Plasmon one-way coupling device and method based on super-structure grating and application of plasmon one-way coupling device and method

Abstract

The invention provides a plasmon one-way coupling device and method based on a super-structure grating and application, the device comprises a substrate, a groove and a dielectric plate, the groove is periodically etched on the substrate, and the dielectric plate covers the groove; the periodic structure comprises a first groove and a second groove, and the periodic structure is bilaterally symmetrical about the second groove by taking an axis which passes through the center of the second groove and is vertical to the extending direction of the groove as a symmetry axis; when the structure is symmetrical, a Bloch resonance mode is excited, and when the second groove shifts along a direction parallel to the extension direction of the groove and perpendicular to the symmetry axis, the symmetry of the structure is destroyed, the antisymmetric non-radiative BIC mode is converted into a quasi-BIC mode, and after the quasi-BIC mode and the Bloch resonance mode are excited, the Bloch resonance mode is excited. Through destructive interference, the surface plasmon has high unidirectivity. According to the invention, by adjusting the transverse offset of the second groove, the propagation directivity of the surface plasmon along the metal dielectric surface is significantly enhanced, efficient directional coupling is realized, and the propagation efficiency is improved at the same time.

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 way to manipulate light on a subwavelength scale and are promising candidates for highly integrated photonic circuits, and have attracted extensive attention in highly integrated photonic devices such as plasmonic circuits, waveguides, and subwavelength imaging technologies. In order to control the directionality of SPPs 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 to the SPPs mode.

[0003] The emergence of metasurfaces has opened up new possibilities for manipulating SPPs. Recent studies have shown 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, as well as unidirectional emission of light beams carrying polarization-dependent and orbital angular momentum into unpolarized photoluminescence in spatially symmetric nanostructures. However, these unidirectional emitters are limited in practical application due to their complex geometries and require expensive manufacturing processes. In addition, the integration of such plasmonic devices into optical systems faces important challenges, mainly the generation of SPPs modes with high conversion efficiency and the control of the propagation direction.

[0004] In the past decade, a variety of methods for designing couplers have been proposed to convert incident light into sub-wavelength nanoscale surface plasmons (SPs) in a unidirectional manner, including magneto-optical effects, optical gratings, non-Hermitian systems with parity-time symmetry, topological photonic crystals, and gradient metasurfaces. Although these methods have made great progress in theory and experiment, they have their own limitations in efficiency. At the same time, some studies are committed to using specific light sources with spin or orbital angular momentum (OAM) to excite SPs. Although obvious unidirectional SPs can be obtained, most of the incident light is scattered into the background due to the momentum mismatch problem, resulting in a 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 symmetry breaking in time or space. 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 have not taken 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 purpose, the present disclosure provides a plasmon unidirectional coupling device based on a metagrating, including a metagrating, wherein the metagrating includes:

[0009] A substrate, which is made of metal;

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

[0011] Wherein, a periodic structure of the supercell unit includes a first groove and a second groove, with an axis passing through the geometric center of the second groove and perpendicular to the groove extension direction 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 groove depth is d 1 , the second groove depth is d 2 , the depth difference |d 1 -d 2 |≥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, and the substrate thickness is t 2 , the thickness of the dielectric plate is t 1 , the width of the first trench and the second trench is w, and its value p is 300-350nm, t 2 450-550nm, t 1 is 200-300nm, and w is 40-60nm.

[0016] In one embodiment, the period p = 328 nm, the substrate thickness t 2 =500nm, dielectric plate thickness t 1 =250nm, the width of the first trench and the second trench w=50nm.

[0017] In one embodiment, the offset of the second groove is Δ, and its value is |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, the method comprising the following steps:

[0020] Step S1, selecting a substrate, periodically etching grooves on the substrate, and covering the grooves with a dielectric plate to construct a supercell unit, so as 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 a periodic structure is left-right 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 is used to make the surface plasmon highly unidirectional.

[0023] In one embodiment, in step S3, the method 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 to the fields of optical communications, optical sensing and nanophotonics imaging.

