A MIM waveguide sensor based on Fano resonance and its application method

Abstract

This invention discloses a MIM waveguide sensor based on Fano resonance and its application method, relating to the field of optical device technology. The method includes a substrate; a metal coating is deposited on the substrate; a U-shaped resonant cavity is etched on the metal coating, and a metal-dielectric-metal MIM straight waveguide is etched below the U-shaped resonant cavity; by incident broadband light from the left side of the straight waveguide and exiting from the right side, when the resonance is excited, the beam energy corresponding to the resonance wavelength is coupled into the U-shaped resonant cavity, causing the transmittance of that wavelength to decrease and forming a transmission peak; the sensor has a wide and stable low refractive index detection range.

Description

Technical Field

[0001] This invention relates to the field of optical device technology, and specifically to a MIM waveguide sensor based on Fano resonance and its application method. Background Technology

[0002] With the development of the communication society, research on integrated circuits has received increasing attention, and the requirements for equipment integration and high precision are constantly increasing. Traditional photonic devices are difficult to integrate on a large scale due to the diffraction limit, while surface plasmon resonance technology can overcome the subwavelength size limitation, which is beneficial for the integration of photonic devices. With the continuous expansion and deepening of the field of biochemical detection, the demand for refractive index detection of drugs or materials is constantly growing, such as disodium hydrogen phosphate solution, aerogels in solar cells, and some polluted water bodies. How to design miniaturized and integrated sensor components has become an urgent problem to be solved.

[0003] As a planar optical waveguide sensor, the Metal-Insulator-Metal (MIM) waveguide sensor has many advantages compared with traditional sensors. The MIM waveguide sensor uses the waveguide on a silicon substrate as an ideal platform for exciting resonance. This design is easy for energy coupling and has a smaller volume, making it more adaptable to some application scenarios with space constraints. Secondly, compared with other optical sensing principles such as Surface Plasmon Resonance (SPR), Fano resonance can give a resonance line shape that is extremely sensitive to structural parameters and the surrounding environment and has obvious asymmetry. This makes the positioning of data better, which is beneficial for sensor detection and enhances its application value in fields such as sensors and optical switches. Currently, many MIM waveguide sensors based on Fano resonance with different structures have been proposed. Some scholars have proposed a MIM waveguide structure with a circular split ring, achieving a sensitivity of up to 1250 nm / RIU and a figure of merit (FOM) of 54. Some scholars have proposed a MIM waveguide sensor composed of a semi-elliptical ring resonator and a baffle. The sensitivity and figure of merit of this sensor can reach 1783 nm / RIU and 27 respectively, and it also has an optical delay of 0.887 ps, which has great advantages in the design of slow light systems. Some scholars have proposed a MIM waveguide sensor with a circular protrusion and a rectangular triangular resonator. The sensitivity of this sensor structure is 3060 nm / RIU, and the FOM is 53.68. In addition, the application of this structure in temperature sensors has also been studied, and its sensitivity is 1.493 nm / ℃. Some scholars have proposed a MIM waveguide sensor structure with a regular octagonal ring cavity on the top and a circular split ring resonator on the bottom, which has three Fano resonance peaks available for sensing, with sensitivities of 650 nm / RIU, 1000 nm / RIU, 1250 nm / RIU and FOMs of 1.6047×10 4 、3.8852×10 4 、1842.54 respectively. Some other scholars have proposed a MIM waveguide sensor structure with a "day"-shaped resonator, which can achieve up to 6 Fano resonance peaks at most.

[0004] However, although the currently adopted MIM waveguide sensors have a low detection range of analyte refractive index, their refractive index detection width is still not large enough, making it difficult to adapt to the detection of various analytes with different refractive indices in practical applications. Summary of the Invention

[0005] To address the shortcomings of existing technologies in detecting various analytes with different refractive indices, this invention proposes a MIM waveguide sensor based on Fano resonance and its application method. By proposing a U-shaped resonant cavity MIM waveguide sensor based on the Fano resonance principle, it has a wide and stable low refractive index detection range, thereby solving the problems existing in the prior art.

[0006] A MIM waveguide sensor based on Fano resonance, comprising:

[0007] The substrate is plated with a metal plating layer; a U-shaped resonant cavity is etched on the metal plating layer, and the U-shaped resonant cavity is formed by a rectangular cavity and two semi-circular annular cavities that are respectively connected at both ends of the rectangular cavity;

[0008] A metal-dielectric-metal MIM straight waveguide is etched below the U-shaped resonant cavity; the U-shaped resonant cavity is used to place the analyte to be tested.

