A refractive index sensor based on metal nanowire plasmon microcavity

Abstract

The present invention relates to a refractive index sensor based on a metal nanowire plasmon microcavity, which primarily comprises a substrate, a metal film, and metal nanowires. The substrate and metal film are both lamellar, with the metal film positioned between the substrate and the metal nanowires, the two layers being in contact. The metal nanowire is a regular pentagonal prism positioned on the other side of the metal film, with one cylindrical surface of the regular pentagonal prism in contact with the metal film. The two metal nanowires are positioned in parallel. The substrate is made of silicon or sapphire, the metal film is made of gold or silver, and the metal nanowires are made of gold or silver. The microcavity structure of the present invention can flexibly perform sensing tests on two-dimensional materials of different sizes by modulating the cavity length, offering the advantage of flexible adjustability. The present invention has a simple structure and utilizes obliquely incident p-polarized white light excitation, resulting in simple excitation conditions. The metal nanowires, two-dimensional materials, and cavity length are all on the micrometer scale, facilitating integrated applications.

Description

Technical Field

[0001] The present invention belongs to the technical field of optical refractive index measuring equipment, and in particular relates to a refractive index sensor based on a metal nanowire plasmon microcavity. Background Art

[0002] With the development of science and technology, refractive index sensing technology has received increasing attention in fields such as physics, chemistry, and biology. Designing smaller, more sensitive, and more stable sensor devices is a current research focus. Surface plasmon polariton (SPP) is the collective oscillation of free electrons on the surface of a metal. SPP is highly sensitive to tiny changes in the near field of a metal surface and has therefore attracted widespread attention in the field of refractive index sensors. SPP-based refractive index sensors can overcome the difficulty of traditional optical refractive index sensors in breaking the diffraction limit, achieving miniaturization and integration while ensuring high sensitivity.

[0003] Currently, SPP-based refractive index sensors are mainly divided into three categories: grating-coupled, prism-coupled, and waveguide-coupled refractive index sensors. These sensors are relatively large, which is not conducive to device integration. Moreover, most sensors are only used for sensing the real part of the refractive index and have not been applied to sensing the complex refractive index of materials.

[0004] In recent years, two-dimensional materials have been widely used in fields such as photodetectors, field-effect transistors, and composite materials. Therefore, the characterization of the physical properties of two-dimensional materials, especially their complex refractive index, is extremely important. Currently, the methods for characterizing the complex refractive index of two-dimensional materials mainly rely on methods such as ellipsometry and optical contrast analysis. However, these methods are difficult to miniaturize and integrate, and are not easy to flexibly manipulate, and have certain limitations.

[0005] In the June 2019 issue of Optics Letters, Volume 44, Issue 12, SiQing Dai et al. published "Complex refractive index measurement for atomic-layer materials via surface plasmon resonance holographic microscopy." The paper describes a complex refractive index sensor. This sensor measures the complex refractive index of two-dimensional materials by measuring changes in the reflected phase. However, the sensor's prism-coupled structure hinders miniaturization and integration. Summary of the Invention

[0006] In order to overcome the shortcomings of complex refractive index sensors that are difficult to miniaturize and integrate, the present invention proposes a refractive index sensor based on a metal nanowire plasmon microcavity.

[0007] The technical solution adopted by the present invention to solve its technical problems is:

[0008] A refractive index sensor based on a metal nanowire plasmon microcavity mainly includes a substrate, a metal film, and a metal nanowire. The substrate and the metal film are both in a lamellar shape. The metal film is located between the substrate and the metal nanowire. The substrate and the metal film are in contact with each other. The metal nanowire is a regular pentagonal prism and is located on the other side of the metal film, with one cylindrical surface of the regular pentagonal prism in contact with the metal film. Two metal nanowires are placed in parallel.

[0009] In the above-mentioned refractive index sensor, the substrate and the metal film layer are rectangular, the substrate and the metal film layer rectangles are equal in size, matched and opposite to each other up and down; the length of the two parallel metal nanowires is smaller than the side length of the substrate and the metal film layer rectangles, and is located in the middle of the metal film layer rectangle.

[0010] In the above-mentioned refractive index sensor, the substrate is made of silicon or sapphire, the metal film is made of gold or silver, and the metal nanowires are made of gold or silver.