[0026] From the above, it can be seen that the present disclosure provides a plasmon unidirectional coupling device, method and application based on a meta-grating, which cleverly destroys the geometric symmetry of the meta-grating 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] The present invention provides a large degree of freedom for the operation of surface plasmons (SPPs) through a unique design, and utilizes the lateral offset of the second groove in the supercell unit as a key factor to effectively control the propagation path and characteristics of SPPs. This innovative control method significantly improves the flexibility of unidirectional control of SPPs, enables the introduction of new physical mechanisms in the construction of optical waveguide sensors and systems, brings new opportunities and possibilities for the development and breakthroughs in related technical fields, and helps to promote optical waveguide sensors and systems towards a more efficient and accurate direction.

[0028] The present invention adopts a one-dimensional complex grating based on 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 the fields of optical communication and sensing. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] In order to more clearly illustrate the technical solutions in the present disclosure or related technologies, the drawings required for use in the embodiments or related technical descriptions are briefly introduced below. 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 creative work.

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

[0031] Figure 2 A numerical simulation electric field variation diagram of the second groove provided in the present disclosure exciting SPPs at different lateral offsets;

[0032] Figure 3 The total efficiency of SPPs excited by the present disclosure varies with the first trench depth d 1 The difference between the two groove depths △d The change relationship diagram of

[0033] Figure 4 The efficiency characterization diagram of the unidirectional excitation SPPs of the metagrating disclosed in the present invention when the incident wavelength is 550nm, where (a) represents the SPPs efficiency on the left side of the metagrating and d 1 and Δ, (b) shows the relationship between the SPPs efficiency on the right side of the metagrating and d 1 The relationship between Δ and

[0034] Figure 5 Schematic diagram of the interference model provided in the present invention, wherein (ab) represent the effect of the change in the value of the lateral offset △ on the excitation mode, (c) represents the relationship between the amplitude coefficient 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; (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 invention.

[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 combination 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 be 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. "Including" or "comprising" and similar words mean that the elements or objects appearing before the word cover 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 can include electrical connections, whether direct or indirect. "Up", "down", "left", "right" and the like are only used to indicate relative positional relationships. When the absolute position of the described object changes, the relative positional 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 known 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, wherein an axis passing through the geometric center of the second groove and perpendicular to the extending direction of the groove is a symmetry axis, and the periodic structure is bilaterally symmetric about the second groove; the second groove can The structure is offset in a direction parallel to the groove extension direction and perpendicular to the symmetry axis. When the second groove is offset in 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 a Bloch resonance mode. When the symmetry of the periodic structure is destroyed by offsetting the second groove to the 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 is used to make the surface plasmons highly unidirectional.

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

[0042] When the second groove is offset by a preset distance range in a specific direction, the symmetry of the structure is broken. This process changes the electromagnetic field distribution inside the metagrating, and the electric and magnetic field distribution laws originally based on the symmetrical structure are changed, affecting the area and mode of interaction between light and matter. From the perspective of optical modes, the destruction of symmetry causes the original antisymmetric non-radiative BIC mode to change, because the symmetry of the structure is one of the important factors to maintain the stability of the BIC mode. Once it is destroyed, the mode characteristics will also change. The Bloch resonance mode when the periodic structure is symmetrical and the quasi-BIC mode after the symmetry is broken, both of which produce highly unidirectional surface plasmons through destructive interference after being excited. 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, thereby achieving unidirectional propagation. This characteristic has important application value in the fields of optical communication and optical sensing. For example, in optical communication, it can realize unidirectional and efficient transmission of signals and reduce 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, 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.

[0044] In this embodiment, the depth of the first groove is d 1 , the second groove depth is d 2 , the depth difference |d 1 -d 2|≥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, which provides 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, and the thickness of the substrate 1 is t. 2 , the thickness of the dielectric plate 22 is t 1 , the width of the first trench and the second trench is w, and its value p is 300-350nm, t 2 450-550nm, t 1 is 200-300nm, w is 40-60nm. Preferably, the period p=328nm, the substrate thickness t 2 =500nm, dielectric plate thickness t 1 = 250nm, the width of the first groove and the second groove w = 50nm, the clear values ​​provide specific goals and standards for the manufacturing process of the meta-grating, and strict quality inspection and control can be carried out during the manufacturing process. There are also clear values ​​that can be used for accurate theoretical analysis and calculation. By substituting these values ​​into relevant optical theoretical models, the optical properties of the meta-grating, such as the excitation efficiency of the surface plasmon and the unidirectional coupling efficiency, can be predicted. For example, according to the given period p and the thickness t of the dielectric plate 22, 1 , the resonant frequency and electromagnetic field distribution of the metagrating under a specific incident light wavelength can be calculated. And based on clear numerical values, parameter optimization can be performed to improve the performance of the metagrating, that is, by adjusting one or more parameters and observing their effects on the performance, the best parameter combination can be found to achieve the best performance. For example, by changing the groove width w or the substrate thickness t 2 To optimize the unidirectional coupling efficiency of surface plasmons. Therefore, providing clear numerical values ​​is of great significance for the manufacture, performance optimization and application of metagratings, and helps to improve the performance and reliability of metagratings.

