High-sensitivity and high-figure-of-merit refractive index sensor based on all-dielectric metasurface and preparation method thereof

Abstract

The invention belongs to the technical field of refractive index sensors, and particularly discloses a high-sensitivity and high-figure of merit refractive index sensor based on an all-dielectric metasurface and a preparation method thereof.The high-sensitivity and high-figure of merit refractive index sensor comprises a substrate and metasurface unit structures distributed on the substrate in an array mode, and each metasurface unit structure comprises a cubic column type silicon block with a through hole formed in the center; a slit is formed in the center of the cubic-column-shaped silicon block in the y-axis direction, a cylindrical silicon block is arranged in the center of the through hole, the center of the cylindrical silicon block moves in the y-axis direction to break structural symmetry, and the center line of the cylindrical silicon block does not coincide with the center line of the through hole. Compared with the prior art, the sensor provided by the invention has the advantages of high sensitivity, high merit index and strong anti-noise capability, and is beneficial to distinguishing the position of a resonance valley; in addition, the sensitivity and the FOM value can reach 865.91 nm / RIU and 31294.2 RIU <-1 > respectively, and the near-infrared optical fiber has more excellent performance in the near-infrared band.

Description

Technical Field

[0001] The present invention relates to the technical field of refractive index sensors, and in particular to a high-sensitivity and high-merit refractive index sensor based on an all-medium metasurface and a preparation method thereof. Background Art

[0002] As one of the important methods for identifying substances and monitoring changes in substances, refractive index sensing technology plays an important role in environmental monitoring, chemical analysis, biomedicine and other fields. Optical refractive index sensors supported by metasurfaces have the advantages of small size, easy integration, high sensitivity, and non-destructive testing, and have attracted widespread attention. In addition, to evaluate the performance of optical refractive index sensors, it is necessary to comprehensively consider two indicators: sensitivity (sensitivity, S, S is defined as S = Δλ / Δn, Δλ is the offset of the resonant wavelength caused by the change of the external refractive index n) and figure of merit (FOM, FOM = S / FWHM, FWHM is the half-peak width). Sensitivity reflects the degree of response of the sensor to the change of refractive index, while the FOM value comprehensively considers the balance between the sensitivity and resolution of the sensor. The higher the FOM value, the more outstanding the sensor is in terms of response speed and accuracy to the change of refractive index. These two indicators are crucial for refractive index detection. It is difficult to find a design method for a metasurface refractive index sensor with high sensitivity and high FOM value.

[0003] Initially, people often used metal structures to design sensors because surface plasmons can localize light fields and enhance the interaction between light and the medium around the structure. However, the inherent ohmic loss of metals will lead to a decrease in the Q factor and FOM value, which is not conducive to resolving the shift of the resonance spectrum. All-dielectric metasurfaces with high refractive index and low loss solve this problem well and have been widely proposed recently. All-dielectric metasurfaces can not only support high-quality resonances, but also excite electromagnetic multipolar resonances, which provides a good platform for high-performance sensing applications.

[0004] Enhancing the light-matter interaction at the nanoscale is very important for high-sensitivity sensing applications. Bound states in the continuum (BIC) are an effective way to achieve ultra-high Q resonance. BIC is a special mode that exists in a continuous frequency range but is completely decoupled from the radiation mode. There are two ways to achieve BIC, depending on the way the local mode and the radiation mode are decoupled: the first is based on symmetry-induced radiation suppression, and the other is accidental decoupling from the radiation mode due to continuous tuning of system parameters. These two types of BIC are called symmetry-protected BIC and accidental BIC. BIC cannot be excited by external excitation, so the resonance line width in the spectrum is zero and cannot be observed. In practical applications, people usually introduce breaks in micro-nano structures to transform BIC into quasi-BIC with sharp resonance peaks. These quasi-BICs have been used in high-performance sensing. Although some achievements have been made in the field of refractive index sensing so far, some of these structures do not pay attention to the location of the introduction of breaking when using symmetry protection quasi-BIC, resulting in changes in the degree of symmetry breaking of the unit structure while affecting the interaction between the local electromagnetic field and the surrounding medium. This affects S when adjusting the FOM value, making it difficult to achieve dual highs in these two performance parameters. Summary of the invention

[0005] In order to solve the above technical problems, the present invention provides a high-sensitivity and high-merit refractive index sensor based on an all-dielectric metasurface and a preparation method thereof.

[0006] To achieve the above object, the present invention is implemented according to the following technical solutions:

[0007] One of the technical solutions of the present invention is a high-sensitivity and high-merit refractive index sensor based on an all-dielectric metasurface, comprising a substrate and a metasurface unit structure arrayed on the substrate, wherein the metasurface unit structure comprises a cubic columnar silicon block with a through hole in the center, a slit in the center of the cubic columnar silicon block in the y-axis direction, a cylindrical silicon block in the center of the through hole and the center of the cylindrical silicon block is moved in the y-axis direction to break the structural symmetry, and the center line of the cylindrical silicon block does not coincide with the center line of the through hole.

