Metasurface refractive index sensor based on bimetallic composite periodic structure and manufacturing method thereof

Abstract

The present invention relates to the technical field of micro-nano optical devices, and in particular to a metasurface refractive index sensor based on a bimetallic composite periodic structure and a method for manufacturing the same. The sensor comprises a substrate and a bimetallic composite periodic structure distributed on the surface of the substrate in a periodic array, wherein the bimetallic composite periodic structure comprises a nanopore membrane and a nanocolumn, wherein the nanopore membrane is provided with circular holes that penetrate up and down and are distributed in an array, wherein the nanocolumn is a cylindrical structure and is nested inside the circular hole, wherein a space filled with a substance to be tested is formed between the nanocolumn and the circular hole, wherein the nanopore membrane and the nanocolumn are composed of different metal materials, wherein the nanopore membrane and the nanocolumn are used to independently excite surface plasmons of different modes and couple with incident light to generate Fano resonance. Surface plasmons of different modes are excited by the bimetallic composite periodic structure, thereby coupling with incident light, thereby improving the sensitivity and quality factor of the refractive index sensor.

Description

Technical Field

[0001] The present invention relates to the technical field of micro-nano optical devices, and in particular to a metasurface refractive index sensor based on a bimetallic composite periodic structure and a manufacturing method thereof. Background Art

[0002] Micro-nano structure optical sensors are not only one of the important development directions of the optoelectronics industry, but also the frontier research direction in the current optical field. Micro-nano structure optical sensors have the advantages of high sensitivity, small size, and easy integration. They can monitor and analyze different chemical substances and biological samples in real time, and have played a great role in different fields such as diagnostics, pharmacy, food safety, environmental monitoring, and homeland security, and have broad application prospects.

[0003] Localized surface plasmons (LSPs) are electron density waves formed by the interaction between electromagnetic waves and free electrons on the metal surface. When the frequency of the incident electromagnetic wave is within the metal plasmon frequency range, it will trigger the collective oscillation of the conduction electrons in the metal, resulting in sub-wavelength localization of the electric field and significant electric field enhancement. When the sample is placed on the metal surface, the interaction between the electric field and the sample will also be enhanced.

[0004] Therefore, it is extremely important to design a sensor based on this principle to solve the problems of low sensing sensitivity and quality factor of existing sensors. Summary of the invention

[0005] The purpose of the present invention is to address the defects of the prior art and provide a metasurface refractive index sensor based on a bimetallic composite periodic structure and a method for manufacturing the same, which can excite surface plasmons of different modes through the bimetallic composite periodic structure, thereby coupling with the incident light to achieve improved sensitivity and quality factor of the refractive index sensor.

[0006] The present invention proposes a metasurface refractive index sensor based on a bimetallic composite periodic structure, and its technical solution is as follows:

[0007] The invention comprises a substrate and a bimetallic composite periodic structure distributed in a periodic array on the surface of the substrate, wherein the bimetallic composite periodic structure comprises a nanoporous membrane and a nanocolumn, wherein the nanoporous membrane is arranged on the surface of the substrate and is provided with circular holes penetrating up and down and distributed in an array, wherein the nanocolumn is a cylindrical structure and is nested inside the circular hole, wherein a space filled with a substance to be tested is formed between the nanocolumn and the circular hole, wherein the nanoporous membrane and the nanocolumn are composed of different metal materials, and wherein the nanoporous membrane and the nanocolumn are used to independently excite surface plasmons of different modes and couple with incident light to generate Fano resonance.

[0008] Preferably, the nanoporous membrane is a gold nanoporous membrane, and the nanocolumns are silver nanocolumns.

[0009] Preferably, the pore-column complex formed by the circular holes and the nanocolumns is distributed in a close-packed hexagonal array.

[0010] Preferably, the height of the nanocolumn is greater than the thickness of the nanoporous membrane.

[0011] Preferably, the height of the nanocolumn is 80-130 nm, and the thickness of the nanoporous membrane is 20-50 nm.

[0012] Preferably, the radius of the nanocolumn is 110-130 nm, and the radius of the nanoporous membrane is 160-210 nm.

