Tetramer refractive index sensor based on continuous domain bound state and manufacturing method thereof

Abstract

The invention discloses a tetramer refractive index sensor based on continuous domain bound states (BICs) and a manufacturing method of the tetramer refractive index sensor. The sensor comprises a silicon dioxide substrate and tetramer unit cells distributed on the upper surface of the substrate in an array mode. The tetramer unit cell comprises four positive polygonal cylinders which are identical in size, the positive polygonal cylinders are perpendicular to the silicon dioxide substrate, and the positive polygonal cylinders are arranged in a matrix mode and rotate by an angle theta towards the center of the unit cell. Along with the change of theta, the metasurface unit cell is converted from a monomer to a tetramer and then converted from the tetramer to the monomer, so that energy band folding is induced in a wave vector space, and then the high-Q-value BICs metasurface with the Q value insensitive to the rotation angle is excited. The refractive index sensor also has the characteristic of high refractive index sensing sensitivity, and the detection efficiency, flexibility and accuracy of the refractive index sensor in the biomedical field are improved.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of optical devices, and particularly relates to a tetramer refractive index sensor based on bound states in the continuum and a manufacturing method thereof. BACKGROUND

[0002] Super surface, full name super material surface, is an artificial structure surface with special physical properties. It is composed of periodically arranged sub-wavelength size units, which can flexibly regulate electromagnetic waves. The emergence of super surface has brought creative changes to the field of electromagnetic wave manipulation. Compared with traditional materials, super surface has the characteristics of small size, light weight, high flexibility and easy integration, and with the continuous development of micro-nano processing technology, the preparation and experimental research of super surface have made remarkable achievements, which is expected to provide strong impetus for the development of information technology, national defense technology and other fields in China.

[0003] Bound States in the Continuum (BICs) is a unique physical phenomenon, which was first proposed in quantum mechanics and has been widely studied and applied in the fields of optics and electromagnetics. The unique feature of BICs is that although its wave function (or electromagnetic wave) is in the continuous spectrum, it is similar to a bound state without radiation leakage, i.e., cannot be coupled with the continuous domain, so the ideal BICs have infinite quality factor and infinitesimal line width, which are theoretically unobservable and cannot be applied. Therefore, quasi-BICs (q-BICs) mode is used in practical applications. This mode has the characteristics of narrow bandwidth and high Q value, which is extremely attractive for imaging, sensing, filtering, light emission and other scenarios. For super surface, by finely adjusting the perturbation parameters of the super surface, BICs can be converted into q-BICs, for example, most designs are to obtain q-BICs mode and adjust Q value by destroying the mirror symmetry of super surface unit cell, at this time, the perturbation parameter corresponds to the degree of destruction of the mirror symmetry of the super surface unit cell.

[0004] An ultra-surface sensor is a new type of sensor that uses the unique electromagnetic properties of ultra-surface to achieve rapid and high-sensitivity detection of biological molecules and chemical substances. This sensor can achieve precise detection and analysis of biological signals in a very small size by designing sub-wavelength structures with specific functions. The main features of the ultra-surface sensor include high sensitivity, low detection limit, fast response, and good specificity. It can directly detect biological molecules such as DNA, proteins, enzymes, and various chemicals without the need for labeling, providing strong technical support for the fields of biomedicine, environmental monitoring, food safety, and others. According to the type of the substance to be detected, the on-chip sensing can be generally divided into two categories: bulk sensing and surface sensing. Surface sensing is suitable for detecting various substances attached to the surface of the waveguide, such as protein molecules, while bulk sensing can be applied to the detection of various liquid and gas substances. The working principle of the on-chip sensing unit can be summarized as follows: by analyzing the disturbance of the measured substance to the optical parameters (such as intensity, phase, polarization, frequency, etc.) of the sensing unit, the information of the measured substance can be determined. With the continuous development and improvement of ultra-surface technology, ultra-surface sensors are expected to play an important role in the future of precision medicine and biological analysis, and promote the technological progress of related industries. Since q-BICs can enhance the electromagnetic field strength near the surface of the ultra-surface, and their resonance wavelength is very sensitive to the refractive index of the medium around the ultra-surface, the shape, size, and other parameters of the ultra-surface unit can be carefully designed to control the q-BICs resonance mode, thereby realizing an ultra-surface sensor with high quality factor and high sensitivity.

