Double-ring air hole metasurface quintuple fano resonance sensor and design method

Abstract

The application discloses a kind of double-circle air hole super surface five Fano resonance sensors and design method, the sensor includes silica substrate and the periodic array arrangement multiple super surface unit structures on silica substrate, each super surface unit structure is silicon nanometer cube, silicon nanometer cube is etched with two different radius circular air hole, the radius difference of two circular air holes forms asymmetric structure.Advantages are: through the radius difference of two circular air holes asymmetric structure, excite five sharp Fano resonance in near infrared wave band, with high Q value and high sensitivity advantage;With polarization-dependent electromagnetic induced transparency effect and bidirectional optical switch control ability, can be used as high-performance refractive index sensor and tunable optical switch simultaneously, provide new design ideas for multifunctional integrated photonic device;Adopt silicon-based all-dielectric structure, fully compatible with silicon-based CMOS technology, simple preparation process, easy for integrated application.

Description

Technical Field

[0001] This invention belongs to the field of metasurface optical sensing technology, specifically relating to a double-circular air-hole metasurface fivefold Fano resonance sensor and its design method. This sensor can excite fivefold Fano resonance in the near-infrared band and has polarization modulation function, and can be applied to fields such as biomedical detection and environmental monitoring. Background Technology

[0002] Near-infrared light has been applied in many fields such as biomedicine, food safety testing, environmental monitoring, 3D printing, etching, and camera sensing. In these imaging and detection fields, optical sensing lenses are essential core components. However, traditional optical sensors are limited by the diffraction limit, making it difficult to meet the requirements of non-destructive, high-sensitivity micro-detection. Traditional optical lenses are based on the principle of refraction, using the thickness variation of curved media to control the phase distribution of light waves, thereby achieving functions such as beam focusing and imaging. Optical devices with traditional lenses are usually bulky and inefficient, and also have inherent chromatic aberration problems, which can lead to incomplete or blurry images.

[0003] Metasurfaces are two-dimensional artificial structures capable of manipulating electromagnetic waves on a subwavelength scale, offering new insights for high-performance sensing. Current research categorizes them into metallic and all-dielectric types. The former achieves optical field manipulation by exciting surface plasmon resonances through metallic nanostructures; however, non-radiative losses lead to resonance peak broadening, reduced quality factor (Q) and FOM (Form Factor of Quality) values, and may also cause system temperature rise, affecting analyte stability and limiting the application of SPR-based micro / nano devices in sensing. In contrast, all-dielectric metasurfaces fundamentally avoid ohmic losses by utilizing the displacement current mechanism of high-refractive-index dielectric materials. They possess narrower linewidths, higher quality factors, and stronger optical field localization capabilities, and are well-suited for CMOS processes. Their excellent integration capabilities and large-scale production potential are driving the development of micro / nano optoelectronic devices towards chip-based and integrated designs.

[0004] Currently, metasurfaces have wide applications in the field of sensing, and Fano resonance is an effective way to realize metasurface sensing. However, most existing metasurface structures can only excite a single Fano resonance, which to some extent limits their sensing range and overall performance. Realizing multiple Fano resonance phenomena in metasurfaces has become an important focus of this research direction. At the same time, existing structures generally suffer from weak polarization control and limited functionality, making it difficult to simultaneously achieve high-sensitivity refractive index sensing and optical switching, among other multifunctional integrations. Therefore, there is an urgent need to design an all-dielectric metasurface refractive index sensor that can excite multiple Fano resonances, possesses both high Q-value and high sensitivity, and has polarization-tunable characteristics. In view of this, this invention proposes a five-fold Fano resonance sensor with a dual-circular air-hole metasurface and its design method. Summary of the Invention

[0005] The technical problem this invention aims to solve is to provide a double-circular air-hole metasurface fivefold Fano resonance sensor and its design method, addressing the shortcomings of existing technologies. This sensor utilizes the radius difference between two circular air holes to form an asymmetric structure, exciting a fivefold sharp Fano resonance in the near-infrared band. It combines high Q-value and high sensitivity, achieving a maximum quality factor of 988 and a maximum refractive index sensitivity of 273 nm / RIU. Simultaneously, it possesses polarization-dependent electromagnetically induced transparency (EIT)-like effect and bidirectional optical switching capability. By changing the incident light polarization angle, the fivefold Fano resonance and EIT-like effect can be switched, achieving an EIT-like window sensitivity of 215 nm / RIU and a maximum quality factor (FOM) of 54.28 RIU. -1 This allows it to be used simultaneously as a high-performance refractive index sensor and a tunable optical switch, providing a new design approach for multifunctional integrated photonic devices. Furthermore, the sensor adopts a silicon-based all-dielectric structure, is fully compatible with silicon-based CMOS processes, has a simple fabrication process, and is easy to integrate into applications.

[0006] The technical solution adopted by the present invention to solve the above-mentioned technical problems is as follows: a dual-circular air-hole metasurface fivefold Fano resonance sensor, including a silicon dioxide substrate and multiple metasurface unit structures arranged in a periodic array on the silicon dioxide substrate. Each metasurface unit structure is a silicon nanocube, and two circular air holes with different radii are etched on the silicon nanocube. The difference in radii of the two circular air holes forms an asymmetric structure.