[0008] Preferably, the side length L of the cubic columnar silicon block is 690nm-770nm, the thickness t is 460nm-540nm, the radius R of the through hole is 80nm-160nm, the slit width g is 20nm-60nm, and the radius r of the cylindrical silicon block is 80nm-160nm.

[0009] Preferably, the period P of the metasurface unit structure is 1200 nm.

[0010] Preferably, the material of the substrate is silicon dioxide.

[0011] The second technical solution of the present invention is a method for preparing a high-sensitivity and high-merit refractive index sensor based on an all-dielectric metasurface, comprising the following steps:

[0012] S1. Designing the above-mentioned metasurface unit structure;

[0013] S2. Depositing silicon atoms onto a substrate to form a metasurface unit structure, thereby obtaining a high-sensitivity and high-merit refractive index sensor based on an all-dielectric metasurface.

[0014] Compared with the prior art, the sensor of the present invention has both high sensitivity and high figure of merit refractive index. Different liquids to be tested are respectively filled on the top of the metasurface, and the metasurface is vertically irradiated with near-infrared polarized light whose polarization direction is perpendicular to the slit opening direction. The change of the refractive index will cause the movement of the resonance valley in the transmission spectrum, and different liquid components can be distinguished by observing the movement of the resonance valley. Through simulation, it can be seen that when the external refractive index changes, the modulation depth of the resonance valley is always close to 100%, indicating that the anti-noise ability is strong and it is conducive to distinguishing the position of the resonance valley. In addition, the sensitivity and FOM value can reach 865.9nm / RIU and 31294.2RIU respectively. -1 , the present invention has more superior performance in the near-infrared band. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Figure 1 Model diagram of a high-sensitivity and high-merit refractive index sensor based on an all-dielectric metasurface established using the COMSOL Multiphysics commercial multi-physics simulation software: (a) is the overall structure diagram; (b) is the distance the hole at the center moves along the y-axis; (c) is the angle at which the silicon block tilts toward the negative direction of the x-axis; (d) is the angle at which the silicon block tilts toward the y-axis.

[0016] Figure 2 The simulation results of high-sensitivity and high-merit refractive index sensors based on all-dielectric metasurfaces using COMSOL Multiphysics commercial multi-physics simulation software: (a) The movement of the resonance valley with the change of the external refractive index; (b) The relationship between the resonance wavelength and the external refractive index, the blue solid line is a linear fit; (c) The change of FOM with the external refractive index; (d) The relationship between the sensitivity S and FOM value and the asymmetry parameter α. DETAILED DESCRIPTION

[0017] In order to make the purpose, technical solution and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with embodiments. The specific embodiments described herein are only used to explain the present invention and are not used to limit the invention.

[0018] In this embodiment, firstly, according to the idea of ​​decoupling the sensitivity and FOM value for regulation, a metasurface unit structure supporting quasi-BIC is designed. Since the sensitivity is related to the ratio of the electromagnetic field acting on the medium around the structure, a model of a high-sensitivity and high-merit refractive index sensor based on an all-medium metasurface is established using COMSOL Multiphysics commercial multi-physics simulation software, as shown in FIG. Figure 1 As shown, a cubic columnar silicon block with a through hole in the center is first designed, a slit is opened in the center of the cubic columnar silicon block in the y-axis direction, a cylindrical silicon block is arranged in the center of the through hole, and the center of the cylindrical silicon block is moved in the y-axis direction to break the structural symmetry, and the center line of the cylindrical silicon block does not coincide with the center line of the through hole. Exemplarily, the side length L of the cubic columnar silicon block is 750nm, the thickness t is 500nm, the radius R of the through hole is 280nm, the slit width g is 30nm, and the radius r of the cylindrical silicon block is 100nm.

[0019] This structure has a high proportion of eigenmodes in which the electromagnetic field interacts with the medium surrounding the structure, thus ensuring high sensitivity. Secondly, by introducing reasonable breaks, we allow the external environment to excite the eigenmode while ensuring that the distribution of the local electromagnetic field is affected as little as possible when the degree of breakage is changed, that is, the sensitivity is kept as unchanged as possible. Since the resonant linewidth is proportional to the degree of breakage, according to FOM=S / FWHM, we can realize a high-performance refractive index sensor with both high sensitivity and FOM values.

[0020] The large difference in refractive index between the metasurface unit structure and the substrate is conducive to the localization of the light field, producing a sharper resonance valley and improving the resolution. Since silicon has a negligible light absorption coefficient in the near-infrared band and a high refractive index of 3.45, it is selected as the material for the metasurface unit structure. In addition, silicon dioxide has a low refractive index of only 1.44 in the near-infrared band, so it is used as a substrate material.

[0021] Illustratively, electron beam lithography can be used to define a surface pattern in a photoresist, followed by development and fixing; the metasurface is etched by inductively coupled plasma; and finally, the remaining photoresist is removed in an ultrasonic bath to form a metasurface unit structure having a period P of 1200nm, thereby obtaining a high-sensitivity and high-merit refractive index sensor based on an all-dielectric metasurface.