[0005] Existing medium ultra-surface sensors based on BICs are usually designed based on the destruction of the in-plane symmetry of the unit cell. The performance of such devices is usually sensitive to the perturbation parameters of the ultra-surface, and the shape of the nanostructure is complex, which leads to a greater impact of processing errors on the performance of the device and poor repeatability. Ultra-surfaces that are robust to perturbation parameters can significantly reduce the requirements for processing technology, promote the application of ultra-surface sensors, and provide more stable sensing performance. SUMMARY

[0006] The present application aims to solve the defects of the above-mentioned existing technology, and proposes a four-polymer refractive index sensor based on continuous domain bound state and a manufacturing method thereof, which has the characteristics of easy processing, Q value robustness, high sensitivity, and polarization insensitivity.

[0007] The technical solution for achieving the purpose of the present application is: a four-polymer refractive index sensor based on continuous domain bound state, characterized by comprising a silicon dioxide substrate and a four-polymer ultra-surface unit cell composed of silicon positive even polygonal columns. The four-polymer ultra-surface unit cell is a square cell, and the four-polymer ultra-surface unit cell contains four positive even polygonal columns of the same size arranged in a square array.

[0008] Preferably, the even polygonal prism is perpendicular to the silicon dioxide substrate.

[0009] Preferably, the four even polygonal prisms are first placed on the substrate in the same orientation, and then rotated by an angle θ toward the geometric center of the tetramer cell, with the straight line located at the geometric center of each even polygonal prism perpendicular to the substrate as the axis.

[0010] Preferably, the thickness of the silicon dioxide substrate (1) is t, where t is greater than 2000 nm.

[0011] This invention also proposes a method for fabricating a tetramer refractive index sensor based on continuous domain bound states, characterized by comprising:

[0012] Amorphous silicon thin films were grown on silicon dioxide substrates using SiH4 gas as the Si source via low-pressure chemical vapor deposition.

[0013] A layer of photoresist was spin-coated onto a polycrystalline silicon thin film using a spin coater, and after heating and curing, it was exposed to electron beam lithography to fabricate a photoresist array of tetramer metasurfaces on an amorphous silicon thin film.

[0014] The Si film was vertically etched using selective dry etching with SF6 and C4F8 gases to form tetramer metasurface structural units. The photoresist residue was then removed by dry oxygen etching to complete the fabrication of the tetramer refractive index sensor.

[0015] Compared with the prior art, the significant advantages of this invention are:

[0016] 1. By exciting q-BICs through band folding, compared with the conventional method of breaking the mirror symmetry of the structure by controlling the geometry or displacement parameters of the structure to achieve q-BIC, the tetramer metasurface does not need to destroy the symmetry of the structure, adopts a simple even polygonal prism, and has no specific requirements for displacement parameters, thus reducing the requirements for processing technology.

[0017] 2. By employing the unique rotation mechanism proposed in this invention, the metasurface cell undergoes a process of transformation from a monomer cell to a tetramer cell and then back to a monomer cell as the rotation angle increases. This corresponds to the process of BICs transforming into q-BICs and then back to BICs. This design makes the Q-value of the q-BICs mode insensitive to the rotation angle at a specific angle, thereby relaxing the requirements for processing technology.

[0018] 3. Employing the unique rotation mechanism proposed in this invention, when the number of side lengths of the even polygonal prism is 4*n, where n is a positive integer, the tetramer cell can still possess polarization insensitivity even if it does not have 90-degree rotational symmetry. This reduces the complexity of the system and expands its application scenarios.

[0019] 4. This invention can achieve significant near-field enhancement, which is beneficial for improving sensing sensitivity.