[0007] The sensor of this invention forms an asymmetric structure through the radius difference of two circular air holes, breaking the structural symmetry of silicon nanocubes and transforming the symmetry-protected continuous-domain bound state into a quasi-continuous-domain bound state mode (i.e., BIC). This results in two new Fano resonance curves on the basis of the triple Fano resonance of the double-symmetric air holes, achieving a fivefold Fano resonance with both high Q value and high FOM value. The sensor can simultaneously excite a fivefold sharp Fano resonance dominated by a magnetic quadrupole (P1), a magnetic dipole (P2), an electric quadrupole (P3), an electric quadrupole (F1), and an electric dipole (F2) in the near-infrared band of 750 nm-2500 nm, with the highest quality factor reaching 988 and the highest refractive index sensitivity reaching 273 nm / RIU. At the same time, this asymmetric structure endows the sensor with polarization-dependent characteristics. Furthermore, regarding polarization characteristics, when the incident light polarization angle increases from 0° to 90°, the original fivefold Fano resonance completely disappears, and an electromagnetically induced transparent window effect appears. The refractive index sensitivity of the resonance peak of the electromagnetically induced transparent window reaches 215 nm / RIU, and the highest quality factor FOM value reaches 54.28 RIU. -1 This further provides the design with bidirectional optical switch control capability.

[0008] Preferably, the multiple metasurface unit structures are arranged in a periodic array along the x-axis and y-axis, with the period Px in the x-axis direction being equal to the period Py in the y-axis direction. This technical solution enables the metasurface to have symmetrical lattice constants in the x-axis and y-axis directions, ensuring the periodic symmetry of the structure in two orthogonal directions. It also helps to eliminate the additional anisotropy that may be introduced by the periodic difference in the x-axis and y-axis directions, making the sensor's control characteristics of the incident light polarization angle clearer and more controllable.

[0009] Preferably, Px = Py = 700 nm; the width of the silicon nanocube is L = 600 nm, and the structural thickness is t = 150 nm. The above periodic parameters, combined with the silicon nanocube width L = 600 nm, maintain an appropriate gap, enabling the metasurface to stably excite a fivefold Fano resonance in the near-infrared band. Simultaneously, this periodic setting ensures suitable coupling strength between metasurface unit structures, avoiding excessively strong coupling between adjacent units due to an excessively small period or increased radiation loss due to an excessively large period. These are preferred parameters for achieving high Q-value resonance. Furthermore, the parameter design of L = 600 nm and t = 150 nm allows the silicon nanocube to support Mie resonance in the near-infrared band, providing a structural basis for the excitation of various multipole modes such as magnetic quadrupoles, magnetic dipoles, electric quadrupoles, and electric dipoles. The thickness t = 150 nm results in a constant and extremely low full width at half maximum (FWHM) for the resonance peak at P1, corresponding to an extremely high quality factor, and this thickness also provides high tolerance for processing errors.

[0010] Preferably, the center distance between the two circular air holes is d1 = 280 nm, and the radii of the two circular air holes are ra and rb, respectively, where ra = 100 nm and rb = 60 nm, and the asymmetry parameter α = ra - rb = 40 nm. In the symmetric structure with α = 0 nm, only a triplet Fano resonance can be excited, and the positions of F1 and F2 correspond to ideal continuous-domain bound states with approximately zero linewidth. When α = 40 nm, the structural symmetry is broken, establishing a radiation path between the free-space continuous spectrum and the non-radiative bound state. Two new Fano resonance curves appear at λ = 1351.5 nm and λ = 1506.9 nm. The original three Fano resonance curves undergo a blue shift and the Q value increases. Finally, a quintet Fano resonance is excited in the 1000 nm-1600 nm band, and the Q value satisfies the inverse square relationship Qrad ∝ β. -2 We define a new concept, asymmetry β = ΔS / S, where ΔS is the area of ​​the fractured portion and S is the area of ​​the silicon dielectric portion.

[0011] Preferably, the sensor excites a five-fold Fano resonance in the near-infrared band. The quality factors (Q values) corresponding to the five-fold Fano resonance are 988, 233, 322, 223, and 718, respectively, with corresponding refractive index sensitivities of 165 nm / RIU, 273 nm / RIU, 94 nm / RIU, 259 nm / RIU, and 216 nm / RIU, and corresponding resonance wavelengths of 1061.7 nm, 1140.1 nm, 1402.6 nm, 1351.5 nm, and 1506.9 nm. This sensor possesses multi-channel high-sensitivity detection capabilities, with a Q value close to 1000 at P1 and approximately 700 at F2. The five resonance wavelengths exhibit a good linear relationship with the change in ambient refractive index, enabling wide-band, high-precision refractive index sensing.

