High-sensitivity refractive index measurement method based on silver-gold plasmon resonance structure

Abstract

The invention belongs to the field of optical precision measurement, and particularly relates to a high-sensitivity refractive index measurement method based on a silver-gold plasmon resonance structure. On the basis of a phase detection type surface plasmon resonance (SPR) sensing technology, a single-layer metal film in a traditional Kretschmann excitation structure is optimized to be a silver-gold double-layer metal film, and the silver-gold double-layer metal film is used as a metal film of a Kretschmann excitation structure of the Kretschmann excitation structure of the Kretschmann excitation structure of the Kretschmann excitation structure of the Kretschmann excitation structure. The silver layer has higher SPR excitation efficiency, so that the excitation structure has high refractive index measurement sensitivity, and the gold layer above the silver layer can effectively prevent the silver layer from being oxidized, so that the excitation structure has high stability, and the excitation structure has high refractive index measurement sensitivity and high stability. By designing a structure parameter optimization algorithm and taking the refractive index measurement sensitivity as a target function, the optimal silver-gold thickness of a to-be-measured sample is calculated, and by combining surface plasmon resonance holographic microscopy, extremely-high-sensitivity measurement of the micro change process of the refractive index of the to-be-measured dielectric sample can be realized. The actual measurement refractive index change amplitude magnitude can reach 10 <-7 > RIU.

Description

TECHNICAL FIELD

[0001] The application belongs to the field of optical precision measurement, and particularly relates to a high-sensitivity refractive index measurement method based on a silver-gold double-layer metal film plasmonic resonance structure. BACKGROUND

[0002] Refractive index is an important optical parameter for characterizing the state attribute of transparent or semi-transparent matter. By accurately measuring the small change of refractive index, the small change of related physical property parameters of the measured matter can be reflected. Therefore, high-sensitivity measurement of refractive index has important application value in related basic scientific research and industrial fields, such as characterizing the composition of matter, analyzing the attribute and characteristic parameter distribution of matter, etc. Surface plasmon resonance (SPR) is a collective ordered oscillation phenomenon of metal surface electrons excited by light waves. When SPR occurs, the metal will absorb the energy of the excited light wave, causing a dramatic change in the intensity and phase of the reflected light wave. Since this change is very sensitive to the refractive index and other physical parameters of the measured sample at the metal interface and their changes, the small changes of related parameters can be detected by analyzing the change of the complex amplitude of the reflected light wave. The SPR sensing technology developed based on the SPR effect can realize high-sensitivity measurement of refractive index. According to the different detection parameters of the reflected light wave, it can be divided into four detection modes of intensity, angle, wavelength and phase. The intensity detection mode is simple in structure, but the detection sensitivity is relatively low, so it is relatively less used in practical applications. Zeng et al. proposed a prism-coupled SPR sensing technology based on angle scanning, which realized the scanning of the incident angle of the excitation light wave by means of a galvanometer system. By measuring the resonance curves of NaCl solutions with different concentrations and calculating the resonance angle shift, it is verified that the refractive index measurement sensitivity of the system is 1.52×10 -6 RIU (Y. Zeng, et al. “A speckle-free angular interrogation SPR imaging sensor based on galvanometer scan and laser excitation,” Plasmonics, 14(6), 1497-1504 (2019)). Majidi et al. based on wavelength scanning SPR sensing technology, by designing a labyrinth type resonance structure in a metal-insulator-metal sandwich waveguide, the refractive index measurement sensitivity is calculated to be 2.99×10 -6RIU (Z. Majidi, et al. "Multi-resonance plasmonic refractive index sensor based on maze-shaped resonators for biological applications," Journal of Optics, 26(9), 095002 (2024)). Compared with the first three detection methods, phase detection has higher measurement sensitivity. Huang et al. based on prism-coupled SPR sensor, using signal processing algorithm to demodulate the phase difference information of different polarization state light beams, makes the system measurement sensitivity of refractive index reach 2.2 x 10 - 7 RIU (Y. H. Huang, et al. "Phase sensitive SPR sensor for wide dynamic range detection," Optics Letters, 36(20), 4092-4094 (2011)), but the phase processing algorithm involved is more complicated and the system measurement phase accuracy is low. Digital holographic microscopy can measure the phase distribution of the object light field with wide field, non-label, non-contact, high precision and dynamic. Combining it with the high sensitivity measurement advantage of SPR, Chinese patent CN115523852A discloses a surface plasmon resonance holographic microscopy (SPRHM) and a method for measuring two-dimensional distribution of double-layer nanometer metal film thickness. By calculating the reflection phase shift difference surface of two models, two monotonic curves with two metal film thicknesses as variables are obtained, and the intersection coordinate value is extracted, which can realize high sensitivity and accurate measurement of double-layer metal film thickness. However, the invention is aimed at measuring the thickness of metal film, and does not solve the technical problem of high sensitivity measurement of dielectric refractive index. Using the technical advantages of wide field, non-label, high throughput and high sensitivity of SPRHM, the invention designs a high sensitivity measurement method for refractive index, which has important industrial technical value.