[0020] Other features and advantages of the invention will be set forth in the following description, and will be apparent in part from the description, or may be learned by practicing the invention. The objects and other advantages of the invention may be realized and obtained by means of the structures particularly pointed out in the written description, claims, and drawings. Attached Figure Description

[0021] The accompanying drawings are for illustrative purposes only and are not intended to limit the invention. Throughout the drawings, the same reference numerals denote the same parts.

[0022] Figure 1 This is a schematic diagram of the metasurface cell structure of the tetramer refractive index sensor based on the continuous domain bound state in this invention.

[0023] Figure 2 This is a top view of the xy plane of the polymer metasurface unit of the present invention. The dashed lines in the figure are only used as reference lines for angles, and the circular arrows in the figure only indicate the direction of rotation and have no practical significance.

[0024] Figure 3 This is a schematic diagram of the resonance peak of the present invention in air.

[0025] Figure 4 This is a schematic diagram illustrating why the present invention possesses polarization insensitivity. The array within the solid frame represents the array before and after rotation, while the dashed frame is obtained by shifting the solid frame by Px / 2.

[0026] Figure 5 This is a schematic diagram of the unit cell transformation mechanism of the present invention, and the energy band diagram corresponding to each unit cell. The arrows and text in the energy band diagram are for indication only and have no practical meaning.

[0027] Figure 6 This is a schematic diagram illustrating that the Q value of the q-BICs mode of the present invention is insensitive to the rotation angle.

[0028] Figure 7 This is a schematic diagram of the sensor sensitivity fitting of the present invention, where the slope of the straight line represents the sensor sensitivity.

[0029] Figure 8 This is a schematic diagram of the unit cell transformation when the even polygonal nanopillars in this invention are regular hexagonal and regular octagonal nanopillars.

[0030] Figure reference numerals: 1. Silicon dioxide substrate; 2. Square pillar at the upper left corner of the tetramer cell; 3. Square pillar at the upper right corner of the tetramer cell; 4. Square pillar at the lower left corner of the tetramer cell; 5. Square pillar at the lower right corner of the tetramer cell; 6. Height of the square pillar; 7. Thickness of the silicon dioxide substrate; 8. Side length of the base of the square pillar; 9. Period of the tetramer cell in the x-direction; 10. Period of the tetramer cell in the y-direction; 11. Rotation angle of the square pillar; 2. Detailed Implementation

[0031] It is readily understood that, based on the technical solution of this invention, various embodiments of the invention can be conceived by those skilled in the art without altering the essential spirit of the invention. Therefore, the following detailed embodiments and accompanying drawings are merely illustrative examples of the technical solution of this invention and should not be considered as the entirety of the invention or as limitations or restrictions on the technical solution of this invention. Rather, these embodiments are provided to enable those skilled in the art to gain a more thorough understanding of the invention. Preferred embodiments of the invention are described below in conjunction with the accompanying drawings, which form part of this application and, together with the embodiments of the invention, serve to illustrate the innovative concept of the invention.

[0032] It should be noted that if the embodiments of the present invention involve directional indicators such as up, down, left, right, front, back, etc., the directional indicators are only used to explain the relative positional relationship and movement of the components in a specific posture as shown in the attached figure. If the specific posture changes, the directional indicators will also change accordingly.

[0033] In this embodiment, as Figure 1 As shown, a tetramer refractive index sensor based on continuous domain bound states is disclosed. The tetramer refractive index sensor includes a silicon dioxide substrate and a unit cell of a tetramer metasurface composed of silicon square prisms. The unit cell has a square outline and contains four square prisms of the same size arranged in a square array. The four square prisms are first placed on the substrate in the same orientation, and then rotated by the same angle about a straight line perpendicular to the substrate and located at the center of each square prism as an axis toward the geometric center of the tetramer unit cell.

