Intermediate infrared absorption enhancing structure based on Fano resonance and application and preparation method thereof
Through the multi-layer film structure and Fano resonance effect designed by the full dielectric material, the enhancement problem of traditional mid-infrared absorption materials is solved, and efficient mid-infrared absorption enhancement and biomolecular detection are achieved, reducing the detection limit and adapting to the detection needs of different molecules.
Patent Information
- Application Number
- CN202510904341.X
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- Filing Date
- 2025-07-01
- Publication Date
- 2025-08-19
AI Technical Summary
Traditional mid-infrared absorption materials are difficult to achieve efficient spectral absorption enhancement, and the existing Fano resonance structure preparation process is complex and cannot meet the molecular absorption needs of different bands.
A multi-layer film structure designed with a full dielectric material, including prism, MgF2, MgO, and SiO2 layers, was prepared by sol-gel combined with spin coating method, using Fano resonance effect to enhance molecular absorption in the mid-infrared band, and precisely aligned molecular characteristic peaks are adjusted by incident angle.
The ultra-high resonance characteristic with a quality factor Q value of 1900 was achieved. The local electric field intensity at the target molecule interface was enhanced by nearly 9 times, and the detection limit was reduced to 5ng/mL, which was suitable for biomolecular detection and complex scene analysis.
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Figure CN120507822A_ABST
Abstract
Description
Technical Field
[0001] The present application relates to the field of optical sensing technology, and in particular to a mid-infrared absorption enhancement structure based on Fano resonance, and its application and preparation method. Background Art
[0002] Mid-infrared band (wavelength range is usually 2-25μm, corresponding to the wave number range of 500-4000cm -1 The mid-infrared region is a characteristic region of many molecular vibrational and rotational energy level transitions and is known as the "molecular fingerprint region." Achieving high-performance signal absorption enhancement in the mid-infrared band is of great significance for the detection and identification of biomolecules.
[0003] Currently, traditional mid-infrared absorption materials (such as silicon and germanium) are limited by their inherent optical properties, making it difficult to achieve efficient enhanced spectral absorption. Optical micro-nanostructures offer a new approach to addressing this challenge. By designing the shape, material, and arrangement of structures at the micrometer or nanometer scale, they can effectively concentrate light energy and enhance absorption in the mid-infrared band. They can also precisely target specific wavelengths, improving detection accuracy. Fano resonance, a special optical phenomenon resulting from the interference between discrete and continuous states, has a unique asymmetric line shape and high quality factor.
[0004] Through searching existing technical literature, it was found that Professor Tan Jishu and others from Huazhong University of Science and Technology disclosed a long-wave infrared absorber structure based on local field enhancement. This technology designed a graphene absorption enhancement structure based on Fano resonance, which can improve the absorption rate of graphene and is expected to be used to achieve long-wave infrared detection enhancement. First, the simulation model of graphene was introduced, including the band structure, surface conductivity, and dielectric constant. Then, the numerical simulation results of the structure were presented, confirming that the Fano resonance introduced by the micron hole array structure of the intrinsic silicon material can confine the light field inside the structure, so that the absorption rate of graphene is increased to 36%. Further search found that the document number is: Optics Express, 2019, 27(5):6320, Algorri JF, Zografopoulos DC et al. proposed an ultrahigh-quality factor resonant dielectric metasurfaces based on hollow nanocuboids. This technology utilizes an all-dielectric metasurface composed of hollow dielectric nanocubes with an ultra-high quality factor. Varying the size of the square holes influences the dipole resonance mode in the nanoparticles, resulting in a Fano resonance in the transmission spectrum with a very high quality factor. However, due to the complex fabrication process and stringent nanoscale requirements, this structure cannot enhance the absorption of mid-infrared molecules across different wavelengths, reducing the device's practicality. Summary of the Invention
[0005] In order to circumvent the loss problem of traditional metal-dielectric structures, this application proposes a mid-infrared absorption enhancement structure based on Fano resonance and its application and preparation method for enhancing molecular absorption in the mid-infrared band. By designing the structure with all-dielectric materials and utilizing a simple thin film preparation process, efficient absorption enhancement of biological molecules in the mid-infrared band is achieved.
[0006] The technical solution adopted in this application is: a mid-infrared absorption enhancement structure based on Fano resonance, including a prism, a substrate, a first MgF2 layer, a MgO layer, a second MgF2 layer and a SiO2 layer arranged in sequence from top to bottom, generating a Fano resonance effect between the first MgF2-MgO-second MgF2-SiO2 with an alternating refractive index distribution.