[0011] The above-mentioned refractive index sensor has a thickness of h1, h1 = 200 μm to 1000 μm; a thickness of h2, h2 = 50 nm to 150 nm; a distance between the two farthest points of the regular pentagon in the cross section of the metal nanowire is d, d = 200 nm to 300 nm, and a length is s, s = 8 μm to 24 μm.

[0012] In the above-mentioned refractive index sensor, the distance between the two parallel metal nanowires can be adjusted according to measurement requirements.

[0013] The beneficial effects of the present invention are:

[0014] A refractive index sensor based on a metal nanowire plasmon microcavity utilizes a surface plasmon microcavity composed of micrometer-scale metal nanowires, an ultra-smooth metal film, and a two-dimensional material layer. Obliquely incident p-polarized white light excites the microcavity's plasmon standing wave mode. Changes in the spectral peak position of this standing wave mode effectively reflect changes in the real part of the refractive index of the two-dimensional material or liquid. The relative modulation depth of this standing wave mode also effectively reflects changes in the imaginary part of the refractive index of the two-dimensional material.

[0015] The microcavity structure proposed in the present invention can flexibly perform sensing tests on two-dimensional materials of different sizes by modulating the cavity length, and has the advantage of flexibility and adjustability.

[0016] The structure of the present invention is easy to prepare and uses oblique incident p-polarized white light excitation, which has simple excitation conditions. The metal nanowires, two-dimensional materials, and cavity length are all in the micrometer range, which is conducive to integrated applications. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] The present invention will be further described below with reference to the accompanying drawings and examples.

[0018] Figure 1 It is a three-dimensional diagram of the present invention;

[0019] Figure 2 This is the main view of the present invention;

[0020] Figure 3 Measuring the complex refractive index stereogram of a two-dimensional material for the present invention;

[0021] Figure 4 The main view of measuring the complex refractive index of two-dimensional materials in the present invention;

[0022] Figure 5 The present invention measures the top view of the complex refractive index of a two-dimensional material;

[0023] Figure 6 The normalized scattering spectra of Example 1 in different glycerol solutions;

[0024] Figure 7 This is a data fitting diagram showing the change of the microcavity resonance peak position with the refractive index in Example 1;

[0025] Figure 8 This is a schematic diagram of the relative modulation depth described in Example 1.

[0026] In the figure: 1. Substrate; 2. Metal film; 3. Metal nanowires; 4. Two-dimensional material layer; 5. Deionized water spectrum; 6. Spectrum of 20% by mass glycerol solution; 7. Spectrum of 40% by mass glycerol solution; 8. Spectrum of 60% by mass glycerol solution; 9. Measured value; 10. Fitted value. DETAILED DESCRIPTION

[0027] Example

[0028] A refractive index sensor based on a metal nanowire plasmon microcavity mainly includes a substrate 1, a metal film 2, and a metal nanowire 3. Figure 1 、 Figure 2 As shown, the refractive index sensor has a layered structure. The substrate 1 and the metal film 2 are both in the form of layers. The metal film 2 is located between the substrate 1 and the metal nanowire 3. The substrate 1 and the metal film 2 are connected to each other. The metal nanowire 3 is a regular pentagonal prism located on the other side of the metal film 2, and one cylindrical surface of the regular pentagonal prism is connected to the metal film 2. The two metal nanowires 3 are placed in parallel.

[0029] The substrate 1 and metal film 2 are rectangular. The two rectangles are equal in size, facing each other from top to bottom, and their sides are connected. Two parallel metal nanowires 3 are shorter than the sides of the rectangle and are located in the middle of the rectangle.

[0030] The material of the substrate 1 is silicon or sapphire, the material of the metal film 2 is gold or silver, and the material of the metal nanowire 3 is gold or silver.

[0031] The thickness of substrate 1 is h1, which ranges from 200 μm to 1000 μm. The thickness of metal film 2 is h2, which ranges from 50 nm to 150 nm. The distance between the two farthest points of the regular pentagonal cross section of metal nanowire 3 is d, which ranges from 200 nm to 300 nm, and the length is s, which ranges from 8 μm to 24 μm. The distance between the two metal nanowires 3 can be adjusted based on measurement requirements.

[0032] Table 1 Materials of components

[0033] Example 1 Example 2 Example 3 base silicon sapphire silicon metal film gold gold silver Metal nanowires silver silver gold

[0034] Table 2 Structural parameters

[0035] Example 1 Example 2 Example 3 <![CDATA[h1]]> 200μm 500μm 1000μm <![CDATA[h2]]> 50nm 100nm 150nm d 200nm 240nm 300nm s 14μm 8μm 24μm

[0036] The refractive index of the liquid is measured using a refractive index sensor based on a metal nanowire plasmon microcavity.