[0046] In this embodiment, the offset of the second groove is Δ, and its value is |Δ|≤Δ max , where Δ maxRepresents the preset maximum offset. Clarifying the range of the offset value helps to make reasonable parameter selection when designing and applying meta-gratings. On the one hand, limiting the offset can avoid the instability of the meta-grating structure or a sharp drop in performance due to excessive offset; on the other hand, by presetting the maximum offset, the influence of different offset degrees on the surface plasmon characteristics can be explored within a certain range, so as to find the optimal offset parameter. When the second groove is offset, the moving 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 an optical communication system, the optical signal can be guided to transmit in a specific direction by adjusting the offset of the second groove, thereby improving communication efficiency and stability. At the same time, this controllability also provides a guarantee for the flexibility and adaptability of meta-gratings in different application scenarios.

[0047] This implementation method gives a large degree of freedom to the operation of surface plasmons (SPPs) through a unique design, and uses the lateral offset of the second groove in the supercell unit as a key factor to effectively control the propagation path and characteristics of SPPs. This innovative control method significantly improves the flexibility of unidirectional control of SPPs, allowing the introduction of new physical mechanisms in optical waveguide sensors and system construction, bringing new opportunities and possibilities for the development and breakthroughs in related technical fields, and helping to promote optical waveguide sensors and systems towards a more efficient and precise direction.

[0048] This embodiment adopts a one-dimensional complex grating based on 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 communication, sensing and other fields.

[0049] The performance of the plasmon unidirectional coupling device based on the metagrating provided by the present invention is verified by experimental simulation below.

[0050] Figure 1 (a) shows the principle diagram of the plasmon unidirectional coupling device based on the metagrating of this embodiment, in which the substrate (preferably a silver sheet) is surrounded by a dielectric plate (preferably SiO 2 At an operating frequency of 545THz (i.e., an incident wavelength of 550nm), this SiO 2 The wave vector of SPPs at the -Silver interface is Among them, k 0 =2π / λ represents the incident wave vector, ε m and ε d Represent the dielectric constants of metal and dielectric respectively. For convenience, Figure 1The area between the two dotted lines in the figure is designed with a groove structure, such as Figure 1 As shown in (b), the supercell contains two grooves with the same width w but different depths, denoted by d 1 and d 2 In the experiment, t 1 =250nm, t 2 =500nm,w=50nm,d 2 =d 1 +Δd,d 1 is a variable. To satisfy the momentum matching when the transverse magnetic polarized light is incident only along the y direction, the wave vector is β spp =k 0 sinθ in +mG, where θ in is the angle between the incident beam and the z direction, G = 2π / p represents the reciprocal lattice vector of the periodic unit structure, and if the period p = 328nm, then m = ±1 represents the diffraction order of the grating structure. Due to the mirror symmetry of the structure, and this symmetrical structure can excite the Bloch resonance mode of symmetrical radiation, we can see the typical SPPs bidirectional excitation such as Figure 2 (b) shows that the coupling efficiency can be controlled by the depth difference. It is found through numerical calculation that when Δd = 39.5nm, the total efficiency can be as high as 90%. In addition, the unidirectional characteristic is achieved by breaking the mirror symmetry by lateral displacement of 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 meta-grating 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 of the two grooves was explored. Figure 3From 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 the mirror-symmetric characteristics of the structure, 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 properties of metagratings, and has potential application value in optical communications, optical sensing and other fields.

