Refractive index sensor based on MIM chiral meniscus structure array

Abstract

The invention provides a refractive index sensor based on an MIM chiral meniscus structure array. A chiral metal nanostructure array of the refractive index sensor comprises a plurality of MIM meniscus units which are periodically arranged; each MIM meniscus unit comprises two metal meniscus structures with the same thickness and a medium meniscus structure arranged between the two metal meniscus structures; each single-layer meniscus structure has an asymmetric shape and has a chiral optical response. According to the invention, three metal / medium meniscus structures with the same size are longitudinally stacked to form an MIM chiral structure. According to the invention, a metal-insulator-metal configuration and a chiral structure are combined, and plasmon coupling in a z direction is introduced, so that a double-peak-shaped CD signal is realized. The optical refractive index sensor has the advantages of high sensitivity, high quality factor and the like, and the problems that a traditional optical refractive index sensor is generally large in size, limited in application range and the like are solved.

Description

Technical Field

[0001] The invention relates to the technical field of micro-nano optics. Background Art

[0002] With the development of nanoscience and the advancement of processing technology, the preparation of metal nanostructures of various configurations has been achieved by using electron beam lithography, molecular self-assembly technology, template lithography technology and chemical synthesis. Therefore, people have shown great research interest in the optical properties and intrinsic photophysical mechanisms exhibited by metal nanostructures, among which localized surface plasmon resonance is one of the research focuses. Localized surface plasmon resonance is a phenomenon in which the free electrons on the surface of a metal nanostructure resonate with the incident light field under the excitation of the incident light field, thereby generating strong absorption and scattering at the resonance frequency, and generating a great field enhancement in a very small volume near the surface of the metal nanostructure. The localized surface plasmon resonance mode is very sensitive to the constituent materials, geometric configuration, structural size and changes in the surrounding environment, making metal nanostructures one of the research hotspots in the fields of micro-nano optical device design and environmental sensing detection.

[0003] Chirality refers to the fact that an object in three-dimensional space cannot be translated or rotated to make it coincide with its mirror image, just like the left hand cannot coincide with the right hand. Chirality is a hot topic in scientific research not only because it is closely related to human characteristics, but also exists at all scales in daily life, from the vast universe to single molecules, such as spiral galaxies, shells on the seashore, vines of plants and the helical structure of DNA molecules.

[0004] Chirality can also be manifested by specific optical properties, such as circular dichroism (CD), which is the difference in absorption between left-handed circularly polarized light and right-handed circularly polarized light, and optical dispersion, which causes the polarization direction of linearly polarized light to rotate after passing through a chiral structure. Therefore, CD spectroscopy and optical dispersion spectroscopy are important characterization methods for studying molecular configurations and distinguishing molecular species in the fields of analytical chemistry and detection. However, the optical response of natural chiral molecules is limited to the ultraviolet band, and the mismatch between the molecular scale and the order of magnitude of the response wavelength leads to a weak optical response, which limits practical applications in terms of high sensitivity and high resolution. Chiral plasmonic structures can achieve CD responses that are several orders of magnitude larger than those of natural chiral molecules, and the response is broadened to the visible-infrared spectral range. Therefore, by designing artificial chiral metal nanostructures with novel structures and simple preparation, introducing optical mechanisms such as plasmon near-field / far-field coupling in the z direction, it is of great significance to achieve significantly enhanced chiral optical responses and use CD signals as sensing signals for realizing fast, label-free and miniaturized high-sensitivity optical sensing platforms. Summary of the invention

[0005] Based on the above-mentioned problems that traditional optical sensors usually have, such as large size, low sensitivity and limited application range, the present invention proposes a refractive index sensor based on a MIM chiral meniscus structure array (where MIM is Metal-Insulator-Metal, metal-insulator-metal), which enhances the refractive index sensing performance through the double-peak CD signal of the chiral plasmon nanostructure. The present invention mainly utilizes three metal / dielectric meniscus structures of the same size stacked longitudinally into a MIM chiral structure. The metal-insulator-metal configuration is combined with the chiral structure, and plasmon coupling in the z direction is introduced to achieve a double-peak CD signal with excellent performance such as high sensitivity and high quality factor.

[0006] In order to achieve the above technical objectives, the technical solutions adopted by the present invention include:

[0007] A refractive index sensor based on a MIM chiral meniscus structure array comprises a substrate and a chiral metal nanostructure array arranged on the substrate, wherein:

[0008] The chiral metal nanostructure array comprises a plurality of periodically arranged MIM meniscus nanounits;

[0009] Each MIM meniscus nanounit includes three stacked single-layer meniscus nanounits, namely, two metal layer meniscus nanounits with the same thickness and a dielectric layer meniscus nanounit disposed between the two metal layer meniscus nanounits;

[0010] Each single-layer meniscus structure has the same chiral shape.

[0011] Furthermore, the thickness of the metal layer in the metal layer meniscus nanounit is 10-100 nm; the thickness of the dielectric layer in the dielectric layer meniscus nanounit is 0-3000 nm, which is used to determine the plasmon coupling strength, thereby affecting the chiral optical response of the structure.

