.e., γ AB ≠γ BA , thus generating unidirectional SPs. Figure 2 As shown in (a), when the lateral offset of the second groove is Δ=-25nm, the excited SPs surface wave moves along the -x direction, as shown in Figure 2 As shown in (c), when the lateral offset of the second trench is Δ=25 nm, the excited SPs surface wave moves along the +x direction.

[0051] At an operating frequency of 545 THz (incident wavelength of 550 nm), the relationship between the depth of the first groove and the depth difference between the two grooves is explored. Figure 3 From the above, we can see that there is a bright and efficient surface plasmon (SPPs) excitation efficiency band, which is mainly formed by the radiation-symmetric mode that excites the metagrating resonance band. This mode excitation leads to the emergence of a high-efficiency SPPs band. However, given that the structure exhibits mirror symmetry, the SPPs excited in this case propagate in two directions, not unidirectionally. This characteristic is an important phenomenon when studying the excitation and propagation of SPPs in metagratings. Subsequent research can further explore how to break the symmetry to achieve goals such as unidirectional propagation. At the same time, in-depth research on the relationship between the groove depth difference and the SPPs excitation efficiency band will help to more accurately control the optical performance of the metagrating, and has potential application value in optical communications, optical sensing and other fields.

[0052] Figure 4The relationship between the asymmetric excitation of SPPs efficiency and the groove depth and lateral shift is clearly demonstrated, and it is worth mentioning that the highest one-side propagation efficiency can reach 75%. In actual situations, due to the ohmic loss of the metal itself, the SPPs propagating along the interface will inevitably dissipate, and thus the amplitude will decrease after a certain distance of propagation. However, in this study, the main focus is on the coupling process of incident light to SPPs, rather than the subsequent propagation process of SPPs along the interface. In order to clarify the potential mechanism of one-way coupling, it is assumed that the metal has no loss in the study, and the coupling efficiency is simply defined as the ratio of the energy of the excited SPPs to the energy of the incident light. Through these research results, it is further confirmed that the one-way propagation of SPPs constructed by the interference between intrinsic modes has robust characteristics, which provides stronger theoretical basis and data support for the application of superlattices in related fields, and lays a solid foundation for subsequent research, which is helpful to promote the further development of SPPs related technologies based on superlattices in the fields of optical communication, sensing and other fields.

[0053] In order to further explore the process of one-way coupling, an interference model is used to explain the principle of manipulating the propagation direction of surface plasmons (SPPs) by the mutual interference of two intrinsic modes. Since BIC mode belongs to non-radiation mode, only symmetric mode can be excited under normal incidence, which leads to the directional and symmetric propagation of SPPs mode (as shown in Figure 5 (a)). According to the momentum matching condition, only m = ±1 diffraction order dominates, so the x-direction space field of SPPs at the interface can be simply described as where β s is the propagation wave vector, a L = a R is the coefficient. When the symmetry of the superlattice is slightly broken (i.e. Δ ≠ 0), the BIC mode is converted into quasi-BIC mode, and symmetric and anti-symmetric modes can be coupled and produce incident (as shown in Figure 5(b)). On the other hand, once the orthogonality of the two eigenmodes is broken, they can interact with each other. When the offset of the second groove is Δ = ±25nm, the absolute values ​​of the coefficients of the two eigenmodes are equal, and the phase distribution difference of the corresponding two modes is π, which is determined by the sign of Δ. Specifically, at Δ = 25nm, the absolute values ​​of the two modes are equal but their phases are inconsistent, with a difference of π, which causes the SPPs to propagate only along the +x direction; on the contrary, when the offset of the second groove is Δ = -25nm, their amplitudes are the same but the phase difference is π, which will cause the SPPs to propagate completely along the -x direction. In fact, the coupling coefficient and Q value between the two eigenmodes and the incident wave are inversely proportional to each other. Therefore, when the lateral offset Δ = 0, the coupling coefficient between the BIC mode and the incident wave is 0. As the lateral offset Δ increases, the coupling efficiency of the antisymmetric mode corresponding to the red line gradually increases, while the coupling efficiency of the symmetric mode corresponding to the blue line gradually decreases.