[0003] The excitation structure of SPR sensing technology is generally Kretschmann structure, mainly composed of "prism-metal film-dielectric". The metal film in the traditional structure is generally composed of a single layer of single metal material, such as gold, silver, etc. Under the premise of the same thickness of the metal film, the excitation structure composed of a silver layer has higher refractive index measurement sensitivity than a gold layer, but silver is easily oxidized in air; the gold layer has higher stability than the silver layer, that is, the existing SPR excitation structure has the problem that high refractive index measurement sensitivity and high stability are difficult to achieve. In addition, although the refractive index measurement sensitivity of the traditional phase detection type SPR sensing technology based on the single layer metal film excitation structure can theoretically reach ≤10 -7 RIU, due to experimental system errors and measurement noise, etc., the actual measurable refractive index change is only 10 -4 RIU. SUMMARY

[0004] Technical problems to be solved

[0005] In order to overcome the limitation of low refractive index measurement sensitivity in the prior art, the present application proposes a high-sensitivity refractive index measurement method based on a silver-gold double-layer metal film surface plasmon resonance structure, based on surface plasmon resonance holographic microscopy technology. The structure parameters are calculated by an optimization algorithm to improve the refractive index measurement sensitivity.

[0006] Technical scheme

[0007] The idea of the present application is to optimize the traditional single-layer metal film in the Kretschmann excitation structure to a silver-gold double-layer metal film based on the phase detection type SPR sensing technology. Since the silver layer has higher SPR excitation efficiency, the optimized excitation structure can maintain high refractive index measurement sensitivity, and the gold layer above the silver layer can effectively prevent the silver layer from being oxidized, so that the optimized excitation structure has good stability, and thus has high sensitivity and high stability. Further, by means of the high-precision measurement of the reflected light wave phase in the SPR process by surface plasmon resonance holographic microscopy, high-sensitivity measurement of the small change process of the refractive index of the sample to be measured is realized.

[0008] In a first aspect, the present application provides a high-sensitivity refractive index measurement method based on a silver-gold plasmon resonance structure, comprising the following steps:

[0009] Step 1: constructing a four-layer surface plasmon excitation model including a base layer, a dielectric sample to be measured, a silver layer, and a gold layer;

[0010] Step 2: obtaining the optimal film thickness parameters d * Ag , d * Au;

[0011] Step 3: refractive index demodulation

[0012] based on the optimal film thickness parameter d * Ag , d * Au obtaining the reflection phase shift difference in the dynamic change process of the refractive index of the dielectric sample to be measured in the excitation model; and demodulating the refractive index value of the dielectric sample to be measured at each moment in the dynamic change process according to the relationship between the refractive index and the reflection phase shift difference in the refractive index demodulation curve D.

[0013] Further, the structure parameter optimization algorithm in step 2 specifically includes:

[0014] Step 21: obtaining the refractive index measurement sensitivity S when the excitation model occurs surface plasmon resonance;

[0015] Step 22: calculating the resonance angle θ R for each silver layer and gold layer thickness combination.

[0016] Step 23: taking the resonance angle θ R calculated in step 22 for each film thickness combination as the incident angle, calculating the corresponding refractive index measurement sensitivity S, and traversing the index to obtain the maximum value Max(S), and the film thickness combination value corresponding to the maximum value is the optimal film thickness parameter d * Ag , d * Au .