[0007] Furthermore, the thickness of the first MgF2 layer is smaller than the incident wavelength.
[0008] Furthermore, the thicknesses of the first MgF2 layer, the second MgF2 layer, and the SiO2 layer are in the micrometer order, and the thickness of the MgO layer is in the nanometer order.
[0009] Furthermore, the prism is fixed to the substrate via UV curing adhesive, and the prism and the substrate are made of the same material.
[0010] Furthermore, when the incident angle of the incident light is in the range of 65°-66.2°, the mid-infrared enhancement band is 2800-4000 nm.
[0011] Furthermore, a SiO2 layer is placed on the object to be tested.
[0012] A mid-infrared absorption enhancement structure based on Fano resonance is used to enhance molecular absorption in the mid-infrared band.
[0013] Furthermore, by changing the incident angle of the incident light, the position of the Fano resonance peak is precisely aligned with the characteristic absorption peak of the target molecule on the analyte.
[0014] A method for preparing a mid-infrared absorption enhancement structure based on Fano resonance comprises the following steps: S1: Preparation of multilayer thin film structure: Preparation of MgF2 optical thin film, using a two-step spin coating method to form a first MgF2 layer on the substrate, annealing to prepare MgO thin film, high-speed spin coating to form an MgO layer on the first MgF2 layer, annealing, and then repeating the spin coating process to prepare a second layer of MgF2 thin film, and finally preparing SiO2 sol, also by spin coating to form a thin film, and annealing to achieve densification; S2: Prism coupling and multi-layer thin film structure integration: After spin coating the multi-layer thin film structure, use UV curing adhesive to align and bond the prism with the substrate where the multi-layer thin film structure is located, and use nitrogen to spread the UV curing adhesive over the entire prism surface; then align the coated surface of the substrate parallel to the prism surface and press to ensure that bubbles are expelled; after completing the bonding of the substrate and prism, use UV lamp to irradiate the bonding surface.
[0015] Furthermore, the substrate was ultrasonically cleaned three times using acetone, isopropyl alcohol, and deionized water before spin coating.
[0016] The beneficial effects of this application compared to the prior art are: 1. In terms of performance, by precisely controlling the thickness parameters of multilayer thin films (such as the combination of a 360nm MgO layer and a 3.6μm SiO2 layer), an ultra-high resonant quality factor (Q) of 1900 is achieved, representing a more than 10-fold improvement over traditional waveguide structures. Simultaneously, the local electric field intensity at the target molecule interface is enhanced by nearly 9-fold, lowering the detection limit of biomarkers such as alpha-fetoprotein (AFP) to the order of 5ng / mL. This all-dielectric design not only addresses the loss issues of surface-enhanced infrared absorption (SEIRA) technology but also enables wavelength adaptive matching within the mid-infrared band by fine-tuning the incident angle from 66.1° to 66.4°, overcoming the limitation of traditional nanostructures that require re-fabrication to detect different molecules.
[0017] 2. This application utilizes a sol-gel combined with spin-coating process, resulting in low unit cost and the ability to achieve uniform fabrication over large areas. The biocompatibility of the SiO2 surface layer enhances protein immobilization efficiency, and combined with a replaceable dielectric layer design, it offers flexible adaptation to complex scenarios such as body fluid analysis and biomolecule detection. BRIEF DESCRIPTION OF THE DRAWINGS
[0018] The present application will be further described below with reference to the accompanying drawings: Figure 1 A three-dimensional stereogram of the Fano resonance structure provided in an embodiment of the present application.
[0019] Figure 2 for Figure 1 Side view of the structure shown.
[0020] Figure 3 This is the process flow chart for preparing sol-gel films.
[0021] Figure 4 Design diagram of the experimental system for mid-infrared enhanced absorption.
[0022] Figure 5 This is the angle scan debug diagram of the Fano structure.
[0023] Figure 6 This is a comparison chart of the AFP protein absorption enhancement effects of the Fano resonance structure and the ordinary waveguide structure.
[0024] Figure 7 This is a diagram showing the wavelength range of the enhanced mid-infrared absorption of the enhanced structure of this application.