[0052] Figure 4 The relationship between the efficiency of asymmetric excitation of surface plasmons (SPPs) by metagratings and the groove depth and lateral offset is clearly demonstrated. It is worth mentioning that the highest single-sided propagation efficiency can reach 75%. In actual situations, due to the ohmic loss of metals themselves, SPPs propagating along the interface will inevitably dissipate, so that the amplitude decreases after propagating for a certain distance. However, in this study, the focus is mainly on the coupling process of incident light to SPPs, rather than the subsequent propagation of SPPs along the interface. In order to clarify the potential mechanism of unidirectional coupling, it is assumed in the study that there is no loss in the metal, and the coupling efficiency is simply defined as the ratio of the excited SPPs energy to the incident light energy. Through these research results, it is further confirmed that the unidirectional propagation of SPPs constructed by interference between eigenmodes has a robust characteristic, which provides a stronger theoretical basis and data support for the application of metagratings in related fields, and also lays a solid foundation for subsequent research, which will help promote the further development of SPPs-related technologies based on metagratings in optical communications, sensing and other fields.

[0053] In order to further explore the process of unidirectional coupling, an interference model is used to explain the principle of mutual interference between two eigenmodes and thus control the propagation direction of surface plasmons (SPPs). Since the BIC mode is a non-radiative mode, it can only excite the symmetric mode under normal incident light, resulting in the SPPs mode showing directional and symmetric propagation (such as Figure 5 (a)). According to the momentum matching condition, only m = ±1 diffraction orders dominate, so the x-direction spatial field of SPPs at the interface can be simply described as In the formula, β s is the propagation wave vector, a L =a R When the symmetry of the superlattice is slightly destroyed (i.e., △≠0), the BIC mode transforms into a quasi-BIC mode, and the symmetric and antisymmetric modes can couple simultaneously and generate incident light (e.g., 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 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 makes the SPPs 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 the strongest state, and at this time, the unidirectional propagation efficiency of surface plasmons (SPPs) is the highest. Substituting these two coupling coefficients into the formula 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, where the efficiency of left and right propagation 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 physical curve in (f) shows the corresponding analytical magnetic field distribution, which completely coincides with the simulation results (dots), clearly explaining the situation of unidirectional SPPs. The physical diagram shows that the antisymmetric QBIC mode and the symmetric radiation mode are in the same phase along the +x (-x) direction, which will produce constructive interference; while in the other direction, they are in opposite phases and produce destructive interference. Therefore, by using the interference of two different symmetric modes, the directionality of surface waves can be freely controlled, opening up a new way for the propagation of surface waves.

[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 comprises the following steps:

[0056] Step S1, selecting a substrate 1, periodically etching a groove 21 on the substrate 1, and covering the groove 21 with a dielectric plate 22 to construct a supercell unit, so as 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 a periodic structure is left-right symmetrical about the second groove;

[0057] Step S2, offsetting the second groove in a direction parallel to the extending direction of the second groove and perpendicular to the symmetry axis;

[0058] 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 to the left and right, the 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 is used to make the surface plasmon highly unidirectional.

[0059] The present disclosure provides a plasmon unidirectional coupling method 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.

[0060] Among them, in step S3, it also includes adjusting the offset Δ of the second groove to regulate the unidirectionality of the surface plasmon. Specifically, the offset of the second groove directly changes the symmetry of the supercell unit of the meta-grating. The originally symmetrical structure is destroyed after the second groove is offset. This change in symmetry affects the distribution of the electromagnetic field inside the meta-grating. When the structure is symmetrical, the optical mode presents a characteristic. After the symmetry is destroyed, the originally existing antisymmetric non-radiative BIC mode is converted into a quasi-BIC mode, and then interacts with the Bloch resonance mode. That is, by adjusting the offset, the degree of symmetry of the structure can be accurately changed, thereby regulating the excitation and interaction of the optical mode related to the unidirectionality of the surface plasmon.

[0061] Moreover, the change of the offset will change the coupling between different eigenmodes. Since the offset of the second groove will cause the change of the electromagnetic field distribution, 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 the two eigenmodes is the same, the coupling efficiency is the strongest at this time, and the unidirectional propagation efficiency of the surface plasmon reaches the highest. Therefore, by adjusting the offset, the coupling process can be precisely controlled, thereby achieving the control of the unidirectionality of the surface plasmon.