[0054] In particular, when the coupling strengths of the two eigenmodes are the same, the coupling efficiency reaches its strongest state, and at this time the unidirectional propagation efficiency of surface plasmons (SPPs) is the highest. Substituting these two coupling coefficients into Eq. (3), the efficiency of stimulated SPPs can be obtained. Figure 5 (d) It can be seen that the analytical results are presented as curves, which are consistent with the simulation results (represented by dots). It is obvious that the excitation efficiency of SPPs along the +x and -x directions will show different changes with the increase of the lateral offset △. The maximum unidirectional efficiency appears at the position of equal coupling coefficient, and the efficiency of left and right propagation at this position can reach 75%. And when the left and right propagation efficiency is about 10%, the unidirectional efficiency is greater than 50%. For example, when the lateral offset △ is ±25nm, the efficiency of unidirectional SPPs can reach 52%. In addition, according to the decomposition of the interference field diagram of the two eigenmodes obtained semi-analytically by COMSOL Multiphysics, Figure 5 (e) and Figure 5 The solid curve in (f) shows the corresponding analytical magnetic field distribution, which completely overlaps with the simulation results (circles), clearly explaining the unidirectional SPPs. The physical diagram shows that the antisymmetric QBIC mode and the symmetric radiation mode are in phase along the +x (-x) direction, resulting in constructive interference; while in the other direction, they are in opposite phases, resulting in destructive interference. Therefore, utilizing the interference of two different symmetric modes can freely control the directionality of surface waves, opening up a new path for surface wave propagation.

[0055] Based on the same inventive concept, the present disclosure also provides a plasmon unidirectional coupling method based on a metagrating, such as Figure 6 As shown, the method includes the following steps:

[0056] In step S1, a substrate 1 is selected, grooves 21 are periodically etched on the substrate 1, and a supercell unit is constructed by covering the grooves 21 with a dielectric plate 22 to prepare a superstructure grating, wherein one periodical structure of the supercell unit includes a first groove and a second groove, and an axis passing through the geometric center of the second groove and perpendicular to the extension direction of the second groove is taken as a symmetry axis, so that the periodical structure is symmetric about the second groove left and right;

[0057] In step S2, the second groove is offset in a direction parallel to the extension direction of the second groove and perpendicular to the symmetry axis.

[0058] In step S3, the superstructure grating is irradiated with incident light, when the periodical structure is symmetric, the superstructure grating is excited to generate Bloch resonance modes, when the symmetry of the periodical structure is destroyed by offsetting the second groove left and right, the originally existing anti-symmetric non-radiation BIC mode is converted into a quasi-BIC mode, and after the quasi-BIC mode and the Bloch resonance mode are excited, the surface plasmon has high unidirectionality through destructive interference.

[0059] The present disclosure provides a plasmonic unidirectional coupling method based on a superstructure grating, which cleverly destroys the geometric symmetry of the superstructure grating by the lateral offset of the second groove in the supercell unit, promotes the conversion of the BIC mode into a quasi-BIC mode, and further interacts with the Bloch resonance mode. This interaction significantly enhances the directionality of surface plasmons (SPPs) propagating along the metal-dielectric surface, while improving the propagation efficiency, and successfully realizes high-efficiency directional coupling. It is worth mentioning that the unidirectional coupling efficiency can reach 0.75, which is a relatively high efficiency level in the visible spectrum.

[0060] In step S3, the offset amount Δ of the second groove is adjusted to control the unidirectionality of the surface plasmon. Specifically, the offset of the second groove directly changes the symmetry of the supercell unit of the superstructure grating. The originally symmetric structure is destroyed after the offset of the second groove. This change in symmetry affects the distribution of electromagnetic fields inside the superstructure grating. When the structure is symmetric, the optical mode exhibits a certain characteristic. After the symmetry is destroyed, the originally existing anti-symmetric non-radiation BIC mode is converted into a quasi-BIC mode, which further interacts with the Bloch resonance mode. That is, by adjusting the offset amount, the symmetry degree of the structure can be accurately changed, thereby controlling the excitation and interaction of the optical mode related to the unidirectionality of the surface plasmon.