[0012] Preferably, when the incident light polarization angle increases from 0° to 90°, the original five-fold Fano resonance disappears, while an electromagnetically induced transparency effect is excited and a new triple resonance mode is generated, and the wavelength remains unchanged. When the incident light polarization angle γ=0°, the transmission spectrum exhibits a single Fano resonance line shape, the bright mode is directly excited, and the dark mode is not effectively excited; as γ gradually increases to 90°, destructive interference occurs between the bright mode and the dark mode, and a transparency window gradually appears. When γ=90°, the modulation amplitude of the transparency window resonance peak approaches 100%, the electromagnetically induced transparency effect reaches a better state, and the original five-fold Fano resonance peak gradually disappears, realizing polarization control of the resonance mode.

[0013] Furthermore, the resonant peak refractive index sensitivity of the electromagnetically induced transparent window is 215 nm / RIU, and the highest quality factor (FOM) is 54.28 RIU. -1 The electromagnetically induced transparency peak mainly originates from the coupling between the ring dipole and the electric quadrupole, exhibiting extremely high density of states and intense dispersion characteristics. When the ambient refractive index increases from 1.31 to 1.35, the EIT-like peak position undergoes a significant redshift. The wavelength shift is 2.15 nm when the refractive index change is only 0.01, demonstrating high refractive index sensitivity.

[0014] Preferably, the bidirectional switching function is achieved by adjusting the polarization angle of the incident light: when the polarization angle is 0°, the fivefold Fano resonance is activated and the electromagnetically induced transparency effect is deactivated; when the polarization angle is 90°, the fivefold Fano resonance is deactivated and the electromagnetically induced transparency effect is activated. By adjusting the polarization angle of the incident light, the switching state of the resonance mode can be achieved. The sensor can be used simultaneously as a high-performance refractive index sensor and a tunable optical switch, realizing multifunctional integration.

[0015] A design method for the above-mentioned double-circular air-hole metasurface fivefold Fano resonance sensor includes the following steps: Step S1: Select the material system and configuration of the metasurface. The substrate is made of silicon dioxide, and the multiple metasurface unit structures are made of silicon material. Each metasurface unit structure is determined to be a silicon nanocube configuration with two circular air holes etched in it. Step S2: Simulate the multiple metasurface unit structures using the finite-difference time-domain method and set the simulation conditions; Step S3: Define the asymmetry parameter as the radius difference between the two circular air holes. By adjusting the radius difference, the structural symmetry is broken to excite the fivefold Fano resonance. Step S4: Scan and optimize the structural parameters of the metasurface to obtain the optimal structural geometric parameters, thus completing the design of the metasurface structure.

[0016] This invention's design method is based on Mie scattering theory, Fano resonance theory, and continuous-domain bound state theory. It simulates the metasurface unit structure using the finite-difference time-domain method and optimizes geometric parameters such as structural thickness, distance between the centers of the two circular holes, unit period, and unit width to obtain the optimal structural geometric parameters. Simultaneously, it quantitatively analyzes the influence of material absorption loss and geometric parameters on radiation loss: analyzing the evolution of the transmission spectrum under different extinction coefficients verifies the metasurface's low sensitivity to optical loss; analyzing the influence of geometric parameters on the coupling leakage rate between bound and continuous states reveals the regulation of radiation loss, demonstrating that the structure has a high tolerance for processing errors. This ensures that the designed metasurface can simultaneously excite a fivefold Fano resonance in the near-infrared band and possess polarization control capabilities.

[0017] Preferably, the simulation conditions set in step S2 include: the multiple metasurface unit structures are periodic boundary conditions along the x-axis and y-axis, 64 perfectly matched layers are set in the z-axis, the mesh size is set to 3 nm × 3 nm × 3 nm, and the energy convergence criterion is set to 10. -9 In the following, the light source is a plane wave incident perpendicularly along the negative z-axis, with the polarization direction parallel to the x-axis; the structural parameters of the metasurface in step S4 include: the period of the multiple metasurface unit structures along the x-axis and the period along the y-axis, the width and structural thickness of the silicon nanocube, the center distance, radius, and asymmetric parameters of the two circular air holes. The above simulation settings ensure the reliability and stability of the high-Q mode simulation results. Parameter scanning quantitatively analyzes the control law of material absorption loss and geometric parameters on radiation loss; wherein the extinction coefficient k is less than 10. -5 The resonance line shape remains basically unchanged at the order of magnitude, proving that the sensor has a high tolerance for processing errors. By analyzing the influence of four core geometric dimensions—structural thickness, center distance, unit period, and unit width—on the Q value, the high-sensitivity detection potential of the two channels, P1 peak and F2 peak, was clarified.