[0017] Further, the process of obtaining the refractive index measurement sensitivity S in step 21 specifically includes:

[0018] using the Fresnel formula to calculate the curve C of the reflection phase shift φ of the reflected light wave of the excitation model in step 1 with respect to the refractive index n of the dielectric sample when the surface plasmon resonance occurs, calculating the first derivative Δφ / Δn of the curve C and the maximum value Max(Δφ / Δn) thereof, and defining the maximum value as the refractive index measurement sensitivity S.

[0019] Further, the resonance angle θ R in step 22 specifically includes: R setting the thickness scanning range and scanning step interval of the silver layer and the gold layer, calculating the intensity reflectance curve of the reflected light wave with respect to the incident angle for each thickness combination, and taking the incident angle corresponding to the minimum value of the intensity reflectance as the resonance angle θ

[0020] Further, the step 3 of obtaining the refractive index demodulation curve D specifically comprises: calculating the change relationship between the reflection phase shift of the reflected light wave and the dielectric refractive index when the fourth layer in the excitation model is respectively the dielectric sample to be measured and air D' and φ0 by using the Fresnel formula, and taking the difference D'-φ0 as the refractive index demodulation curve D.

[0021] Further, the step 3 of obtaining the reflection phase shift difference of the dielectric sample to be measured in the dynamic change process of the refractive index specifically comprises:

[0022] Step 31: setting the fourth layer in the excitation model to be respectively the dielectric sample to be measured and air, and obtaining the refractive index demodulation curve D;

[0023] Step 32: based on the optimal film thickness parameter d * Ag , d * Au , and the silver-gold double-layer metal film is plated on the excitation model base layer.

[0024] Step 33: measuring the reflection phase shift difference of the dielectric sample to be measured in the dynamic change process of the refractive index by using the optical interference method and combining the secondary exposure principle.

[0025] Further, in the four-layer surface plasmon excitation model of the step 1, the base layer is a cover glass, which is made of glass, and the refractive index of the dielectric sample is less than the refractive index of the cover glass.

[0026] Further, the dielectric constant ε p , ε Ag , and ε Au of the cover glass, the silver layer and the gold layer are known parameters, the initial refractive index n s of the dielectric sample is an initial calculation condition, and the thickness d Ag , d Au of the silver layer and the gold layer are optimization parameters.

[0027] Secondly, the application provides a system for realizing the high-sensitivity refractive index measurement method based on the silver-gold plasmon resonance structure, which is characterized by comprising a solid laser, a fiber coupler, a negative lens, a collimating lens, a converging lens, a polarizer, a beam splitter, a microscope objective, a four-layer surface plasmon resonance excitation model, an imaging lens, a Wollaston prism, a polarizer, and an image acquisition device which are sequentially aligned and built.

[0028] The polarized light emitted by the solid laser is coupled out through the fiber coupler, expanded and collimated by a negative lens and a collimating lens, and becomes parallel light; the parallel light is converged on the back focal plane of a microscope objective through a converging lens, a polarizer and a beam splitter plate, and is incident on a four-layer surface plasmon resonance excitation model through the microscope objective; the reflected light is incident on a Wollaston prism and a polarizer through a beam splitter plate and an imaging lens, and the hologram formed by off-axis interference is recorded by an image acquisition device.

[0029] Further, the polarizer modulates the polarization state of the light beam to 45° polarization.

[0030] Advantages

[0031] Compared with the prior art, the high-sensitivity refractive index measurement method based on a silver-gold plasmonic resonance structure has at least the following advantages:

[0032] 1. The refractive index measurement method proposed in the application optimizes the silver-gold plasmonic resonance structure model parameters with the maximum refractive index measurement sensitivity as the objective function, and combines the refractive index demodulation curve and the refractive index change information demodulated by the reflection phase shift difference measured by the optical interference method. This method has the characteristics of high refractive index measurement sensitivity and high stability.