[0009] Furthermore, the metal coating has a rectangular structure, and its length is... x =800nm, width y =700nm.

[0010] Furthermore, the length of the rectangular cavity in the U-shaped resonant cavity is L =100nm, width is W =60nm; the inner and outer diameters of the semi-circular annular cavity in the U-shaped resonant cavity are 80nm and 140nm, respectively.

[0011] Furthermore, the width of the straight waveguide w 0 = 50nm, the distance between it and the bottom of the U-shaped resonant cavity. g 0 = 20nm.

[0012] Furthermore, the metal coating is made of metallic silver; the refractive index of the metallic silver material is... Determined using the Debye-Drude model, it is represented as follows:

[0013] ;

[0014] in, ε ∞ The high-frequency dielectric constant of silver is... ε s The static dielectric constant is τ For relaxation time, σ The electrical conductivity of silver, i The imaginary unit, Where is the dielectric constant of silver. ω Angular frequency, ω 0 is the free space dielectric constant.

[0015] This invention also proposes an application method for a MIM waveguide sensor based on Fano resonance, comprising the following steps:

[0016] The analyte to be tested is filled into the U-shaped resonant cavity;

[0017] After broadband light is incident from one end of the MIM straight waveguide, when the beam energy corresponding to a specific wavelength can be coupled into the U-shaped resonant cavity, a spectrometer is used to perform spectral detection on the outgoing light from the other end of the MIM straight waveguide to obtain the resonant wavelength value corresponding to the analyte; wherein the specific wavelength is the resonant wavelength corresponding to the refractive index of the analyte.

[0018] This invention provides a MIM waveguide sensor based on Fano resonance, which has the following advantages:

[0019] The U-shaped resonant cavity proposed in this invention consists of a rectangular cavity and two semi-circular annular cavities on the left and right. The rectangular structure is designed to maintain a consistent coupling distance between the resonant cavity and the straight waveguide, simplifying the coupling process and avoiding coupling analysis between curved waveguides. Furthermore, by changing geometric parameters such as the bending radius of the two semi-circular annular cavities, the resonant wavelength and spectral characteristics such as the FOM (Form Oscillator Membrane Oscillator) can be adjusted, making the sensor design more flexible. Based on the Fano resonance principle, the U-shaped resonant cavity MIM waveguide sensor proposed in this invention has a wide and stable low refractive index detection range, superior wavelength sensitivity, high linearity, and a superior quality factor, as well as excellent sensing performance. It is expected to be applied in the detection of low refractive index pharmaceutical materials. Attached Figure Description

[0020] Figure 1 This is a top view of a U-shaped resonant cavity MIM waveguide sensor in an embodiment of the present invention;

[0021] Figure 2 This is a schematic diagram showing the electric field distribution and transmittance distribution for different incident light wavelengths in an embodiment of the present invention; Figure 2 (a) in the diagram shows the distribution of the high-transmittance electric field. Figure 2 (b) in the diagram shows the distribution of the low-permeability electric field;

[0022] Figure 3 In the embodiments of the present invention, the total transmittance varies with g 0 change spectrum;

[0023] Figure 4 In the embodiments of the present invention, the total transmittance varies with W Variation spectrum;

[0024] Figure 5 This is a spectrum showing the total transmittance as a function of L in an embodiment of the present invention; Figure 5In (a), L = 100-140 nm; Figure 5 In (b), L = 450 nm;

[0025] Figure 6 This is a graph showing the total transmittance curve as a function of the refractive index of the analyte in an embodiment of the present invention.

[0026] Figure 7 This is a schematic diagram of the total transmission peak wavelength sensitivity analysis in an embodiment of the present invention. Detailed Implementation

[0027] The technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings of the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments.

[0028] This invention proposes a Fano resonance-based MIM (Metal-Insulator-Metal) waveguide sensor. The MIM sensor employs a U-shaped resonant cavity structure with silicon as the substrate and silver as the plasma metal. Electromagnetic field mode simulation analysis of the designed sensing structure is performed using the full-vector finite element method, achieving [the desired result]. n a =1.05-1.2 analyte refractive index range with a wavelength sensitivity of 1079 nm / RIU, possessing R 2 High linearity of 0.9993, and Δ n a It exhibits a wide and stable detection range of 0.15, while maintaining a refractive index of 60.71 RIU at an analyte refractive index of 1.05. -1 It has the largest FOM and superior sensing performance.