[0018] Compared with the prior art, the present invention has the following advantages: 1. This invention forms an asymmetric structure by the radius difference of two circular air holes, breaking the structural symmetry of silicon nanocubes and transforming the symmetry-protected continuous-domain bound state into a quasi-continuous-domain bound state mode. This simultaneously excites five sharp Fano resonances in the near-infrared band, enabling wide-band, multi-channel refractive index sensing. The five resonance modes combine high Q value and high sensitivity, with the highest quality factor reaching 988 and the highest refractive index sensitivity reaching 273 nm / RIU, significantly improving sensing performance. 2. This invention excites quasi-continuous bound state modes based on a symmetry-protected continuous bound state mechanism. The radiation quality factor Q and the structural asymmetry β satisfy an inverse square relationship: Qrad ∝ β. -2 The resonance peak position, linewidth, and Q value can be flexibly adjusted by regulating the radius difference between the two circular air holes. At the same time, by adjusting the geometric parameters such as the structural thickness, unit period, and center distance between the two circular holes of the silicon nanocube, active tuning of the resonance characteristics can be achieved, exhibiting good structural tunability. 3. This invention exhibits significant polarization-dependent characteristics. By changing the incident light polarization angle, the original fivefold Fano resonance and the electromagnetically induced transparency effect can be switched. The refractive index sensitivity of the resonance peak of the electromagnetically induced transparency window is 215 nm / RIU, and the highest quality factor (FOM) reaches 54.28 RIU. -1 Based on this, the switching state of the resonance mode was realized. When the polarization angle is 0°, the fivefold Fano resonance is turned on and the electromagnetic-induced transparency effect is turned off. When the polarization angle is 90°, the fivefold Fano resonance is turned off and the electromagnetic-induced transparency effect is turned on, so that the device can be used as a high-performance refractive index sensor and a tunable optical switch at the same time, realizing multi-functional integration. 4. This invention adopts a silicon-based all-dielectric structure, which effectively avoids ohmic loss, has low optical loss and strong field localization ability compared with traditional metal metasurfaces. At the same time, it is fully compatible with silicon-based CMOS technology, has a simple fabrication process, and controllable cost. It has the potential for large-scale integration and industrial application and can be widely used in fields such as biomedical detection, food safety analysis, and environmental monitoring, providing a new design idea for multifunctional integrated photonic devices. 5. This invention exhibits high tolerance for material absorption loss; when the extinction coefficient k of the silicon material is less than 10... -5 At the order of magnitude, the resonance profile remains essentially unchanged, exhibiting strong robustness to processing errors and ensuring the performance stability of the device after actual fabrication. Attached Figure Description

[0019] Figure 1 This is a schematic diagram of the basic structure and geometric parameters of the double-circular air-hole all-dielectric metasurface described in this invention; Figure 2 This is a comparison of the transmission spectra of the metasurface described in this invention under a symmetric dual-pore structure (α=0 nm) and an asymmetric dual-pore structure (α=40 nm). The dashed line is the spectrum at α=0 nm, and the solid line is the spectrum at α=40 nm. The illustration depicts the specific structure of the constituent molecules under the two different conditions. Figure 3 This is an evolution trend diagram of the transmission spectrum of the metasurface described in this invention as a function of the asymmetry parameter α. The F1, F2, BIC modes and q-BIC modes are marked in the corresponding positions in the figure. The q-BIC mode is a quasi-BIC state with high Q value and finite linewidth. Figure 4 This is a graph showing the relationship between the Q factor of the metasurface described in this invention and the structural asymmetry ΔS / S, along with a fitting curve. The red solid line represents the fitting result, showing the inverse square relationship between the Q factor and ΔS / S. The area of ​​the upper half of the hollow circle in the inset shows the magnitude of ΔS. Figure 5 This is a graph showing the relationship between the resonant wavelength and the full width at half maximum (FWHM) of the metasurface described in this invention as a function of structural thickness t, where (a) is the relationship between the resonant wavelength and the thickness; and (b) is the relationship between the full width at half maximum (FWHM) and the thickness. Figure 6 This is a graph showing the relationship between the resonant wavelength and the full width at half maximum (FWHM) of the metasurface described in this invention as a function of the distance d1 between the centers of the two circular holes. (a) shows the relationship between the resonant wavelength and d1; (b) shows the relationship between the full width at half maximum (FWHM) and d1. Figure 7 This is a graph showing the relationship between the resonant wavelength and the full width at half maximum (FWHM) of the metasurface described in this invention as a function of the unit cell period p, where (a) shows the relationship between the resonant wavelength and the unit cell period p; and (b) shows the relationship between the full width at half maximum (FWHM) and the unit cell period p. Figure 8 This is a graph showing the relationship between the metasurface resonance wavelength and the full width at half maximum (FWHM) as a function of the unit structure width L, where (a) is the relationship between the resonance wavelength and L; and (b) is the relationship between the FWHM and L. Figure 9 This is a transmission spectrum of the metasurface resonance wavelength described in this invention as a function of the environmental refractive index n; Figure 10 , Figure 11 , Figure 12 , Figure 13 This is a magnified view showing the change of resonant wavelength shift of each resonant mode of the metasurface described in this invention with the background refractive index, as well as sensitivity fitting curves at each location. Among them, (a)(b)(c)(d)(e) are magnified views of the change of resonant wavelength shift with the background refractive index at P1, P2, P3, F1, and F2, respectively; (a1)(b1)(c1)(d1)(e1) are sensitivity fitting curves at P1, P2, P3, F1, and F2, respectively. Figure 14 This is an evolution diagram of the transmission spectrum of the metasurface described in this invention as a function of the polarization angle of the incident light. (a) shows the change of the transmission spectrum with the polarization angle when α=40 nm, and the part in the dashed box is the process of the EIT window appearing; (b) shows the transmission spectrum at 1000-1200 nm when the polarization angle γ=90°; (c) shows a top view of the metastructure and a schematic diagram of the polarization angle. Figure 15 The present invention describes the sensing performance of the metasurface with an EIT-like peak at a polarization angle of 90°, where (a) shows the variation characteristics of the EIT-like resonance peak with the surrounding refractive index, and (b) shows the corresponding curves of the wavelength shift of the EIT-like peak and the refractive index. Figure 16 These are sensitivity fitting curves of the metasurface-like EIT resonance peak as a function of the refractive index of the environment, as described in this invention. (a) shows the effect of the refractive index of the surrounding medium on M1, M2, and M3; (b) shows the sensitivity fitting curve at M1; (c) shows the sensitivity fitting curve at M2; and (d) shows the sensitivity fitting curve at M3. Figure 17 This is a transmission spectrum evolution diagram of the metasurface described in this invention under different extinction coefficients k; Figure 18 The structural radiation loss caused by thickness t for the Q values ​​of each mode of the metasurface described in this invention, where (a) is the relationship between Q and thickness; and (b) is the trajectory of the poles in the complex plane. Figure 19 The structural radiation loss caused by the center distance d1 in the Q value of each mode of the metasurface described in this invention is shown in (a) as the relationship between Q and the center distance; (b) is the trajectory of the poles in the complex plane. Figure 20 The Q values ​​of each mode of the metasurface described in this invention are caused by the structural radiation loss due to the unit period p, where (a) is the relationship between Q and the unit period; and (b) is the trajectory of the complex plane pole. Figure 21 The Q values ​​of each mode of the metasurface described in this invention are caused by the structural radiation loss due to the element length L, where (a) is the relationship between Q and element length; and (b) is the trajectory of the complex plane pole. Detailed Implementation