[0033] 2. The refractive index measurement method proposed in the application has high experimental result precision, and the actual measured refractive index change amplitude can reach 10 -7 RIU, which significantly improves the refractive index measurement sensitivity and will play an important value in biomedical and chemical substance detection. BRIEF DESCRIPTION OF DRAWINGS

[0034] Figure 1 is the optical path diagram of the surface plasmon resonance holographic microscopic measurement system involved in the application.

[0035] Figure 2 is a silver-gold surface plasmon resonance model schematic diagram.

[0036] Figure 3 is a refractive index measurement sensitivity calculation principle diagram.

[0037] Figure 4 is a silver-gold film thickness parameter corresponding resonance angle calculation result schematic diagram.

[0038] Figure 5 is a silver-gold film thickness parameter corresponding refractive index measurement sensitivity S calculation result schematic diagram.

[0039] Figure 6 is a flowchart of the high-sensitivity refractive index measurement method based on a silver-gold plasmonic resonance structure.

[0040] REFERENCE NUMERALS:

[0041] 1 - solid state laser, 2 - fiber coupler, 3 - negative lens, 4 - collimating lens, 5 - converging lens, 6 - polarizer, 7 - beam splitter plate, 8 - microscope objective, 9 - cover glass, 10 - silver layer, 11 - gold layer, 12 - dielectric sample, 13 - imaging lens, 14 - Wollaston prism, 15 - polarizer, 16 - image acquisition device. DETAILED DESCRIPTION

[0042] In order to make the purpose, technical solutions and advantages of the present application more clear, the present application is further described in detail below in combination with the drawings and examples. It should be understood that the specific examples described herein are only used to explain the present application and do not limit the present application.

[0043] The present application is further described in combination with examples and drawings:

[0044] Example 1

[0045] The embodiment of the present application provides a very high sensitivity refractive index measurement method based on silver-gold plasmon resonance structure, which is characterized by the following steps:

[0046] Step 1: Construct a four-layer surface plasmon excitation model composed of "base layer-silver layer-gold layer-dielectric sample".

[0047] In the four-layer surface plasmon excitation model, the base layer is preferably a cover glass, which can be of glass material; wherein the dielectric constant ε p , ε Ag , ε Au of the cover glass, silver layer and gold layer are known parameters, and the initial approximate refractive index n s of the dielectric sample is used as the initial calculation condition; the thickness d Ag , d Au of the silver layer and gold layer are used as optimization parameters; the refractive index of the dielectric sample is less than that of the cover glass.

[0048] Step 2: Obtain the optimal film thickness parameters d * Ag , d * Au of the silver layer-gold layer metal film through a structure parameter optimization algorithm.

[0049] The curve C of the change of the reflection phase shift φ of the reflection light wave of the excitation model in step 1 with the refractive index n of the dielectric sample when surface plasmon resonance occurs is calculated by Fresnel formula, and the first derivative Δφ / Δn and its maximum value Max(Δφ / Δn) of the curve C are further calculated, and the maximum value is defined as the refractive index measurement sensitivity S.

[0050] The thickness scanning range of the silver layer and the gold layer is set to 1nm~50nm, and the scanning step interval is 1nm. The Fresnel formula is used to calculate the change curve of the intensity reflectivity of the reflected light wave with the incident angle under each thickness combination. The incident angle corresponding to the minimum value of the intensity reflectivity is taken as the resonance angle θ under this film thickness combination. R .

[0051] The resonance angle under each film thickness combination is used as the incident angle, and the corresponding refractive index measurement sensitivity S is calculated. The maximum value Max(S) is traversed and indexed, and the corresponding film thickness combination value is the optimal film thickness parameter d * 1.d * 2. When the double metal film is a silver layer and a gold layer respectively, the optimal film thickness parameter is recorded as d * Ag d * Au .

[0052] Step 3: Refractive index demodulation:

[0053] Based on the optimal film thickness parameter d * 1.d * 2. Obtain the reflection phase shift difference during the dynamic change of the refractive index of the dielectric sample to be measured in the excitation model; demodulate the refractive index value according to the corresponding relationship between the refractive index and the reflection phase shift difference in the refractive index demodulation curve D.