[0029] The U-shaped resonant cavity MIM waveguide sensor proposed in this invention uses silicon as a substrate, with metallic silver plated on the substrate, followed by etching of the U-shaped resonant cavity and straight waveguide. For example... Figure 1 The image shows a top view of a U-shaped resonant cavity MIM waveguide sensor. The white area represents the analyte-filled region, the blue area is the silver plating, and the gray area is the perfect matching layer used in the simulation analysis. The sensor has a refractive index detection range of 1.05–1.2, and analytes include, for example, disodium hydrogen phosphate solution and aerogel in solar cells.

[0030] In this structure, the length of the rectangular waveguide is... x _ span =800nm, rectangular waveguide width y _ span =700nm, perfectly matching layer width t _ pml =100nm, length of the rectangular segment of the resonant cavityL =100nm, width of the rectangular segment of the resonant cavity W =60nm, the inner and outer diameters of the semicircular ring are respectively r _ small =80nm and r _ big =140nm, distance between the straight waveguide and the resonant cavity g 0 = 20nm. To ensure that only light of the TM0 mode is incident and propagates, the width of the straight waveguide is designed to be... w 0 = 50nm.

[0031] This invention proposes an application method for a MIM waveguide sensor based on Fano resonance, comprising the following steps: After filling the resonant cavity with an analyte, the refractive index of the U-shaped resonant cavity changes. Broadband light is incident from the left port of a straight waveguide, and the beam energy corresponding to a specific wavelength is coupled into the U-shaped resonant cavity. This specific wavelength is the resonant wavelength corresponding to the refractive index of the analyte. At this time, the spectrum received on the right side of the straight waveguide will show a valley at this resonant wavelength. Then, by using a spectrometer to perform spectral detection on the light emitted from the right side of the straight waveguide, the resonant wavelength value corresponding to the analyte can be obtained. In practical applications, firstly, transmittance spectral analysis is performed using multiple analytes with known refractive indices to obtain the resonant wavelength corresponding to that refractive index, thereby deriving the calculation relationship between the refractive index of the analyte and the resonant wavelength. Next, spectral analysis is performed on the analyte with an unknown refractive index to obtain its corresponding resonant wavelength. Based on the previously obtained calculation relationship, the refractive index of the analyte can be calculated accordingly, completing the sensing application process.

[0032] When the refractive index of the liquid to be tested is n a When = 1.1, different incident light wavelengths have different electric field distributions and transmittances, such as Figure 2 As shown. Among them Figure 2 (a) in the diagram represents the electric field distribution at high transmittance when the incident light wavelength is 1100 nm, indicating that this wavelength is not the resonant wavelength. Figure 2 (b) shows a schematic diagram of the electric field distribution with low transmittance when the incident light wavelength is 1200 nm. At this time, the incident waveguide mode and the upper cavity mode resonate, and energy cannot be transmitted.

[0033] In waveguide sensor design, commonly used plasma metals include gold and silver. This invention uses silver as the plasma metal. The refractive index of silver is determined by the Debye-Drude model (a theoretical model used to describe the optical and electrical responses of materials (especially metals and plasmas) under electromagnetic fields), as shown in the following equation:

[0034] ;

[0035] in, ε ∞ =3.8344 is the high-frequency dielectric constant of silver. ε s =-9530.5 is the static dielectric constant. τ =7.35×10 -15 s is the relaxation time. σ =1.1486×10 7 s / m is the electrical conductivity of silver. i The imaginary unit, Where is the dielectric constant of silver. ω Angular frequency, ω 0 is the free space dielectric constant.

[0036] In this sensor, the Fano resonance peak appears as a transmission peak. Broadband light enters from the left side of the straight waveguide and exits from the right side. When the resonance is excited, the beam energy corresponding to the resonance wavelength is coupled into the resonant cavity, resulting in a decrease in the transmittance of that wavelength, thus forming a transmission peak. The excitation of the Fano resonance can be analyzed using the Multimode Interference Coupled Mode Theory (MICMT), specifically including the following steps:

[0037] S1. Multimode Interference Coupled-Mode Theory Analysis: This invention uses MICMT to explain the Fano resonance phenomenon in a plasma resonant cavity system. This theory includes phase coupling analysis. In a plasma resonant cavity system, Fano resonance is mainly caused by interference between resonant modes, which includes two parts: interference between different resonant modes within the same resonant cavity and interference between resonant modes in different resonant cavities. Furthermore, when the resonant cavity system exhibits asymmetry, the resonant modes cannot be degenerate, resulting in degenerate interference Fano resonance. The coherent transmission characteristics between plasma resonant cavity systems are studied using the MICMT equation, which includes phase and mode coupling. The MICMT equation is as follows:

[0038] (2)

[0039] In the formula, j The imaginary unit; and They represent the first n The amplitude and frequency of each mode; Indicates the first n The internal loss decay time of each resonant mode and This indicates the coupling decay time between the resonator and the waveguide; and It is the coupling coefficient. and express and The complex conjugate; and Indicates the first n The coupling phase of each resonant mode; and They are respectively for and The normalized coefficient; Indicates the first n The phase difference between the output and input terminals of each resonant mode; Let be the amplitudes of the incident and reflected light waves in the waveguide, where "-" indicates light exiting the waveguide and "+" indicates light entering the waveguide. Represents the normalized result In a resonant cavity system, SPPs enter from the left side. Since there is no incident light at the output port, When the resonant cavity has a symmetrical structure, .

[0040] According to the MICMT equations, the complex amplitude transmission coefficient at the waveguide output port is:

[0041] (3)

[0042] The transmittance of the system can be expressed as , φ n Indicates the first n The total coupling phase difference of each resonant mode; φ n1 Indicates the first n The coupling phase difference between the resonator and waveguide 1 in each resonant mode; and Indicates to and Modulus taking. When the input and output terminals of the resonant cavity system are symmetrical about the circular resonant cavity, then... and The transmittance formula can then be simplified as follows:

[0043] .

[0044] S2. In this invention, the sensitivity of the sensor is determined by the change in resonant wavelength and the change in the refractive index of the analyte, i.e., the wavelength sensitivity S, which can be calculated by the following formula, where Δ λ Δ represents the change in resonant wavelength corresponding to the change in refractive index of the analyte. n a This is to analyze the change in the refractive index of the material.

[0045] ;

[0046] At the same time, quality factor FOM It is also an important parameter for measuring sensor performance. FOM The larger the value, the narrower and sharper the resonance peak, indicating that the structure has more accurate sensing characteristics. The quality factor of the sensor can be calculated as shown in the following formula, where... FWHM This refers to the full width at half maximum (FWHM) of the resonance peak, and S is the wavelength sensitivity mentioned above.

[0047] .

[0048] S3. Sensor Structural Parameter Analysis: The structural parameters of the sensor will be optimized, and the changing patterns of characteristics such as the total transmission peak wavelength will be investigated by altering these parameter values. This will be done in conjunction with the analysis of the refractive index of the material. n a When the distance is 1.1, consider the distance between the straight waveguide and the resonant cavity respectively. g 0. Resonant cavity width W and the length of the rectangular segment of the resonant cavity L Parametric scan optimization is performed, and all subsequent optimized parametric scans follow the control variable method. Except for the three parameters mentioned above, all other parameters retain their initial values.

[0049] a) Distance between the straight waveguide and the resonant cavity g 0:

[0050] Distance between straight waveguide and resonant cavity g The wavelength was optimized to 0, with values ​​of 5nm, 10nm, 20nm, 30nm, and 40nm, and the incident wavelength variation step was 5nm. Other parameters, L=100nm and W=60nm, remained constant. The total transmittance was compared to... Figure 3 As shown. It can be seen that when g As the wavelength of the total transmission peak increases from 0, there is a blue shift in the wavelength of the total transmission peak. A smaller FWHM (field wavelength of light) is beneficial for improving the FOM (field wavelength of light), but at the same time, the total transmission peak gradually increases, which is detrimental to subsequent spectral analysis. To balance FWHM and the total transmission peak, the optimized distance between the straight waveguide and the resonant cavity is... g 0 = 20nm.

[0051] b) Resonant cavity width W :

[0052] The resonant cavity width W was optimized, with values ​​of 40nm, 45nm, 50nm, 55nm, and 60nm, and the incident wavelength variation step was 5nm. The remaining parameters were set to L=100nm. g 0 = 10nm, and remains constant, the total transmittance is compared to, for example Figure 4As shown, the FWHM decreases slowly as W increases. However, from the perspective of the model structure, the ring structure is gradually lost when W is too large. Finally, the optimized resonant cavity width W = 60nm was obtained.