[0020] The present invention will be further described in detail below with reference to the accompanying drawings and embodiments.

[0021] Example 1: This example provides a dual-circular air-hole metasurface fivefold Fano resonance sensor, the structure of which is as follows. Figure 1As shown, the sensor comprises a silicon dioxide substrate and multiple metasurface unit structures arranged in a periodic array on the substrate. Each metasurface unit structure is a silicon nanocube, and two circular air holes are etched on the silicon nanocube. The two circular air holes have different radii, and the radius difference forms an asymmetric structure.

[0022] In this embodiment, the metasurface unit structures are arranged in a periodic array along the x-axis and y-axis, with the period Px along the x-axis being equal to the period Py along the y-axis, Px = Py = 700 nm. The silicon nanocube has a width L = 600 nm and a structural thickness t = 150 nm. The center-to-center distance between the two circular air holes is d1 = 280 nm, and the radii of the two circular air holes are ra = 100 nm and rb = 60 nm, respectively. The asymmetry parameter α = ra - rb = 40 nm.

[0023] The metasurface of this embodiment was simulated and analyzed using the finite-difference time-domain method. The simulation conditions were set as follows: the metasurface was deposited on a silica substrate and immersed in a liquid with a refractive index n=1.33; periodic boundary conditions were set in the x and y directions; 64 perfectly matched layers were added in the z direction; the mesh size was set to 3 nm × 3 nm × 3 nm; and the energy convergence criterion was set to 1 × 10⁻⁶. -9 The light source is a plane wave incident perpendicularly along the negative z-axis, with the polarization direction parallel to the x-axis; the optical constants of silicon and silicon dioxide are referenced in the Palick manual.

[0024] like Figure 2 As shown, under the initial condition of a double-circular symmetric air hole, when the asymmetry parameter α = 0 nm (ra = rb = 60 nm), three sharp resonance peaks appear at λ = 1076.4 nm, λ = 1150.6 nm, and λ = 1430.6 nm, respectively, defined as P1, P2, and P3. At this time, the positions of F1 and F2 correspond to the ideal continuous domain bound state, with a linewidth approximately zero and a Q value approaching infinity. When the asymmetry parameter α = 40 nm (ra = 100 nm, rb = 60 nm), the planar symmetry of the element is broken, and a radiative pathway is established between the continuous spectrum in free space and the non-radiative bound state. Two new Fano resonance curves appear at λ = 1351.5 nm and λ = 1506.9 nm, respectively, denoted as F1 and F2. The original three resonance curves show a blue shift at λ = 1061.7 nm, λ = 1140.1 nm, and λ = 1402.6 nm, respectively, with a decrease in linewidth and an increase in modulation amplitude. Ultimately, a fivefold Fano resonance was excited in the 1000 nm-1600 nm wavelength range, such as Figure 2As shown by the solid line, the resonance peaks in the transmission spectrum were nonlinearly fitted using the Fano formula. The Q values ​​corresponding to the five resonance modes were 988, 233, 322, 223, and 718, respectively, with modulation depths of 86.3%, 95.3%, 98.4%, 98.2%, and 62.1%, respectively. The modulation depth and spectral contrast at P2, F1, and F2 approached 100%.

[0025] like Figure 3 As shown, the transmission spectrum changes accordingly as the asymmetry parameter α gradually increases from 0 to 60. The smaller the amplitude of symmetry breaking introduced into the structure, the narrower the radiation channel, the less energy leakage, the higher the Q value of the quasi-continuous bound state mode, and the narrower the corresponding Fano resonance spectral line. It has been verified that the Q value of the structure is more ideal when α = 40 nm.