[0061] Further, the change of the offset will change the coupling between different eigenmodes, and the electromagnetic field distribution will change due to the second groove offset, and different offsets will change the electromagnetic field overlap area and phase relationship of each eigenmode. For example, when the offset is appropriate, the coupling strength of two eigenmodes is the same, at this time the coupling efficiency is the strongest, and the unidirectional propagation efficiency of surface plasmons reaches the highest. Therefore, by adjusting the offset, the coupling process can be precisely controlled, and then the control of the unidirectionality of surface plasmons is realized.

[0062] Further, the offset Δ of the second groove is |Δ|≤Δ max wherein Δ max represents the preset maximum offset, and in terms of structural stability, limiting the offset in the range is helpful to maintain the stability of the metasurface grating structure. If the offset is too large, it may cause local stress concentration or structural deformation of the metasurface grating. For example, in the manufacturing process, too large offset may cause cracks or deformation of the dielectric plate or metal substrate, affecting its optical performance and service life, and by setting the maximum value, the structure can be ensured within a certain range of deformation that can be tolerated, ensuring that the metasurface grating can maintain a relatively stable structure under long-term use or different environmental conditions, thereby ensuring the normal realization of its function. In terms of optical performance control, the explicit value range provides a basis for precise control of optical performance. Under different values, the response of the metasurface grating to light will be different, and a small change in the offset may cause fine tuning of optical properties such as surface plasmon unidirectionality, while near may bring more significant changes. Therefore, researchers can conduct value experiments and theoretical analysis within this range to find the most suitable offset value for specific application scenarios, and realize the optimization and control of the optical performance of the metasurface grating such as surface plasmon excitation efficiency, propagation direction and coupling efficiency.

[0063] The method of the above embodiment is realized by the corresponding plasmonic unidirectional coupling device based on metasurface grating in the foregoing embodiment, and has the beneficial effects of the corresponding device embodiment, and the present embodiment will not be repeated here.

[0064] Based on the same inventive concept, the present disclosure also provides an application of the plasmonic unidirectional coupling device based on the super-structured grating as described above, which is applied in the field of optical communication, optical sensing and nanophotonic imaging. In the field of optical communication, the device can realize the unidirectional coupling of surface plasmons, reduce the reflection and scattering of signals in the transmission process, and thus improve the transmission efficiency of signals. For example, by precisely controlling the parameters of the super-structured grating, such as the groove depth, width and offset of the second groove, etc., the light signal can be efficiently propagated in a specific direction, reducing signal loss and extending transmission distance. In the field of optical sensing, the device can utilize the strong localization and sensitivity to environmental changes of surface plasmons to improve the detection sensitivity of sensors. For example, when surface plasmons propagate in the super-structured grating, their propagation characteristics will be affected by physical quantities (such as temperature, pressure, chemical concentration, etc.) in the surrounding environment. By monitoring the changes in the propagation characteristics of surface plasmons, high-precision detection of these physical quantities can be achieved. In the field of nanophotonic imaging, the device can utilize the short-wavelength characteristics of surface plasmons to improve the resolution of imaging. Since the wavelength of surface plasmons is shorter than that of electromagnetic waves in free space, light focusing and imaging can be achieved on a smaller spatial scale. For example, by exciting surface plasmons on the surface of the super-structured grating and utilizing their propagation characteristics for imaging, high-resolution imaging on the nanoscale can be achieved.

[0065] It will be obvious to a person skilled in the art that, as the application is not limited to the details of the foregoing exemplary embodiments but can be implemented in other specific forms, the present application should cover all the technical solutions falling within the scope of equivalents of the claims and their technical features. Therefore, the embodiments should be considered exemplary and non-limiting, and the scope of the present application is defined by the appended claims rather than the foregoing description, and all the changes falling within the meaning and scope of the equivalent elements of the claims should be included in the present application. Any reference signs in the claims should not be considered as limiting the claims involved.

[0066] In addition, it should be understood that although the present specification is described in terms of embodiments, not every embodiment contains only one independent technical solution, and the description of the specification is only for the sake of clarity, and those skilled in the art should consider the specification as a whole, and the technical solutions in each embodiment can be appropriately combined to form other embodiments that those skilled in the art can understand.