[0054] The optimal film thickness parameter d * Ag d * Au The resonance angle θ corresponding to this film thickness combination * R As the initial condition, the Fresnel formula is used to calculate the relationship between the reflection phase shift of the reflected light wave and the refractive index of the dielectric when the fourth layer in the excitation model is the dielectric sample to be measured and air respectively. The difference between the two, D'-φ0, is used as the refractive index demodulation curve D;

[0055] According to the optimal film thickness parameter d * Ag d * Au A silver-gold double-layer metal film is plated on the cover glass using a metal thin film preparation process; the optical interference method is preferentially used in combination with the double exposure principle to measure the reflection phase shift difference during the dynamic change of the refractive index of the dielectric sample to be tested; based on the one-to-one correspondence between the refractive index and the reflection phase shift difference in the refractive index demodulation curve D, the refractive index value at each moment during the dynamic change of the dielectric sample to be tested is calculated from the reflection phase shift difference.

[0056] Example 2

[0057] The initial approximate refractive index of the dielectric sample to be measured is taken as the initial calculation condition, and an SPR excitation model composed of "cover glass-silver layer-gold layer-dielectric sample" is constructed, wherein the dielectric constants of the cover glass, silver layer and gold layer are known parameters, and the thicknesses of the silver layer and gold layer are optimization parameters, as shown in Figure 2 The dielectric constants of the dielectric sample, gold layer, silver layer and cover glass are represented by ε s , ε Au , ε Ag and ε p respectively, the refractive index of the dielectric sample is represented by n s , and the film thicknesses of the gold layer and silver layer are represented by d Au and d Ag .

[0058] Based on the above model, the curve of the reflection phase shift of the reflected light wave with the change of the refractive index of the dielectric is calculated by using the Fresnel formula when SPR occurs, and the first derivative of the curve is further calculated and the maximum value of the first derivative is indexed, which is the refractive index measurement sensitivity under the current model parameters, as shown in Figure 3 The dashed line in the figure is the curve of the change of the reflection phase shift difference φ with the refractive index n of the dielectric, and the solid line is the first derivative Δφ / Δn of the curve, and the circular point indicates the maximum value Max(Δφ / Δn) of the first derivative, which is defined as the refractive index measurement sensitivity S.

[0059] Since the maximum refractive index measurement sensitivity can be obtained only under the condition of effective excitation of SPR, and the SPR excitation condition is generally met by adjusting the system incident angle, it is necessary to first determine the resonance angle of each film thickness combination for subsequent calculation. The thickness scanning range of the silver layer and the gold layer is set to 1 nm to 50 nm, and the scanning step interval is 1 nm. The curve of the intensity reflectivity of the reflected light wave with the change of the incident angle is calculated for each film thickness combination by using the Fresnel formula, and the incident angle corresponding to the minimum value of the intensity reflectivity is extracted as the resonance angle of the film thickness combination, as shown in Figure 4 Further, the incident angle of the excitation model corresponding to each film thickness combination is fixed at its resonance angle, and the corresponding refractive index measurement sensitivity is calculated, and the maximum value of the refractive index measurement sensitivity is indexed, and the thicknesses of the silver layer and the gold layer corresponding to the maximum value are the optimal structure parameters, as shown in Figure 5 The color bar is displayed as the logarithm of S with a base of 10; the five-star symbol indicates the maximum value of S, and the thicknesses of the silver layer and the gold layer corresponding to the maximum value are the optimal film thickness parameters; the Figure 4 and 5 are calculated by taking the initial approximate refractive index of the dielectric sample as 1.3367.

[0060] As shown in Figure 6 , the specific workflow of the refractive index measurement is as follows:

[0061] The optimal structure parameters and the corresponding resonance angle are taken as initial conditions, the relationship between the reflection phase shift of the reflected light and the refractive index of the dielectric is calculated by Fresnel formula when the fourth layer of the resonance structure is the dielectric sample to be measured and air respectively, the difference between the two is taken as the relationship between the difference of the reflection phase shift and the refractive index of the dielectric, which is taken as the refractive index demodulation curve. Then, according to the optimal structure parameters, a silver-gold double-layer metal film is plated on the cleaned cover glass by using a metal thin film preparation process (such as an electron beam evaporation plating system, a vacuum high-temperature evaporation plating system, etc.), and the silver-gold double-layer metal film is coupled to the surface plasmon resonance holographic microscopic measurement system.