[0013] A method for manufacturing a metasurface refractive index sensor based on a bimetallic composite periodic structure of the present invention comprises:

[0014] A layer of gold nanofilm is deposited on the SiO2 substrate;

[0015] Spin coating photoresist on the gold nanofilm;

[0016] Placing a first hole-shaped mask plate on the photoresist and performing exposure, development, and fixing operations, wherein the first hole-shaped mask plate has first circular holes distributed in an array;

[0017] Remove the hole-shaped mask plate and wash away the photoresist remaining inside the first circular hole formed by exposure;

[0018] The gold nanofilm is etched to form a gold nanoporous membrane having an array of vertically penetrating circular holes;

[0019] Spin coating photoresist on the gold nanopore membrane;

[0020] Placing a second hole-shaped mask plate on the photoresist and performing exposure, development, and fixing operations, wherein the second hole-shaped mask plate has second circular holes distributed in an array;

[0021] Remove the second hole-shaped mask plate, and wash away the photoresist remaining inside the second circular hole formed by exposure;

[0022] Sputtering silver liquid in the second circular hole to form a silver nanocolumn concentrically arranged with the second circular hole;

[0023] Washing away the remaining photoresist to form a metasurface refractive index sensor based on a bimetallic composite periodic structure;

[0024] The radius of the first circular hole is greater than that of the second circular hole.

[0025] More preferably, the first circular hole and the second circular hole are concentrically arranged and distributed in a close-packed hexagonal array.

[0026] The beneficial effects of the present invention are:

[0027] 1. A bimetallic composite periodic structure is set on the surface of the substrate. The bimetallic composite periodic structure includes a nanoporous membrane and a nanocolumn. The nanoporous membrane is provided with circular holes that penetrate up and down and are distributed in an array. The nanocolumn is a cylindrical structure and is nested inside the circular hole. The nanoporous membrane and the nanocolumn are made of different metal materials. Through the above structure, the nanoporous membrane and the nanocolumn can independently excite surface plasmons of different modes and couple with the incident light to produce Fano resonance, thereby further enhancing the modulation depth and reducing the full width at half maximum (Full Width at Half Maximum, FWHM) at the resonance wavelength of the reflection spectrum, thereby greatly improving the sensitivity and quality factor of the sensor.

[0028] 2. The nanopore membrane uses gold nanopore membrane and the nanocolumns use silver nanocolumns, which can maximize the coupling between different optical modes and improve the sensing performance of the sensor. The hole-column complex formed by the circular hole and the nanocolumn is distributed in a close-packed hexagonal array, which can further enhance the coupling between different optical modes and improve the sensing performance of the sensor. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 It is a schematic diagram of the three-dimensional structure of the present invention;

[0030] Figure 2 It is a partial top view schematic diagram of a simulation cycle of the present invention;

[0031] Figure 3 The reflection spectrum of the present invention when the refractive index of the substance to be tested is 1.3;

[0032] Figure 4 The reflection spectrum of the refractive index between 1.1 and 1.4 calculated by the finite element method of the present invention;

[0033] Figure 5 It is a fitting curve of the reflection valley of the present invention and the refractive index of different substances to be tested.

[0034] In the figure: 1-substrate, 2-bimetallic composite periodic structure, 201-nanoporous membrane, 202-nanopillar, 203-circular hole, 204-space filled with the substance to be tested. DETAILED DESCRIPTION

[0035] In order to make the technical problems, technical solutions and beneficial effects to be solved by the present application more clearly understood, the present application is further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present application and are not used to limit the present application.

[0036] It should be noted that when an element is referred to as being "fixed to" or "disposed on" another element, it can be directly on the other element or indirectly on the other element. When an element is referred to as being "connected to" another element, it can be directly connected to the other element or indirectly connected to the other element.

[0037] It should be understood that the orientation or position relationship indicated by terms such as "length", "width", "up", "down", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inside" and "outside" are based on the orientation or position relationship shown in the drawings, and are only for the convenience of describing the present application and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore should not be understood as a limitation on the present application.

[0038] In addition, the terms "first" and "second" are used for descriptive purposes only and should not be understood as indicating or implying relative importance or implicitly indicating the number of the indicated technical features. Thus, a feature defined as "first" or "second" may explicitly or implicitly include one or more of the features. In the description of this application, the meaning of "plurality" is two or more, unless otherwise clearly and specifically defined.