Claims

1. A plasmon unidirectional coupling device based on a metagrating, characterized in that: A metagrating is provided, wherein the metagrating comprises: a substrate made of metal; A supercell unit, comprising a groove and a dielectric plate, wherein the groove is periodically etched on the substrate and the dielectric plate covers the groove; A periodic structure of the supercell unit includes a first groove and a second groove, and an axis passing through the geometric center of the second groove and perpendicular to the extending direction of the groove is used as a symmetry axis, and the periodic structure is bilaterally symmetrical about the second groove; The second groove can be offset in a direction parallel to the groove extension direction and perpendicular to the symmetry axis, and when the second groove is offset in this direction within a preset distance range, the symmetry of the periodic structure is destroyed; When the periodic structure is symmetrical, the metagrating is excited into the Bloch resonance mode. When the symmetry of the periodic structure is destroyed by shifting the second groove left and right, the originally existing antisymmetric non-radiative BIC mode of the metagrating is converted into a quasi-BIC mode. After the quasi-BIC mode and the Bloch resonance mode are excited, destructive interference causes the surface plasmon to have a high degree of unidirectionality.

2. The plasmon unidirectional coupling device based on a metagrating according to claim 1, characterized in that: The first trench has a depth of d1 and the second trench has a depth of d2, and their depth difference |d1-d2|≥Δd, where Δd represents the minimum depth difference, and its value is 10nm≤Δd≤50nm.

3. The plasmon unidirectional coupling device based on a metagrating according to claim 1 or 2, characterized in that: The period of a periodic structure of the supercell unit is p, the substrate thickness is t2, the dielectric plate thickness is t1, the width of the first groove and the second groove is w, and the value of p is 300-350nm, t2 is 450-550nm, t1 is 200-300nm, and w is 40-60nm.

4. The plasmon unidirectional coupling device based on a metagrating according to claim 3, characterized in that: The period p=328 nm, the substrate thickness t2=500 nm, the dielectric plate thickness t1=250 nm, and the widths w of the first and second grooves are 50 nm.

5. The plasmon unidirectional coupling device based on a metagrating according to claim 3, characterized in that: The offset of the second groove is Δ, which is set to |Δ|≤Δ max , where Δ max Indicates the preset maximum offset.

6. The plasmon unidirectional coupling device based on a metagrating according to claim 5, characterized in that: When the second groove deflects, the moving direction of the surface plasmon is consistent with the deflection direction of the second groove.

7. A plasmon unidirectional coupling method based on a metagrating, characterized by: The method comprises the following steps: Step S1: Select a substrate, periodically etch grooves on the substrate, and cover the grooves with a dielectric plate to construct a supercell unit to prepare a metagrating, wherein a periodic structure of the supercell unit includes a first groove and a second groove, and an axis passing through the geometric center of the second groove and perpendicular to the extension direction of the second groove is used as a symmetry axis, so that the periodic structure is bilaterally symmetrical about the second groove; Step S2, offsetting the second groove in a direction parallel to the extending direction of the second groove and perpendicular to the symmetry axis; Step S3, using incident light to illuminate the metagrating. When the periodic structure is symmetrical, the metagrating is excited into the Bloch resonance mode. When the symmetry of the periodic structure is destroyed by shifting the second groove left and right, the originally existing antisymmetric non-radiative BIC mode of the metagrating is converted into a quasi-BIC mode. After the quasi-BIC mode and the Bloch resonance mode are excited, destructive interference causes the surface plasmon to have a high degree of unidirectionality.

8. The method for unidirectional plasmon coupling based on a metagrating according to claim 7, characterized in that: In step S3, the method further includes adjusting the offset Δ of the second groove to control the unidirectionality of the surface plasmon.

9. The method for unidirectional plasmon coupling based on a metagrating according to claim 8, characterized in that: The offset Δ of the second groove is set to |Δ|≤Δ max , where Δ max Indicates the preset maximum offset.

10. An application of a plasmon unidirectional coupling device based on a metagrating according to any one of claims 1 to 6, characterized in that: It is used in the fields of optical communications, optical sensing and nanophotonics imaging.

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