[0062] As shown in Figure 1 the 632.8 nm linearly polarized light emitted by the solid laser 1 is coupled out by the fiber coupler 2, expanded and collimated by the negative lens 3 and the collimating lens 4, and becomes parallel light. The parallel light is converged on the back focal plane of the microscopic objective 8 by the converging lens 5, the polarizer 6 and the beam splitter 7, and is incident on the SPR excitation structure in a wide-field illumination manner after the microscopic objective. The polarizer 6 modulates the polarization state of the light beam to 45° polarization. The SPR excitation structure is composed of “cover glass 9-silver layer 10-gold layer 11-dielectric sample 12”. The reflected light passes through the beam splitter 7 and the imaging lens 13, and then passes through the Wollaston prism 14 and the polarizer 15 to realize off-axis interference, and the hologram formed thereby is recorded by the image acquisition device 16. After the sample is placed above the gold layer, the position thereof is adjusted to be half of the sample and the air region in the field of view of the image acquisition device (attention is paid to ensuring that the sample boundary direction is consistent with the propagation direction of the surface plasmon wave to avoid noise interference), and the sample hologram is recorded continuously for a period of time. Finally, the sample is removed from the measurement system by using the physical adsorption method, the medium above the gold layer is air, and a background hologram is taken (the optical and mechanical parameters of the system cannot be changed when the background hologram is collected).

[0063] For the sample and the background hologram, the angular spectrum method is first used to reconstruct the complex amplitude of the light field and extract the phase distribution, and then the background noise is removed by background subtraction based on the principle of double exposure to obtain the phase image. In the long-term measurement process, considering the stability of the system optical and mechanical elements, the background hologram collected at the last moment is taken as the background of the entire measurement process, and the measurement error generated thereby is large, so that additional phase correction is required. The phase image obtained by background subtraction contains the sample and air regions, and the phase correction is completed by subtracting the average phase of the air region from the entire phase image. Finally, the average phase of the sample region is calculated, which is theoretically equal to the difference of the reflection phase shift. According to the one-to-one correspondence between the refractive index and the difference of the reflection phase shift in the demodulation curve, the refractive index of the dielectric sample at each time during the small change process of the refractive index is calculated from the difference of the reflection phase shift.

[0064] In summary, the present application is based on silver-gold double-layer metal film plasmon resonance structure, and uses a surface plasmon resonance holographic microscopic measurement system to perform high-sensitivity measurement on a small change in refractive index during a liquid sample evaporation process. -7 High-sensitivity refractive index change measurement of RIU.

[0065] The technical features of the above embodiments can be combined in any manner, and to make the description concise, all possible combinations of the technical features in the above embodiments are not described, but as long as the combinations of the technical features do not contradict, they should be considered as falling within the scope of the present application.

[0066] The above embodiments only express specific implementation manners of the present application, and the description is relatively specific and detailed, but it should not be understood as a limitation on the patent scope of the present application. It should be pointed out that, for those skilled in the art, several modifications and improvements can be made without departing from the idea of the present application, and these all fall within the protection scope of the present application. Therefore, the protection scope of the present application should be subject to the appended claims.

Claims

1. A high-sensitivity refractive index measurement method based on a silver-gold plasmon resonance structure, characterized in that The steps include: Step 1: Construct a four-layer surface plasmon excitation model including a base layer, a dielectric sample to be tested, a silver layer, and a gold layer; Step 2: Obtain the optimal film thickness parameter d of the silver-gold metal film through the structural parameter optimization algorithm * Ag d * Au ; Step 3: Refractive index demodulation: Based on the optimal film thickness parameter d * Ag d * Au Obtain the reflection phase shift difference during the dynamic change of the refractive index of the dielectric sample to be measured in the excitation model; and demodulate the refractive index value at each moment during the dynamic change of the dielectric sample to be measured according to the relationship between the refractive index and the reflection phase shift difference in the refractive index demodulation curve D.