[0026] like Figure 4 As shown, the asymmetry β is defined as ΔS / S, where ΔS is the area of ​​the fractured portion and S is the area of ​​the silicon dielectric portion. This embodiment verifies that the Q value and the asymmetry β satisfy an inverse square relationship, i.e., Qrad∝β. -2 This indicates that the fivefold Fano resonance is transformed from a symmetry-protected continuous domain bound state.

[0027] The metasurface in this embodiment possesses excellent active tunability. For example... Figure 5 As shown, when the structural thickness t is gradually varied from 140 nm to 160 nm (in 5 nm steps), the transmission spectrum exhibits a redshift at all five resonant wavelengths. The redshifts at P1 and P3 are relatively small, while those at P2, F1, and F2 are more significant. The full width at half maximum (FWHM) at P1, F1, and F2 remains almost unchanged, while the FWHM at P2 and P3 increases. Figure 6 As shown, when the center distance d1 between the two circular apertures is increased from 280 nm to 320 nm (in 10 nm steps), the transmission spectral lines shift slightly with the increase of d1. The shifts at P1 and P2 are almost invisible, a slight blue shift occurs at F1 and F2, and a slight red shift occurs at P3. The full width at half maximum (FWHM) at P3 increases significantly, while the other four locations remain almost unchanged. Figure 7 As shown, when the unit period p is increased from 680 nm to 720 nm (step size 10 nm), the resonance wavelength in the transmission spectrum redshifts, and the redshift range of the spectral lines at P2, F1, and F2 is larger than that at P1 and P3. Simultaneously, the resonance field strength factor decreases, and the full width at half maximum (FWHM) increases. Figure 8 As shown, when the unit cell width L is increased from 580 nm to 620 nm (in 10 nm increments), all five resonance peak wavelengths undergo a redshift, with P2 showing a relatively smaller shift and F1 exhibiting a more pronounced redshift. Increasing L significantly reduces the full width at half maximum (FWHM) at the P3 resonance, thus increasing the Q factor. Figure 9As shown, when the environmental refractive index n is increased sequentially from 1.31 to 1.35 in increments of 0.01, five resonant modes can still be observed within the target wavelength band. The center wavelength shifts towards longer wavelengths as n increases, exhibiting a good linear relationship. Comprehensive calculations yield the following results: Figure 10 , Figure 11 , Figure 12 , Figure 13 The relationship between the resonant wavelength shift at P1, P2, P3, F1, and F2 and the background refractive index, as well as the sensitivity fitting plots at each location, show that the Q value at P1 is close to 1000 and the Q value at F2 is about 700. Therefore, this structure has the capability of a high-performance wide-range refractive index sensor.

[0028] By comprehensively comparing the effects of the above parameters on the optical response, the optimal structural geometric parameters of the metasurface were finally determined as follows: t=150nm, p=700nm, L=600nm, ra=100nm, rb=60nm, α=ra-rb=40nm, d1=280nm.

[0029] The metasurface in this embodiment also exhibits an electromagnetically induced transparency (EIT) effect with adjustable polarization angle. For example... Figure 14 As shown, when the incident light polarization angle γ = 0°, the transmission spectrum exhibits a fivefold Fano resonance line shape. As γ gradually increases to 90°, the fivefold Fano resonance gradually disappears, and a transparent window gradually appears near the original resonance wavelength. The transmittance rises from the bottom and enters a weak electromagnetically induced transparency state. When γ = 90°, the original fivefold Fano resonance completely disappears, and the modulation amplitude of the transparent window resonance peak is close to 100%. At the same time, three new resonance peaks (denoted as M1, M2, and M3) appear in the 1000 nm-1200 nm range. Among them, the peak located at 1137.3 nm exhibits a narrow transparent window characteristic and can be classified as an EIT-like effect. The EIT-like peak excited by this structure mainly originates from the coupling between the ring dipole and the electric quadrupole, and has extremely high density of states and intense dispersion characteristics.

[0030] The sensing performance of the EIT-like effect was analyzed. With γ=90° selected, as the refractive index of the metasurface environment gradually increased from 1.31 to 1.35, a significant redshift occurred in the EIT-like peak position. Figure 15 As shown, when the refractive index change is only 0.01, the wavelength shift of the EIT-like peak is 2.15 nm, corresponding to a sensitivity of 215 nm / RIU; Figure 16 As shown, the calculated sensitivities of M1, M2, and M3 are 76 nm / RIU, 204 nm / RIU, and 215 nm / RIU, respectively, with FOM values ​​of 54.28 RIU. -1 48.57RIU -1 38.39RIU-1 It exhibits high refractive index sensitivity.

[0031] By adjusting the polarization angle of the incident light, a bidirectional switching function can be achieved: when the polarization angle is 0°, the fivefold Fano resonance is activated and the electromagnetically induced transparency (EIT) effect is deactivated; when the polarization angle is 90°, the fivefold Fano resonance is deactivated, generating a new triple resonance mode and simultaneously exciting the EIT effect, without changing the wavelength. Based on this function, this design realizes the ability to switch the resonance mode by adjusting the polarization angle of the incident light, thus possessing bidirectional optical switch control capability.