[0037] The refractive index sensor based on the metal nanowire plasmon microcavity of Example 1 was used to measure the refractive index of deionized water (DI water) and glycerol solutions with mass fractions of 20%, 40%, and 60%, respectively.

[0038] The measurement process is as follows:

[0039] First, the solution to be tested is dropped onto the surface of the refractive index sensor where the metal nanowires 3 are located;

[0040] Secondly, white light is used to irradiate the metal nanowires 3 covered by the test solution;

[0041] Secondly, a spectrometer is used to collect the scattering spectrum of the metal nanowires.

[0042] The measurement results are as follows Figure 6 As shown in the figure, the refractive indices of the measured solutions, deionized water and glycerol solutions with mass fractions of 20%, 40%, and 60%, are 1.333, 1.3572, 1.3841, and 1.4129, respectively. By processing the measurement results, the curve of the refractive index changing with the measurement wavelength is shown in the figure. Figure 7As shown in the figure, its sensing sensitivity is 712nm / RIU, which meets the technical requirements of refractive index measurement in engineering practice.

[0043] The complex refractive index of two-dimensional materials is measured using a refractive index sensor based on metal nanowire plasmon microcavity.

[0044] Graphene is used as a test object. The graphene thickness is h, where h = 0.35 nm to 8.00 nm. It is a two-dimensional material. The complex refractive index of graphene is measured using the refractive index sensor based on a metal nanowire plasmon microcavity in Example 1 as a test device.

[0045] The process of measuring the complex refractive index of graphene with different thicknesses is as follows:

[0046] Step 1: Forming a microcavity structure

[0047] Graphene with different thicknesses is placed between the metal film 2 and the metal nanowires 3, and the metal nanowires 3 are placed in parallel to form a parallel cavity structure, such as Figure 3 、 Figure 4 、 Figure 5 shown.

[0048] Step 2: Collect scattering spectrum

[0049] Using a spectrometer, the scattering spectra of the metal nanowires 3 in the microcavity structure are collected on graphene with different thicknesses. The distance between two parallel metal nanowires 3, that is, the cavity length, is changed to collect the scattering spectra under different cavity length conditions.

[0050] Step 3: Calculate the relative modulation depth

[0051] The spectral data obtained in step 2 can be used to calculate the relative modulation depth of the graphene microcavity at a certain wavelength and a certain thickness when the spacing is different. The relative modulation depth is Figure 8 shown.

[0052] Step 4: Calculate the propagation length L of the SPP SPP

[0053] Relative modulation depth and propagation length L SPP The relationship between is given by formula (1), and L can be obtained by fitting SPP .

[0054]

[0055] In formula (1), is the relative modulation depth, r is the end face reflectivity of SPP at the cavity wall, L SPP is the propagation length of the SPP, and l is the cavity length, i.e., the spacing between two parallel metal nanowires.

[0056] According to formula (1), the propagation length L of SPP is obtained by calculation: SPP .

[0057] Step 5: Calculate the imaginary part of the effective refractive index

[0058] The SPP propagation length is related to the imaginary part of the effective refractive index Im(n eff ) are as follows:

[0059]

[0060] In formula (2), λ is the wavelength selected when measuring the relative modulation depth.

[0061] By calculating from formula (2), we can get the effective refractive index imaginary part Im(n eff ).

[0062] Step 6: Calculate the complex refractive index

[0063] The relationship between the imaginary part of the effective refractive index and the complex dielectric function is as follows:

[0064]

[0065] In formula (3), λ0 is the free space wavelength, ε m is the complex dielectric function of the gold film, ε vac is the dielectric constant in vacuum, ε d is the complex dielectric function of graphene, and h is the thickness of graphene.

[0066] According to formula (3), the complex dielectric function ε of graphene can be obtained d .

[0067] The relationship between the complex dielectric function and the real and imaginary parts of the complex refractive index is as follows:

[0068] ε d =(n 2 -k 2 )+2nk*i (4)

[0069] In formula (4), n is the real part of the refractive index of graphene, k is the imaginary part of the refractive index of graphene, and i is the imaginary unit.