[0012] Furthermore, the metal layers in the metal layer meniscus nanounits are all made of precious metal materials, including but not limited to one or more of gold, silver, and aluminum.

[0013] Furthermore, the dielectric layer in the dielectric layer meniscus nanounit is made of dielectric materials, including but not limited to silicon dioxide and magnesium fluoride.

[0014] Furthermore, the periodic arrangement of the chiral metal nanostructure array is arbitrarily selected, including but not limited to a tetragonal lattice array and a hexagonal lattice array.

[0015] Furthermore, the material of the substrate includes glass, and the thickness of the substrate is set according to the working requirements of the refractive index sensor.

[0016] Compared with the prior art, the present invention has the following advantages:

[0017] 1. The refractive index sensor based on the MIM chiral meniscus structure array provided by the present invention has sensitive sensing characteristics for changes in the refractive index of the environment by utilizing the localized surface plasmon resonance. By detecting the changes in the resonance peak position of the CD spectrum, the changes in the refractive index of the surrounding environment are detected, and it has high sensitivity and accuracy.

[0018] 2. The refractive index sensor based on the MIM chiral meniscus structure array provided by the present invention, when in use, changes in the refractive index of the surrounding environment will cause changes in the structure's transmittance of circularly polarized light, thereby changing the CD. Refractive index sensing is achieved by detecting changes in circular dichroism.

[0019] 3. The refractive index sensor based on the MIM chiral meniscus structure array provided by the present invention is a fast and miniaturized sensor, which overcomes the problems of large size and limited application range commonly existing in traditional optical refractive index sensors, and can play an important role in many fields such as biological monitoring, chemistry, and environmental monitoring.

[0020] Based on the above reasons, the present invention can be widely promoted in the fields of micro-nano optics and the like. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 It is a schematic diagram of the three-dimensional structure of the refractive index sensor based on the MIM chiral meniscus structure array of the present invention.

[0022] Figure 2 It is a schematic diagram of the planar structure of a unit structure in the refractive index sensor based on the MIM chiral meniscus structure array of the present invention on the xz plane or the yz plane.

[0023] Figure 3 This is a CD spectrum diagram in an air environment provided in an embodiment of the present invention.

[0024] Figure 4 Graphs showing CD spectra under different environmental refractive indices in the embodiments of the present invention.

[0025] Figure 5 Graph showing sensitivity characteristics of a refractive index sensor in an embodiment of the present invention.

[0026] Figure 6 Graph showing the functional relationship between quality factor and wavelength in an embodiment of the present invention. DETAILED DESCRIPTION

[0027] like Figure 1-2 As shown, the present invention provides a refractive index sensor based on a MIM chiral meniscus structure array, comprising a substrate, and a chiral metal nanostructure array disposed on the substrate, wherein:

[0028] The chiral metal nanostructure array comprises a plurality of periodically arranged MIM meniscus nanounits;

[0029] Each MIM meniscus nanounit includes three stacked single-layer meniscus nanounits, namely, two metal layer meniscus nanounits with the same thickness and a dielectric layer meniscus nanounit disposed between the two metal layer meniscus nanounits;

[0030] Each single-layer meniscus structure has the same chiral shape.

[0031] When specifically implemented, as a preferred embodiment of the present invention, Figure 2 As shown, the thickness of the metal layer in the metal layer meniscus nanounit is h1=h3=10-100 nm; the thickness of the dielectric layer in the dielectric layer meniscus nanounit is h2=0-3000 nm, which is used to determine the plasmon coupling strength, thereby affecting the chiral optical response of the structure.

[0032] In specific implementation, as a preferred embodiment of the present invention, the metal layer in the metal layer meniscus nanounit is made of precious metal materials, including but not limited to one or more of gold, silver, and aluminum, and the dielectric layer in the dielectric layer meniscus nanounit is made of dielectric materials, including but not limited to silicon dioxide and magnesium fluoride.

[0033] In specific implementation, as a preferred embodiment of the present invention, the periodic arrangement of the chiral metal nanostructure array is arbitrarily selected, including but not limited to a tetragonal lattice array and a hexagonal lattice array.

[0034] In a specific implementation, as a preferred embodiment of the present invention, the material of the substrate includes glass, and the thickness of the substrate is set according to the working requirements of the refractive index sensor.

[0035] Example 1

[0036] The chiral metal nanostructure array has different transmission for left-handed and right-handed circularly polarized light, which is manifested as the presence of chiral resonance peaks in the visible-near infrared spectral region. Since the localized surface plasmon resonance is sensitive to changes in the refractive index of the surrounding environment, as the refractive index increases, the resonance peak in the CD spectrum redshifts, thereby realizing refractive index sensing with high sensitivity and accuracy.

[0037] Based on the finite difference time domain method, the simulation experiment was carried out using FDTD Solutions simulation software. The specific parameters are as follows:

[0038] The chiral metal nanostructure array is placed on a glass substrate; the periodic arrangement of the array is a hexagonal lattice array; the nanostructure is divided into three layers, wherein the upper and lower metal layers are made of gold with a height of h1=h3=30nm, and the dielectric layer in the middle layer is made of silicon dioxide with a height of h1=50nm.