[0053] c) Length L of the rectangular segment of the resonant cavity:

[0054] The length L of the rectangular segment of the resonant cavity was optimized, with values ​​of 100nm, 110nm, 120nm, 130nm, and 140nm, and the incident wavelength variation step was 5nm. The remaining parameters were... g 0 = 10nm W =50nm, and remain constant, the total transmittance for example Figure 5 As shown in (a) above, it can be seen that when L varies within a certain range, the FWHM and total transmission peak value do not change significantly. Meanwhile, as... Figure 5 As shown in (b), the total transmittance is plotted when the length of the rectangular segment of the resonant cavity is L = 450 nm. Since the refractive index model for silver in the simulation software only has values ​​between wavelengths of 187.9-1937 nm, the total transmittance curve data for L = 450 nm is incomplete due to this limitation. Therefore, the length L of the rectangular segment of the resonant cavity cannot be too large. Finally, the optimized length of the rectangular segment of the resonant cavity is L = 100 nm.

[0055] S4. Sensitivity Analysis: The refractive index range of the analyte was selected as 1.05~1.2, and parametric scanning was performed with a refractive index change step size of 0.01 to obtain the corresponding total transmittance curve data, as shown below. Figure 6 As shown. Next, the curves of the total transmission peak wavelength corresponding to the refractive index of the analyte are presented, and their sensitivity and linearity are analyzed through fitting. It can be seen that as the refractive index of the analyte increases, the total transmission peak wavelength redshifts relatively uniformly. Simultaneously, it is calculated that... n a At point = 1.05, there is a minimum. FWHM =18.07nm, which means it has the maximum FOM =60.71RIU -1 .like Figure 7 As shown, the average wavelength sensitivity is 1079 nm / RIU, and the linearity is... R 2 The value is 0.9993, indicating relatively stable sensitivity.

[0056] This invention proposes a Fano resonance-based MIM (Metal-Insulator-Metal) waveguide sensor. The MIM sensor employs a U-shaped resonant cavity structure with silicon as the substrate and silver as the plasma metal. Electromagnetic field mode simulation analysis of the designed sensing structure is performed using the full-vector finite element method, and its sensing performance is characterized by wavelength analytical methods. When the refractive index of the analyte is in the range of 1.05-1.20, the sensor's wavelength sensitivity S = 1079 nm / RIU, and its linearity R0... 2 =0.9993, and has a maximum quality factor (FOM) of 60.71 RIU at a refractive index of 1.05. -1 Furthermore, the distance g0 between the straight waveguide and the resonant cavity, the cavity width W, and the length L of the rectangular segment of the resonant cavity were analyzed and optimized using a parametric scanning method. The study shows that the proposed MIM waveguide sensor possesses a wide and stable low-refractive-index detection range, superior wavelength sensitivity, high linearity, and a superior quality factor, exhibiting excellent sensing performance and promising applications in the detection of low-refractive-index pharmaceutical materials.

[0057] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A MIM waveguide sensor based on Fano resonance, characterized in that, include: substrate; The substrate is plated with a metal coating; a U-shaped resonant cavity is etched on the metal coating, the U-shaped resonant cavity being formed by a rectangular cavity segment and two semi-circular annular cavities connected at both ends of the rectangular cavity; the metal coating is made of metallic silver; the refractive index of the metallic silver is... Determined using the Debye-Drude model, it is represented as follows: ;in, ε ∞ The high-frequency dielectric constant of silver is... ε s The static dielectric constant is τ For relaxation time, σ The electrical conductivity of silver, i The imaginary unit, Where is the dielectric constant of silver. ω Angular frequency, ω 0 The free space dielectric constant; A metal-dielectric-metal MIM straight waveguide is etched below the U-shaped resonant cavity; the U-shaped resonant cavity is used to place the analyte to be tested. The application method of the MIM waveguide sensor based on Fano resonance includes the following steps: filling the analyte to be tested into a U-shaped resonant cavity; after broadband light is incident from one end of the MIM straight waveguide, when the beam energy corresponding to a specific wavelength can be coupled into the U-shaped resonant cavity, the outgoing light from the other end of the MIM straight waveguide is spectrally detected using a spectrometer to obtain the resonant wavelength value corresponding to the analyte; wherein the specific wavelength is the resonant wavelength corresponding to the refractive index of the analyte to be tested.

2. The MIM waveguide sensor based on Fano resonance according to claim 1, characterized in that, The metal coating has a rectangular structure.

Images