[0032] The loss mechanism of the metasurface in this embodiment is analyzed. For example... Figure 17 As shown, the absorption loss of the material is quantified by introducing the extinction coefficient k. When the asymmetry parameter α = 40 nm and the extinction coefficient k is less than 10... -5 At the order of magnitude 10, the resonance line shape of the transmission spectrum remains essentially unchanged; as the value of k gradually increases, the modulation depth of the five resonance peaks decreases, the linewidth broadens, and the Q value decreases; when k increases to 10... -1 At the order of magnitude, the resonant mode is almost completely destroyed. This indicates that the metasurface of this embodiment has low sensitivity to optical loss, high tolerance to processing errors, and strong robustness.

[0033] Furthermore, by changing the four core geometric dimensions of the metasurface (structural thickness t, distance d1 between the centers of the two circular holes, element period p, and element structural width L), the influence of the structural factor on the coupling leakage rate between the bound state and the free-space continuous state is analyzed, thereby affecting the full width at half maximum (FWHM) and Q-value of different modes. Figure 18 As shown, the resonance peak at P1 exhibits a constant and extremely low full width at half maximum (FWHM), corresponding to an extremely high Q value, and its loss shows excellent stability with dielectric layer thickness t. The FWHM of the resonance peak at P2 increases linearly with t, while the Q value decreases. P3, F1, and F2 exhibit non-monotonic FWHM modulation characteristics, first decreasing and then increasing. Figure 19 As shown, the Q value of P1 exhibits a double peak at d1 = 300 nm and 320 nm, while P2, P3, and F1 show relatively low and stable Q values ​​throughout the entire range of d1. Figure 20 As shown, the F2 mode has the highest Q factor, stabilizing at around 3000, and slightly decreasing with increasing period; the P1 mode's Q value is between 500 and 700, slowly increasing with increasing period; the Q factors of the P2, P3, and F1 modes decrease with increasing period. Figure 21 As shown, the Q value at P1 reaches a peak of approximately 977 at L=610 nm, and the F1 and F2 modes exhibit clear non-monotonic characteristics. These results indicate that the P1 peak is thickness-dependent and its Q value is modulated by the width and aperture spacing extremes, while the F2 peak is periodically modulated and possesses an extremely high absolute Q value. Both channels demonstrate high-sensitivity detection potential.

[0034] Example 2: This example provides the design and fabrication methods of the sensor from Example 1 (the fabrication method is only an example), including the following steps: Step S1: Select the material system and configuration of the metasurface. The substrate is silicon dioxide, and the multiple metasurface unit structures are made of silicon material. Each metasurface unit structure is determined to be a silicon nanocube configuration with two circular air holes etched in it.

[0035] Step S2: Simulate multiple metasurface unit structures using the finite-difference time-domain method. The simulation conditions are set as follows: the multiple metasurface unit structures have periodic boundary conditions along the x-axis and y-axis, 64 perfectly matched layers are set in the z-axis, the mesh size is set to 3 nm × 3 nm × 3 nm, and the energy convergence criterion is set to 1 × 10⁻⁶. -9 The light source is a plane wave that is incident perpendicularly along the negative z-axis and has a polarization direction parallel to the x-axis.

[0036] Step S3: Define the asymmetry parameter as the radius difference between the two circular air holes. By adjusting the radius difference, the structural symmetry is broken to excite the fivefold Fano resonance.

[0037] Step S4: The structural parameters of the metasurface are scanned and optimized. The structural parameters of the metasurface include: the period of multiple metasurface unit structures along the x-axis and the period along the y-axis, the width and thickness of the silicon nanocube, the center distance, radius and asymmetric parameters of the two circular air holes. The optimal structural geometry parameters are obtained through scanning optimization: Px=Py=700nm, silicon nanocube width L=600 nm, structural thickness t=150 nm, center distance between the two circular holes d1=280 nm, ra=100nm, rb=60 nm, and asymmetric parameter α=40 nm, thus completing the design of the metasurface structure.

[0038] Step S5: A silicon thin film is deposited on a silicon dioxide substrate using a low-pressure chemical vapor deposition process, and then thoroughly cleaned with acetone and isopropanol in sequence to form a clean process substrate.

[0039] Step S6: A layer of ZEP520A resist is prepared on the substrate surface by spin coating. The spin coating process is first carried out at a low speed for a short time, and then the speed is increased and the rotation time is extended. After the coating is completed, the resist layer is baked to make it firmly adhere to the substrate.

[0040] Step S7: The substrate is exposed using electron beam lithography. After exposure, the sample is sequentially placed in xylene for development, and then fixed with isopropanol to obtain the required mask pattern.

[0041] Step S8: The mask pattern is transferred to the silicon layer using inductively coupled plasma etching (ICP-C) technology. During the etching process, parameters such as temperature, humidity, gas flow rate, and etching time are controlled.