[0039] Embodiment 1

[0040] Figure 1 , 2 The schematic diagram of the structure of a metasurface refractive index sensor based on a bimetallic composite periodic structure provided by a preferred embodiment of the present application is shown. For the sake of convenience, only the part related to the present embodiment is shown, which is described in detail as follows:

[0041] A metasurface refractive index sensor based on a bimetallic composite periodic structure comprises a substrate 1 and a bimetallic composite periodic structure 2 distributed in a periodic array on the surface of the substrate 1, wherein the bimetallic composite periodic structure 2 comprises a nanoporous membrane 201 and nanopillars 202, wherein the nanoporous membrane 201 is arranged on the surface of the substrate 1 and is provided with circular holes 203 which penetrate up and down and are distributed in an array, wherein the nanopillars 202 are cylindrical structures and are nested inside the circular holes 203, wherein a space 204 filled with a substance to be tested is formed between the nanopillars 202 and the circular holes 203, wherein the nanoporous membrane 201 and the nanopillars 202 are made of different metal materials, and wherein the nanoporous membrane 201 and the nanopillars 202 are used to independently excite surface plasmons of different modes and couple with incident light to generate Fano resonance.

[0042] In one embodiment, the nanoporous membrane 201 is a gold nanoporous membrane, and the nanorods 202 are silver nanorods.

[0043] In one embodiment, the hole-column complex formed by the circular holes 203 and the nano-columns 202 is distributed in a close-packed hexagonal array. That is, each circular hole 203 is surrounded by 6 identical circular holes, and the centers of the six circular holes form a regular hexagon. There are silver nano-columns inside the circular holes 203, and the centers of the central circular holes and the bottom circles of the silver nano-columns are located at the center of the regular hexagon; the substance to be tested is filled in the substance-filled space 204 between the gold film and the silver column, as well as on the surface of the gold nano-hole film and the silver nano-column.

[0044] In one embodiment, the unit period is defined as p nm, the thickness of the gold nanopore membrane is d nm, the height of the silver nanocolumn in the center of the circular hole 203 is h nm, wherein the radius of the circular hole 203 is R1 nm, and the radius of the silver nanocolumn is R2 nm. The height of the nanocolumn 202 is higher than the thickness of the nanopore membrane 201. The height of the nanocolumn 202 is 80-130 nm, and the thickness of the nanopore membrane 201 is 20-50 nm. The radius of the nanocolumn 202 is 110-150 nm, and the radius of the nanopore membrane 201 is 150-210 nm.

[0045] This sensor works in the near-infrared band, the incident light wave is incident along the positive direction of the Z axis, the polarization direction of the incident light wave is along the X axis, and the substrate material is a dielectric material. Among them, the X axis direction is the substrate length direction, the Y axis direction is the substrate width direction, and the Z direction is the substrate thickness direction.

[0046] Embodiment 2

[0047] The sensor of the present invention is further described below by taking BK7 glass as an example. In this embodiment, the refractive index of the BK7 glass substrate is 1.4876.

[0048] When selecting materials, we conducted a large number of experiments on different material combinations in the structure, and finally selected four metal elements: silver, copper, gold and aluminum, all of which can be used as materials for localized surface plasmon sensors. Based on a large number of experiments and simulations, we can obtain widely accepted results: gold has the best local field strength effect, silver has the smallest ohmic loss, aluminum can be excited in the near-ultraviolet band, copper is worse than gold, silver and aluminum in terms of local field strength, and the excitation wavelength is in the optical fiber communication band. Due to the insufficient thermal stability of copper, the temperature increase of the device is more likely to cause copper oxidation. Considering the stability of the device, copper is generally not used as the excitation metal material for surface plasmons. Since the excitation bands of gold and silver are closest and both are near the near-infrared, the material selection of gold and silver for the bimetallic micro-nanostructure can enhance the coupling between different optical modes and improve the sensing performance of the sensor.

[0049] In the process of parameter optimization, we conducted a lot of experiments on the combination of different parameters under the selected structure and material, and finally obtained the optimal structural parameters: the period length P along the X-axis direction x is 1000nm, and the period length P along the Y axis is y The thickness of the gold film d is 30nm, the height of the silver column h is 100nm, the radius of the gold film hole R1 is 160nm, the radius of the silver column R2 is 130nm, and the refractive index n of the surrounding material to be tested is 1.3.