[0062] Furthermore, the offset Δ of the second groove is set to |Δ|≤Δ max , where Δ max Represents the preset maximum offset. In terms of structural stability, limiting the offset within the range helps maintain the stability of the meta-grating structure. If the offset is too large, it may cause local stress concentration or structural deformation of the meta-grating. For example, during the manufacturing process, excessive offset may cause cracks or deformation in the dielectric plate or metal substrate, affecting its optical performance and service life. By setting the maximum value, the structure can be guaranteed to be within the tolerable deformation range to a certain extent, ensuring that the meta-grating can still 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 regulation, a clear range of values ​​provides a basis for precise regulation of optical performance. Under different values, the response of the meta-grating to light will be different. Smaller offset changes may cause fine-tuning of optical properties such as surface plasmon unidirectionality, while closer values ​​may bring more significant changes. Therefore, researchers can find the offset value that best suits a specific application scenario by conducting value experiments and theoretical analysis within this range, and optimize the optical performance of the meta-grating, such as surface plasmon excitation efficiency, propagation direction, and coupling efficiency.

[0063] The method of the above-mentioned embodiment is implemented by the corresponding plasmon unidirectional coupling device based on metagrating in the aforementioned embodiment, and has the beneficial effects of the corresponding device embodiment, which will not be described in detail in this embodiment.

[0064] Based on the same inventive concept, the present disclosure also provides an application of a plasmon unidirectional coupling device based on a meta-grating as described above, which is applied to the fields of optical communication, optical sensing, and nanophotonics imaging. In the field of optical communication, the device can realize unidirectional coupling of surface plasmons, reduce reflection and scattering of signals during transmission, and thus improve the transmission efficiency of signals. For example, by precisely controlling the parameters of the meta-grating, such as the groove depth, width, and offset of the second groove, the optical signal can be efficiently propagated in a specific direction, reducing signal loss and extending the transmission distance. In the field of optical sensing, the device can utilize the strong localization of surface plasmons and their sensitivity to changes in the surrounding environment to improve the detection sensitivity of the sensor. For example, when a surface plasmon propagates in a meta-grating, its propagation characteristics are affected by physical quantities in the surrounding environment (such as temperature, pressure, chemical concentration, etc.). By monitoring changes in the propagation characteristics of surface plasmons, high-precision detection of these physical quantities can be achieved. In the field of nanophotonics imaging, the device can use 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 a metagrating and using their propagation characteristics for imaging, high-resolution imaging at the nanoscale can be achieved.

[0065] It will be apparent to those skilled in the art that the invention is not limited to the details of the exemplary embodiments described above and that the invention can be implemented in other specific forms without departing from the spirit or essential features of the invention. Therefore, the embodiments should be considered exemplary and non-limiting in all respects, and the scope of the invention is defined by the appended claims rather than the foregoing description, and it is intended that all variations falling within the meaning and scope of the equivalent elements of the claims be included in the invention. Any reference numeral in a claim should not be considered as limiting the claim to which it relates.

[0066] In addition, it should be understood that although the present specification is described according to implementation modes, not every implementation mode contains only one independent technical solution. This description of the specification is only for the sake of clarity. Those skilled in the art should regard the specification as a whole. The technical solutions in each embodiment may also be appropriately combined to form other implementation modes that can be understood by those skilled in the art.

Claims

1. A plasmon unidirectional coupling device based on a metagrating, characterized in that: A meta-grating is included, wherein the meta-grating includes: A substrate, which is 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 is covered on the groove; Wherein, a periodic structure of the supercell unit includes a first groove and a second groove, with an axis passing through the geometric center of the second groove and perpendicular to the groove extension direction 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 groove depth is d1, the second groove depth is d2, and the depth difference |d1-d2|≥Δd, wherein Δd represents the minimum depth difference, and the value thereof is 10nm≤Δd≤50nm.

3. A 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=328nm, the substrate thickness t2=500nm, the dielectric plate thickness t1=250nm, and the width w of the first groove and the second groove is 50nm.

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, and its value is |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 in that: The method comprises the following steps: Step S1, selecting a substrate, periodically etching grooves on the substrate, and covering the grooves with a dielectric plate to construct a supercell unit, so as 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 a periodic structure is left-right 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 is used to make the surface plasmon highly unidirectional.

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 as claimed in 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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