[0039] In this embodiment, the CD response is CD=T LCP -T RCP Calculated, where T LCP represents the transmission coefficient of the structure for left-handed circularly polarized light, T RCP represents the transmission coefficient of the structure for right-handed circularly polarized light. Figure 3 As shown in the figure, under the condition that the surrounding medium is air, the transmission of the MIM chiral meniscus structure array of this embodiment is greatly different under the incidence of left-handed circularly polarized light and right-handed circularly polarized light, thereby producing an obvious CD response. It can be observed from the figure that there is an obvious resonance peak near the wavelength λ=989nm, and the amplitude is CD=17%; at the resonance valley of wavelength λ=1040nm, the CD signal extreme value is 36%. In addition, the rapid change from the resonance peak to the resonance valley results in a double-peaked signal.

[0040] like Figure 4 As shown, it is a graph of the CD spectrum of this embodiment under different ambient refractive indices. The transmission spectrum of the present invention under different concentrations of ambient refractive index is simulated, and the CD spectrum is calculated, and the ambient refractive index is gradually increased so that the refractive index gradually increases from 1.0 to 1.4, with an increase of 0.1 each time. The horizontal and vertical axes in the figure are wavelength and CD value, respectively. The environmental refractive indices n corresponding to the CD spectrum curve in the figure are 1.0, 1.1, 1.2, 1.3, and 1.4, respectively. It can be seen from the results in the figure that with the increase of the external refractive index, the resonance peak and resonance valley of the CD spectrum are significantly red-shifted. Therefore, a refractive index sensor that accurately senses refractive index changes in the refractive index range of 1.0 to 1.4 is proposed.

[0041] Example 2

[0042] like Figure 5 As shown in FIG. 1 , the corresponding relationship between the CD spectrum resonance wavelength and the ambient refractive index and the fitting curve in Example 1. It can be concluded from the figure that the CD resonance wavelength and the ambient refractive index present a standard linear relationship. The ability of the plasmon sensor to detect changes in the ambient environment is quantified by the sensitivity S, which is defined as:

[0043]

[0044] Where Δλ represents the shift of the spectral resonance peak caused by the change of the ambient refractive index, and Δn represents the change of the ambient refractive index. The calculated resonance peak sensitivity S of the hexagonal lattice array of the MIM meniscus structurepeak =435nm / RIU and resonance valley sensitivity S dip =328nm / RIU.

[0045] In addition, in order to more accurately describe the sensing performance of plasmonic nanostructures, the quality factor FOM* is usually defined as a dimensionless indicator:

[0046]

[0047] Where ΔI(λ) / I0(λ) represents the change in relative intensity caused by the change in the ambient refractive index Δn at a fixed wavelength λ0. In the calculation of FOM* in this embodiment, I(λ) is taken from n env =1.3, I0(λ) is taken from n env = 1.0, and then Δn is n env =1.3 and n env =1.0, the relationship between the calculated FOM* and wavelength λ is as follows Figure 6 As shown in the figure, we can get a high quality factor FOM*=646RIU -1 .

[0048] In summary, the CD spectrum of the present invention is highly sensitive to changes in the ambient refractive index, making it potentially applicable in the fields of chemistry, environment, biology, and other disciplines.

Claims

1. A refractive index sensor based on a MIM chiral meniscus structure array, characterized in that: It includes a substrate and a chiral metal nanostructure array arranged on the substrate, wherein: the chiral metal nanostructure array includes a plurality of periodically arranged MIM meniscus nanounits; each MIM meniscus nanounit includes three stacked single-layer meniscus nanounits, namely, two metal layer meniscus nanounits with the same thickness and a dielectric layer meniscus nanounit arranged between the two metal layer meniscus nanounits; each single-layer meniscus structure has the same chiral shape.

2. The refractive index sensor based on the MIM chiral meniscus structure array according to claim 1, characterized in that: The thickness of the metal layer in the metal layer meniscus nanounit is 10-100 NM; the thickness of the dielectric layer in the dielectric layer meniscus nanounit is 0-3000 NM, which is used to determine the plasmon coupling strength, thereby affecting the chiral optical response of the structure.

3. The refractive index sensor based on the MIM chiral meniscus structure array according to claim 1, characterized in that: The metal layers in the metal layer meniscus nanounits are all made of precious metal materials, including but not limited to one or more of gold, silver, and aluminum.

4. The refractive index sensor based on the MIM chiral meniscus structure array according to claim 1, characterized in that: The dielectric layer in the dielectric layer meniscus nanounit is made of dielectric materials, including but not limited to silicon dioxide and magnesium fluoride.

5. The refractive index sensor based on the MIM chiral meniscus structure array according to claim 1, characterized in that: The periodic arrangement of the chiral metal nanostructure array is arbitrarily selected, including but not limited to a tetragonal lattice array and a hexagonal lattice array.

6. The refractive index sensor based on the MIM chiral meniscus structure array according to claim 1, characterized in that: The material of the substrate includes glass, and the thickness of the substrate is set according to the working requirements of the refractive index sensor.

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