[0050] The reflectivity spectrum of the metasurface refractive index sensor based on the gold hole silver column bimetallic composite periodic structure obtained in this embodiment is calculated by the finite element method at 1122nm~1127nm (as shown in Figure 3 ). Figure 3 It can be seen that there is a resonance at the central wavelength of 1124.9nm with a reflectivity of 0.13. This is because under the illumination of normal incident light at a specific wavelength, coupling occurs between the periodically arranged gold nano-hole array and the silver column in the circular hole inside the gold film, which in turn generates Fano resonance, resulting in an obvious reflection valley in the reflection spectrum of the normal incident light, thereby increasing its modulation depth and reducing the full width at half maximum FWHM.

[0051] Figure 4 The gold-hole-silver-column bimetallic composite periodic structure metasurface refractive index sensor obtained in this embodiment has a fixed gold layer thickness (30nm) and a silver column height (100nm). When the refractive index of the substance to be measured changes, the incident electromagnetic wave is calculated by the finite element method to obtain a reflectivity spectrum curve at 1000nm to 1250nm. Figure 5 It can be seen that, with the increase of the refractive index of the substance to be measured, the measured characteristic reflection spectrum of the metasurface refractive index sensor based on the gold-hole-silver-pillar bimetallic composite periodic structure gradually red-shifts, which is consistent with the sensing characteristic that the surface plasmon excitation wavelength increases with the increase of the dielectric constant of the surrounding medium layer; in addition, each reflection valley corresponding to the refractive index has its own characteristic valley, and there is a wavelength interval of more than 40nm between the characteristic valleys of the adjacent reflection spectra.

[0052] In order to accurately obtain the sensitivity (S) of the sensor and then obtain the quality factor (FOM), it is necessary to fit the corresponding relationship between the characteristic reflection valley wavelength and the refractive index. Figure 5 This is the fitting curve of the reflection valley and the refractive index of different materials to be measured based on the metasurface refractive index sensor with gold hole and silver column bimetallic composite periodic structure. Figure 5According to the sensitivity calculation formula S = Δλ / Δn, it can be concluded that the sensitivity S of the gold-silver bimetallic surface plasmon sensor is 865nm / RIU, which is more than twice the sensitivity of the general metal nanohole array sensor (about 333nm / RIU). The half-width FWHM of the reflection valley is calculated to be 0.14nm, so the quality factor FOM of the sensor is 6178RIU -1 , is the quality factor of an ordinary single metal surface plasmon sensor (about 5-35RIU -1 ) is more than 176 times.

[0053] As can be seen from the above, the present invention studies a surface plasmon sensor combining gold nanoholes and silver nanopillars based on a gold hole silver pillar bimetallic composite periodic structure metasurface refractive index sensor. Coupling occurs between the two different optical modes of the periodically arranged gold nanohole array and the silver pillars in the circular holes inside the gold film, which in turn generates Fano resonance, resulting in an obvious reflection valley in the reflection spectrum of the normal incident light, thereby increasing its modulation depth, reducing the full width at half maximum FWHM, and improving the quality factor of the sensor. By performing parameter scanning on the various structural parameters of the sensor unit structure, the performance index sensitivity and quality factor of the sensor are optimized, and the sensitivity and quality factor are analyzed, providing a reference model for the bimetallic type surface plasmon refractive index sensor.

[0054] Embodiment 3

[0055] This embodiment also provides a method for manufacturing a metasurface refractive index sensor based on a bimetallic composite periodic structure, comprising:

[0056] Step 1, coating a layer of gold nanofilm on a SiO2 substrate;

[0057] Step 2, spin coating photoresist on the gold nanofilm;

[0058] Step 3, placing a first hole-shaped mask plate on the photoresist and performing exposure, development, and fixing operations, wherein the first hole-shaped mask plate has first circular holes distributed in an array;

[0059] Step 4, removing the hole-shaped mask plate, and washing away the photoresist remaining inside the first circular hole formed by exposure;

[0060] Step 5, etching the gold nanofilm to form a gold nanoporous membrane with array-distributed vertically penetrating circular holes;

[0061] Step 6, spin coating photoresist on the gold nanopore membrane;

[0062] Step 7, placing a second hole-shaped mask plate on the photoresist and performing exposure, development, and fixing operations, wherein the second hole-shaped mask plate has second circular holes distributed in an array;

[0063] Step 8, removing the second hole-shaped mask plate, and washing away the photoresist remaining inside the second circular hole formed by exposure;

[0064] Step 9, sputtering silver liquid in the second circular hole to form a silver nanocolumn concentrically arranged with the second circular hole;

[0065] Step 10, washing away the remaining photoresist to form a metasurface refractive index sensor based on a bimetallic composite periodic structure;

[0066] The radius of the first circular hole is greater than that of the second circular hole. The radius of the first circular hole is the radius of the circular hole 203 in the nanoporous membrane 201, and the radius of the second circular hole is the radius of the nanocolumn 202. The first circular hole and the second circular hole are concentrically arranged and distributed in a close-packed hexagonal array.