2. The refractive index measurement method according to claim 1, wherein: The structural parameter optimization algorithm in step 2 specifically includes: Step 21: When the excitation model generates surface plasmon resonance, obtaining a refractive index measurement sensitivity S; Step 22: Calculate the resonance angle θ for each combination of silver and gold layer thicknesses R ; Step 23: The resonance angle θ for each film thickness combination calculated in step 22 is R As the incident angle, calculate the corresponding refractive index measurement sensitivity S, traverse the index to its maximum value Max(S), and the corresponding film thickness combination value is the optimal film thickness parameter d * Ag d * Au .

3. The refractive index measurement method according to claim 2, wherein: The process of obtaining the refractive index measurement sensitivity S in step 21 specifically includes: Use the Fresnel formula to calculate the curve C of the reflection phase shift φ of the reflected light wave when the excitation model in step 1 occurs surface plasmon resonance as a inverse function of the refractive index n of the dielectric sample. Calculate the first-order derivative Δφ / Δn of the curve C and its maximum value Max(Δφ / Δn). Define this maximum value as the refractive index measurement sensitivity S.

4. The refractive index measurement method according to claim 2, wherein: The resonance angle θ in step 22 R The calculation process specifically includes: setting the thickness scanning range and scanning step interval of the silver layer and the gold layer, calculating the intensity reflectivity of the reflected light wave with the incident angle under each thickness combination, and taking the incident angle corresponding to the minimum value of the intensity reflectivity as the resonance angle θ under this film thickness combination. R .

5. The refractive index measurement method according to claim 1, wherein: Obtaining the refractive index demodulation curve D in step 3 specifically includes: using the Fresnel formula to calculate the relationship D' and φ0 between the reflection phase shift of the reflected light wave and the refractive index of the dielectric when the fourth layer in the excitation model is the dielectric sample to be measured and air respectively, and taking the difference D'-φ0 between the two as the refractive index demodulation curve D.

6. The refractive index measurement method according to claim 1, wherein: The step 3 of obtaining the reflection phase shift difference during the dynamic change of the refractive index of the dielectric sample to be measured specifically includes: Step 31: setting the fourth layer in the excitation model to be the dielectric sample to be measured and air, respectively, to obtain a refractive index demodulation curve D; Step 32: Based on the optimal film thickness parameter d obtained in step 2 * Ag d * Au , plating a silver-gold double-layer metal film on the excitation model base layer; Step 33: Use optical interference method combined with the principle of double exposure to measure the reflection phase shift difference during the dynamic change of the refractive index of the dielectric sample to be tested.

7. The refractive index measurement method according to claim 1, wherein: In the four-layer surface plasmon excitation model in step 1, the base layer is a cover glass, which is made of glass, and the refractive index of the dielectric sample is smaller than the refractive index of the cover glass.

8. The refractive index measurement method according to claim 7, wherein: The dielectric constants ε of the cover glass, silver layer and gold layer p , ε Ag , ε Au is a known parameter, the initial refractive index n of the dielectric sample s As the initial calculation condition, the thickness of the silver layer and the gold layer d Ag d Au as an optimization parameter.

9. A system for implementing the high-sensitivity refractive index measurement method based on a silver-gold plasmon resonance structure according to any one of claims 1 to 8, characterized in that: The system includes a solid laser, a fiber coupler, a negative lens, a collimating lens, a converging lens, a polarizer, a beam splitter, a microscope objective, a four-layer surface plasmon resonance excitation model, an imaging lens, a Wollaston prism, a polarizer, and an image acquisition device, which are aligned and constructed in sequence. After the output polarized light of the solid laser is coupled out through the fiber coupler, it is expanded and collimated by the negative lens and the collimating lens and becomes parallel light; the parallel light is converged on the back focal plane of the microscope objective lens through the converging lens, polarizer and beam splitter, and then incident on the four-layer surface plasmon resonance excitation model after passing through the microscope objective lens; the reflected light passes through the beam splitter and imaging lens, and then realizes off-axis interference through the Wollaston prism and polarizer, and the formed hologram is recorded by the image acquisition device.

10. The system according to claim 9, characterized in that: The polarizer modulates the polarization state of the light beam to 45° polarization.

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