[0034] The tetramer metasurface sensor is mirror-symmetrical in the x and y directions and consists of a two-layer structure in the z direction: a bottom layer of silicon dioxide substrate with a thickness greater than 2000 nm, and an upper layer of tetramer metasurface. The height of the square prisms constituting the tetramer is 320 nm. Figure 2 As shown, the square prism has a base side length of 435 nm, and the metasurface cell has a period of 1380 nm in both the x and y directions. Furthermore, without changing the structure of the polymeric metasurface sensor, a proportionally scaled structure can achieve the same function as this invention, only detecting drift in the resonant wavelength.

[0035] like Figure 3 As shown, under the special rotation mechanism proposed in this invention, although the metasurface unit cell does not possess 90-degree rotational symmetry, the array within the dashed frame obtained by translating the solid frame by Px / 2 after rotating the metasurface as a whole by 90 degrees is completely equivalent to the array within the solid frame before rotation. Considering that the number of unit cells contained in the array in reality is enormous, this invention can be considered as the lattice approximately satisfying 90-degree rotational symmetry. Therefore, this invention has the characteristic of being polarization insensitive.

[0036] The simulation of this invention was performed using the "Electromagnetic Waves, Frequency Domain" physics field in COMSOL software. Periodic boundary conditions were used in the x and y directions, and PML boundary conditions were used in the z direction. Electromagnetic waves were input and output through ports. The refractive index of silicon dioxide was set to 1.45, and the refractive index of silicon was set to 3.45. The mesh was controlled by the physics field. Complex frequencies were calculated using the "Characteristic Frequency" solver, Q = ω. r / (2ω i ), where ω r ω is the real part of the complex frequency. i The imaginary part of the complex frequency is represented by the resonant wavelength, which is obtained by converting the real part of the complex frequency. The band structure is obtained by scanning the wave vector using the "characteristic frequency" and "parameter scan," removing the silicon dioxide substrate and filtering out low-Q modes during band calculation. The transmission spectrum is calculated using the "wavelength domain," with the light source band ranging from 1530 nm to 1570 nm and a step size of 0.2 nm.

[0037] like Figure 4 As shown, with increasing rotation angle, the unit cell undergoes a transformation from a monomeric unit cell to a tetrameric unit cell, and then back to a monomeric unit cell. This transformation leads to band folding in wave vector space. When θ is 0°, the unit cell is a monomeric unit cell, and a degenerate bound state mode (corresponding to BICs) exists at point M in the band diagram. When θ is greater than 0° and less than 45°, the degenerate bound state at point M in the monomeric unit cell band diagram folds to point Γ in the tetrameric unit cell band diagram, becoming a continuous state that can couple with free-space radiation, i.e., q-BICs. When θ is 45°, the unit cell transforms back into a monomeric unit cell, and the q-BICs mode is folded back into the bound state, becoming a bound state with infinite Q (i.e., BICs). As θ continues to increase, the reverse process occurs, meaning the Q value changes periodically with θ.

[0038] like Figure 5 As shown, this conversion mechanism between BICs and q-BICs causes the Q value of the q-BICs mode to vary periodically with θ. When θ is 24°, the Q value is insensitive to θ, a characteristic that allows for more relaxed requirements on device fabrication techniques. However, when θ approaches 0° or 45°, the Q value approaches infinity.

[0039] like Figure 6 As shown, the resonance spectrum of this invention in air is a typical Fano line shape with a narrow linewidth. The resonance wavelength is located at 1544 nm.

[0040] like Figure 7 As shown, when the ambient refractive index of the tetramer refractive index sensor changes from 1.2 to 1.4, a significant redshift occurs in the resonant wavelength. The known formula for calculating the sensor's sensitivity is S = Δλ / Δn, where Δλ is the change in resonant wavelength and Δn is the change in refractive index. Simulation data yields a sensor sensitivity of 552.4 nm / RIU.

[0041] like Figure 8 As shown, when the regular even polygonal prism is a regular hexagonal prism or a regular octagonal prism, it is similar to... Figure 4 The phenomenon of unit cell transformation still exists, which means that in wave vector space, similar to Figure 4 Mid-band folding also occurs, thereby exciting q-BICs whose Q-value is insensitive to θ. Therefore, the cylinder in this invention is not limited to square nanopillars, but can be any regular even polygonal cylinder. Furthermore, when the number of side lengths of the regular even polygonal cylinder is 4*n, where n is a positive integer, even if the tetramer cell does not possess 90-degree rotational symmetry, it can still exhibit polarization insensitivity. The principle of this phenomenon has been explained in [the original text]. Figure 3 The explanation is as follows.