[0042] In step S9, residual photoresist is removed sequentially using N-methyl-2-pyrrolidone and acetone solutions, and ultrasonic oscillation is used to promote the peeling of the photoresist layer. After cleaning with deionized water and drying with nitrogen, a metasurface structure with an etched double circular air hole pattern is obtained.

[0043] The aforementioned design method is based on Mie scattering theory, Fano resonance theory, and continuous-domain bound state theory. Structural parameters are optimized using the finite-difference time-domain method. The influence of material absorption loss and geometric parameters on radiation loss is quantitatively analyzed, demonstrating that the structure has high tolerance to fabrication errors. This ensures that the designed metasurface can simultaneously excite a fivefold Fano resonance in the near-infrared band and possess polarization control capabilities. The design and fabrication methods are simple, cost-effective, and compatible with silicon-based CMOS processes, possessing the potential for large-scale integration on semiconductor chips.

Claims

1. A dual-circular air-hole metasurface fivefold Fano resonance sensor, comprising a silicon dioxide substrate and multiple metasurface unit structures periodically arrayed on the silicon dioxide substrate, characterized in that, Each of the metasurface unit structures is a silicon nanocube, on which two circular air holes with different radii are etched, and the difference in radii between the two circular air holes forms an asymmetric structure.

2. The dual-circular air-hole metasurface fivefold Fano resonance sensor according to claim 1, characterized in that, The multiple metasurface unit structures are arranged in a periodic array along the x-axis and y-axis, and the period Px in the x-axis direction is equal to the period Py in the y-axis direction.

3. The dual-circular air-hole metasurface fivefold Fano resonance sensor according to claim 1, characterized in that, Px=Py=700 nm; the width of the silicon nanocube is L=600 nm and the structural thickness is t=150 nm.

4. The double-circular air-hole metasurface fivefold Fano resonance sensor according to claim 1, characterized in that, The center distance between the two circular air holes is d1 = 280 nm, and the radii of the two circular air holes are ra and rb, respectively, where ra = 100 nm, rb = 60 nm, and the asymmetry parameter α = ra - rb = 40 nm.

5. The double-circular air-hole metasurface fivefold Fano resonance sensor according to claim 1, characterized in that, The sensor excites a quintet of Fano resonances in the near-infrared band. The quality factors Q corresponding to the quintet of Fano resonances are 988, 233, 322, 223 and 718, respectively. The corresponding refractive index sensitivities are 165 nm / RIU, 273 nm / RIU, 94 nm / RIU, 259 nm / RIU and 216 nm / RIU, respectively. The corresponding resonance wavelengths are 1061.7 nm, 1140.1 nm, 1402.6 nm, 1351.5 nm and 1506.9 nm, respectively.

6. The double-circular air-hole metasurface fivefold Fano resonance sensor according to claim 1, characterized in that, When the polarization angle of the incident light increases from 0° to 90°, the original fivefold Fano resonance disappears, while an electromagnetically induced transparency effect is excited and a new triple resonance mode is generated.

7. The double-circular air-hole metasurface fivefold Fano resonance sensor according to claim 6, characterized in that, The resonant peak refractive index sensitivity of the electromagnetically induced transparent window is 215 nm / RIU, and the highest quality factor (FOM) is 54.28 RIU. -1 .

8. The double-circular air-hole metasurface fivefold Fano resonance sensor according to claim 1, characterized in that, The bidirectional switching function is achieved by adjusting the polarization angle of the incident light: when the polarization angle is 0°, the fivefold Fano resonance is turned on and the electromagnetic-induced transparency effect is turned off; when the polarization angle is 90°, the fivefold Fano resonance is turned off and the electromagnetic-induced transparency effect is turned on.

9. A design method for a double-circular air-hole metasurface fivefold Fano resonance sensor as described in any one of claims 1 to 8, characterized in that, Includes the following steps: Step S1: Select the material system and configuration of the metasurface. The substrate is made of silicon dioxide, and the multiple metasurface unit structures are made of silicon material. Each metasurface unit structure is determined to be a silicon nanocube configuration with two circular air holes etched in it. Step S2: Simulate the multiple metasurface unit structures using the finite-difference time-domain method and set the simulation conditions; Step S3: Define the asymmetry parameter as the radius difference between the two circular air holes. By adjusting the radius difference, the structural symmetry is broken to excite the fivefold Fano resonance. Step S4: Scan and optimize the structural parameters of the metasurface to obtain the optimal structural geometric parameters, thus completing the design of the metasurface structure.

10. The design method of the double-circular air-hole metasurface fivefold Fano resonance sensor according to claim 9, characterized in that, The simulation conditions set in step S2 include: the multiple metasurface unit structures are periodic boundary conditions along the x-axis and y-axis, 64 perfectly matched layers are set in the z-axis, the mesh size is set to 3 nm × 3 nm × 3 nm, and the energy convergence criterion is set to 10. -9 The light source is a plane wave incident perpendicularly along the negative z-axis, with the polarization direction parallel to the x-axis; the structural parameters of the metasurface in step S4 include: the period of the multiple metasurface unit structures along the x-axis and the period along the y-axis, the width and structural thickness of the silicon nanocube, the center distance, radius and asymmetric parameters of the two circular air holes.

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