[0022] Then, COMSOL Multiphysics commercial multi-physics simulation software is used to simulate the designed high-sensitivity and high-merit refractive index sensor based on the all-dielectric metasurface in the wavelength domain, as follows:

[0023] A high-sensitivity and high-merit refractive index sensor based on an all-dielectric metasurface is placed in water, and the metasurface is vertically irradiated with near-infrared polarized light whose polarization direction is perpendicular to the slit opening direction, and the transmission spectrum is measured. Under the excitation of linear polarized light, two resonance modes will be excited in the metasurface unit structure, and two resonance valleys will appear in the transmission spectrum. One of the resonance modes is a leakage mode with a large resonance line width, which is not conducive to the resolution of the resonance valley offset, so it is not considered. The other resonance mode is a quasi-BIC mode with an extremely narrow resonance line width, which is sensitive to external refractive index changes and can be used for high-precision detection.

[0024] The simulation results are as follows Figure 2 As shown in (a), the smaller the distance of the central cylinder movement, the narrower the resonance line width and the higher the FOM value. However, considering the current processing accuracy limit, we calculated the refractive index sensing capability of the designed metasurface when the central cylinder moves 40nm. In the above case, the line width of the quasi-BIC mode is 0.03nm and the Q factor is 63685. We considered several common liquid refractive indices with external refractive indices between 1.33-1.40 and filled them above the designed metasurface. The resonance valley movement caused by them is shown in Figure 1. Figure 2 As shown in (a) in the figure, when the external refractive index increases, the resonance wavelength undergoes a significant red shift, and the resonance line width remains almost unchanged. In addition, the resonance valley modulation depth is close to 100%, indicating that the anti-interference ability is strong.

[0025] The metasurface is vertically illuminated with near-infrared polarized light whose polarization direction is perpendicular to the slit opening direction, and the object to be tested is changed. The transmission spectrum of the sample is measured, and the sensitivity and FOM value are calculated. The calculation of sensitivity is as follows: Figure 2 As shown in (b), the orange dots mark the resonant wavelengths at different external refractive indices, and then a linear fit is performed, represented by the blue solid line, and finally the slope is calculated to obtain the result. The results show that the sensitivity can reach 865.9nm / RIU. Combined with its resonant line width, the FOM value is obtained, and the calculation results are as follows Figure 2 As shown in (c) in the figure. When the refractive index changes from 1.33 to 1.40, the FOM increases from 26239.7RIU -1 Increased to 31294.2 RIU -1 The designed metasurface has superior performance in refractive index sensing in the near-infrared band, and this good performance comes from the reasonable structural design.

[0026] Furthermore, in order to illustrate the effectiveness of the metasurface unit structure design of the present invention, the geometric parameters of the sample are changed, the metasurface is vertically irradiated with near-infrared polarized light whose polarization direction is perpendicular to the slit opening direction, and the transmission spectrum of the sample is measured. Specifically, this embodiment defines the asymmetry parameter as α=σ / 2R, and calculates the sensitivity and FOM value under different asymmetry parameters α. Figure 2 As shown in (d), since the only purpose is to observe the change of FOM value, the resonance line width when the refractive index n = 1.33 is taken for calculation of FOM value under different asymmetry parameters α. Figure 2 As can be seen from (d) in the figure, when the asymmetry parameter α changes, the sensitivity S does not change significantly, while the FOM value gradually increases as the asymmetry parameter α becomes smaller, so that the sensitivity and FOM can reach a high level when the metasurface is used as a refractive index sensor.

[0027] The technical solution of the present invention is not limited to the above-mentioned specific embodiments. All technical variations made according to the technical solution of the present invention fall within the protection scope of the present invention.

Claims

1. A high-sensitivity and high-merit refractive index sensor based on an all-dielectric metasurface, comprising a substrate and a metasurface unit structure arrayed on the substrate, characterized in that: The supersurface unit structure includes a cubic columnar silicon block with a through hole in the center, a slit in the center of the cubic columnar silicon block in the y-axis direction, a cylindrical silicon block in the center of the through hole and the center of the cylindrical silicon block moves in the y-axis direction to break the structural symmetry, and the center line of the cylindrical silicon block does not coincide with the center line of the through hole.

2. The high-sensitivity and high-merit refractive index sensor based on the all-dielectric metasurface according to claim 1, characterized in that: The side length L of the cubic columnar silicon block is 690nm-770nm, the thickness t is 460nm-540nm, the radius R of the through hole is 80nm-160nm, the slit width g is 20nm-60nm, and the radius r of the cylindrical silicon block is 80nm-160nm.

3. The high-sensitivity and high-merit refractive index sensor based on the all-dielectric metasurface according to claim 1, characterized in that: The period P of the supersurface unit structure is 1200nm.

4. The high-sensitivity and high-merit refractive index sensor based on the all-dielectric metasurface according to claim 1, characterized in that: The material of the substrate is silicon dioxide.

5. A method for preparing a high-sensitivity and high-merit refractive index sensor based on an all-dielectric metasurface, characterized in that: The following steps are involved: S1. Designing a supersurface unit structure as claimed in any one of claims 1 to 4; S2. Depositing silicon atoms onto the substrate to form a metasurface unit structure, thereby obtaining a high-sensitivity and high-merit refractive index sensor based on the all-dielectric metasurface.

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