[0067] The embodiments described above are only used to illustrate the technical solutions of the present application, rather than to limit them. Although the present application has been described in detail with reference to the aforementioned embodiments, a person skilled in the art should understand that the technical solutions described in the aforementioned embodiments may still be modified, or some of the technical features may be replaced by equivalents. Such modifications or replacements do not deviate the essence of the corresponding technical solutions from the spirit and scope of the technical solutions of the embodiments of the present application, and should all be included in the protection scope of the present application.

Claims

1. A metasurface refractive index sensor based on a bimetallic composite periodic structure, characterized in that: The invention comprises a substrate (1) and a bimetallic composite periodic structure (2) distributed in a periodic array on the surface of the substrate (1), wherein the bimetallic composite periodic structure (2) comprises a nanoporous membrane (201) and nanopillars (202), wherein the nanoporous membrane (201) is arranged on the surface of the substrate (1) and is provided with circular holes (203) penetrating from top to bottom and distributed in an array, wherein the nanopillars (202) are cylindrical structures and are nested inside the circular holes (203), wherein a space (204) filled with a substance to be tested is formed between the nanopillars (202) and the circular holes (203), wherein the nanoporous membrane (201) and the nanopillars (202) are made of different metal materials, and wherein the nanoporous membrane (201) and the nanopillars (202) are used to independently excite surface plasmons of different modes and couple with incident light to generate Fano resonance; The nanoporous membrane (201) is a gold nanoporous membrane, and the nanocolumns (202) are silver nanocolumns; The hole-column complex formed by the circular holes (203) and the nano-columns (202) is distributed in a close-packed hexagonal array; The nanoporous membrane (201) and the nanocolumn (202) independently excite surface plasmons of different modes and couple with the incident light to generate Fano resonance.

2. The metasurface refractive index sensor based on a bimetallic composite periodic structure according to claim 1, characterized in that: The height of the nanorod (202) is greater than the thickness of the nanoporous membrane (201).

3. The metasurface refractive index sensor based on a bimetallic composite periodic structure according to claim 2, characterized in that: The height of the nanorod (202) is 80-130 nm, and the thickness of the nanoporous membrane (201) is 20-50 nm.

4. The metasurface refractive index sensor based on a bimetallic composite periodic structure according to claim 1, characterized in that: The radius of the nanocolumn (202) is 110-130 nm, and the radius of the nanoporous membrane (201) is 160-210 nm.

5. A method for manufacturing a metasurface refractive index sensor based on a bimetallic composite periodic structure according to any one of claims 1 to 4, characterized in that: include A layer of gold nanofilm is deposited on the SiO2 substrate; Spin coating photoresist on the gold nanofilm; Placing a first hole-shaped mask plate on the photoresist and performing exposure, development, and fixing operations, wherein the first hole-shaped mask plate has first circular holes distributed in an array; Remove the hole-shaped mask plate and wash away the photoresist remaining inside the first circular hole formed by exposure; The gold nanofilm is etched to form a gold nanoporous membrane having an array of vertically penetrating circular holes; Spin coating photoresist on the gold nanopore membrane; Placing a second hole-shaped mask plate on the photoresist and performing exposure, development, and fixing operations, wherein the second hole-shaped mask plate has second circular holes distributed in an array; Remove the second hole-shaped mask plate, and wash away the photoresist remaining inside the second circular hole formed by exposure; Sputtering silver liquid in the second circular hole to form a silver nanocolumn concentrically arranged with the second circular hole; Washing away the remaining photoresist to form a metasurface refractive index sensor based on a bimetallic composite periodic structure; The radius of the first circular hole is greater than that of the second circular hole.

6. The method according to claim 5, characterized in that: The first circular hole and the second circular hole are concentrically arranged and distributed in a close-packed hexagonal array.

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