[0042] Furthermore, when the number of side lengths of the positive even polygonal prism is 4*n, where n is a positive integer, the tetramer cell can still possess polarization insensitivity even if it does not have 90-degree rotational symmetry.

[0043] The purpose of this invention is to provide a tetramer refractive index sensor based on continuous domain bound states and its fabrication method. This sensor possesses characteristics such as ease of fabrication, Q-value robustness, high sensitivity, and polarization insensitivity. It has broad application prospects in practical fields such as biochemical detection, food processing, and medical monitoring.

[0044] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any changes or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in the present invention should be included within the scope of protection of the present invention.

[0045] It should be understood that, in order to simplify the present invention and help those skilled in the art understand its various aspects, in the above description of exemplary embodiments of the present invention, various features of the present invention are sometimes described in a single embodiment or with reference to a single figure. However, the present invention should not be construed as including all features in the exemplary embodiments as essential technical features of the claims of this patent.

[0046] It should be understood that the modules, units, components, etc., included in the device of one embodiment of the present invention can be adaptively changed to be placed in a device different from that embodiment. Different modules, units, or components included in the device of the embodiment can be combined into a single module, unit, or component, or they can be divided into multiple sub-modules, sub-units, or sub-components.

Claims

1. A tetramer refractive index sensor based on continuous domain bound states, characterized in that, It includes a silicon dioxide substrate (1) and a tetramer metasurface cell composed of silicon positive even polygonal pillars. The substrate (1) is a square substrate, and the tetramer metasurface cell contains four positive even polygonal pillars of the same size arranged in a square array.

2. The tetramer refractive index sensor based on continuous domain bound states according to claim 1, characterized in that, The even polygonal prism is perpendicular to the silicon dioxide substrate (1).

3. The tetramer refractive index sensor based on continuous domain bound states according to claim 2, characterized in that, The four even polygonal prisms are first placed on the substrate in the same orientation, and then rotated by a certain angle θ towards the geometric center of the tetramer cell, with the straight line located at the geometric center of each even polygonal prism perpendicular to the substrate as the axis.

4. The tetramer refractive index sensor based on continuous domain bound states according to claim 3, characterized in that, The even polygonal prism (2) is rotated clockwise by an angle θ, the even polygonal prism (3) is rotated counterclockwise by an angle θ, the even polygonal prism (4) is rotated counterclockwise by an angle θ, and the even polygonal prism (5) is rotated clockwise by an angle θ.

5. The tetramer refractive index sensor based on continuous domain bound states according to claim 4, characterized in that, The thickness of the silicon dioxide substrate (1) is t, where t is greater than 2000 nm.

6. The tetramer refractive index sensor based on continuous domain bound states according to claim 1, characterized in that, The silicon even polygonal prism is a cube.

7. The tetramer refractive index sensor based on continuous domain bound states according to claim 1, characterized in that, The number of sides of the regular even polygonal prism is 4*n, where n is a positive integer.

8. A method for fabricating a tetramer refractive index sensor based on a continuous domain bound state as described in any one of claims 1 to 7, characterized in that, include: Amorphous silicon thin films were grown on silicon dioxide substrates using SiH4 gas as the Si source via low-pressure chemical vapor deposition. A layer of photoresist was spin-coated onto a polycrystalline silicon thin film using a spin coater, and after heating and curing, it was exposed to electron beam lithography to fabricate a photoresist array of tetramer metasurfaces on an amorphous silicon thin film. The Si film was vertically etched using selective dry etching with SF6 and C4F8 gases to form tetramer metasurface structural units. The photoresist residue was then removed by dry oxygen etching to complete the fabrication of the tetramer refractive index sensor.

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