[0070] According to the complex dielectric function ε of graphene d , from formula (4), the real part n and imaginary part k of the graphene refractive index can be calculated.

[0071] At this point, the complex refractive index of graphene is obtained.

Claims

1. A refractive index sensor based on a metal nanowire plasmon microcavity, characterized in that: The invention mainly comprises a substrate (1), a metal film (2), and a metal nanowire (3); the substrate (1) and the metal film (2) are both in the form of lamellar sheets; the metal film (2) is located between the substrate (1) and the metal nanowire (3); the substrate (1) and the metal film (2) are in contact with each other; the metal nanowire (3) is a regular pentagonal prism, located on the other side of the metal film (2), and one cylindrical surface of the regular pentagonal prism is in contact with the metal film (2); and two metal nanowires (3) are placed in parallel; The process of measuring the complex refractive index of graphene with different thicknesses by the refractive index sensor is as follows: Step 1: Forming a microcavity structure Graphene with different thicknesses is placed between a metal film (2) and a metal nanowire (3), and the metal nanowires (3) are placed in parallel to form a parallel cavity structure; Step 2, collecting scattering spectrum; Using a spectrometer, scattering spectra of the metal nanowires (3) in the microcavity structure are collected on graphene with different thicknesses, and the distance between two parallel metal nanowires (3) is changed to collect scattering spectra under different cavity length conditions; Step 3, calculate the relative modulation depth; The spectral data obtained in step 2 can be used to calculate the relative modulation depth of the graphene microcavity at a certain wavelength and a certain thickness when the spacing is different. Step 4: Calculate the propagation length L of the SPP SPP ; Relative modulation depth and propagation length L SPP The relationship between is given by formula (1), and L can be obtained by fitting SPP ; In formula (1), is the relative modulation depth, r is the end face reflectivity of SPP at the cavity wall, L SPP is the propagation length of the SPP, l is the cavity length, i.e., the spacing between two parallel metal nanowires; According to formula (1), the propagation length L of SPP is obtained by calculation: SPP ; Step 5: Calculate the imaginary part of the effective refractive index The SPP propagation length is related to the imaginary part of the effective refractive index Im(n eff ) are as follows: In formula (2), λ is the wavelength selected when measuring the relative modulation depth; By calculating from formula (2), we can get the effective refractive index imaginary part Im(n eff ); Step 6, calculating the complex refractive index; The relationship between the imaginary part of the effective refractive index and the complex dielectric function is as follows: In formula (3), λ0 is the free space wavelength, ε m is the complex dielectric function of the gold film, ε vac is the dielectric constant in vacuum, ε d is the complex dielectric function of graphene, h is the thickness of graphene; According to formula (3), the complex dielectric function ε of graphene can be obtained d ; The relationship between the complex dielectric function and the real and imaginary parts of the complex refractive index is as follows: ε d =(n 2 -k 2 )+2nk*i(4) In formula (4), n is the real part of the refractive index of graphene, k is the imaginary part of the refractive index of graphene, and i is the imaginary unit; According to the complex dielectric function ε of graphene d , from formula (4), the real part n and imaginary part k of the graphene refractive index can be calculated. At this point, the complex refractive index of graphene is obtained.

2. The refractive index sensor based on metal nanowire plasmon microcavity according to claim 1, characterized in that: The substrate (1) and the metal film (2) are rectangular, and the sizes of the rectangles of the substrate (1) and the metal film (2) are equal and the upper and lower parts are matched and opposite; the length of the two parallel metal nanowires (3) is less than the side length of the rectangles of the substrate (1) and the metal film (2) and is located in the middle of the rectangle of the metal film (2).

3. The refractive index sensor based on metal nanowire plasmon microcavity according to claim 1, characterized in that: The material of the substrate (1) is silicon or sapphire, the material of the metal film (2) is gold or silver, and the material of the metal nanowire (3) is gold or silver.

4. The refractive index sensor based on metal nanowire plasmon microcavity according to claim 1, characterized in that: The thickness of the substrate (1) is h1, h1=200μm~1000μm; the thickness of the metal film (2) is h2, h2=50nm~150nm; the distance between the two farthest points of the regular pentagon in the cross section of the metal nanowire (3) is d, d=200nm~300nm, and the length is s, s=8μm~24μm.

5. The refractive index sensor based on metal nanowire plasmon microcavity according to claim 1, characterized in that: The distance between the two parallel metal nanowires (3) can be adjusted according to measurement requirements.

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