[0025] Figure 8 This is the electric field distribution diagram corresponding to different incident light angles after the thickness of each layer is optimized. In the figure, X represents the width of the film within a certain range, and Y represents the thickness of the film. DETAILED DESCRIPTION
[0026] like Figures 1 to 8 As shown, the present application provides a mid-infrared absorption enhancement structure based on Fano resonance. Based on the prism-multilayer optical medium film structure, combined with the Fano resonance theory and the demand for mid-infrared absorption enhancement, a high-performance mid-infrared absorption enhancement structure is designed and implemented.
[0027] The following describes in detail the various components of the Fano resonance-based mid-infrared absorption enhancement structure of this application: The prism is placed on top of the multilayer optical dielectric film. The prism can be a triangular prism or a semi-cylindrical prism. There are no specific material requirements, and glass (for visible light) or silicon (for infrared light) can be selected. To ensure the uniformity of the low-refractive-index dielectric layer, the polished surface is preferably no longer than one inch (2.54 cm) on a side.
[0028] The multilayer optical medium film structure of the present application is respectively a substrate, a first MgF2 layer, a MgO layer, a second MgF2 layer, and a SiO2 layer from top to bottom. Figure 1 and 2 As shown, MgF2 is a material widely used in the field of optical thin films. It has excellent optical transparency in the mid-infrared region and is suitable as a waveguide layer in multilayer optical dielectric thin film structures. Its refractive index in the mid-infrared band is between 1.33 and 1.34. At a wavelength of around 3000nm, its refractive index is about 1.34. In multilayer optical dielectric thin film structures, it can be used as a low-refractive-index layer to achieve phase modulation and interference effects of light waves. MgF2 also has an extremely low extinction coefficient of about 1×10 -5 , which can effectively reduce the energy loss of light waves during propagation. In the structure designed in this application, the MgF2 film is used as the first layer of the multilayer structure (refractive index n 1) and the third layer (refractive index n 3), where the thickness of the first MgF2 layer is d 1 = 1.6 μm, thickness of the second MgF2 layer d 3 = 5 μm. MgF2 thin films are prepared using a sol-gel method combined with spin coating. MgF2 sol is synthesized by hydrofluoric acid-catalyzed hydrolysis and polymerization of magnesium acetate. The sol is then evenly spread onto a substrate to form a liquid film, which is then spun at high speed using a spin coater to form an optical thin film.
[0029] MgO is a high-refractive-index material with excellent optical properties. Its refractive index in the mid-infrared band (at a wavelength of about 3000nm) is about 1.668, and its extinction coefficient is 0.01. It has low absorption loss in the mid-infrared region and supports the excitation of localized modes as a high-refractive-index layer in a multilayer film structure. MgO also has good chemical stability and mechanical strength, and can maintain stable optical properties in complex environments. In the structure designed in this application, the MgO film is used as the second layer of the multilayer structure ( n 2) Its thickness d2 = 380nm. Its high refractive index creates a strong refractive index contrast with the low-refractive index material (MgF2), further enhancing the asymmetric line shape of the Fano resonance. The film is prepared using magnesium nitrate as a precursor, citric acid as a complexing agent, and ammonia water to adjust the pH value. Sol-gel preparation is then performed by spin coating.
[0030] SiO2 is also a material widely used in the field of optical thin films. Its refractive index is 1.395 in the mid-infrared band around 3000nm. It is used as a medium refractive index layer in multilayer thin film structures to achieve continuous state support. Its extinction coefficient is extremely low, about 1×10 -5 , which can effectively reduce the energy loss of light waves during propagation. The film is made by spin coating after preparing a sol using tetraethyl orthosilicate as the silicon source, ammonia as the catalyst, and ethanol as the solvent.
[0031] The Fano resonance effect occurs between several dielectric layers with staggered refractive index distribution (first MgF2-MgO-second MgF2-SiO2). When the low refractive index dielectric layer (MgF2) is very thin ( d 1 is less than the incident wavelength). Light passes through the interface where total internal reflection occurs and interferes within the multilayer thin-film structure. The narrowband waveguide mode (discrete state) supported by the high-refractive-index dielectric layer (MgO) couples and interferes with the broadband radiation mode (continuous state) formed at the second MgF2 / SiO2 interface. Under phase-matching conditions, a Fano resonance with a sharp asymmetric line shape is formed. This resonance highly localizes the incident light energy within the submicron-scale region of the SiO2 surface layer, producing a significant electric field enhancement effect. By precisely controlling the thickness and refractive index gradient of each dielectric layer, the system can achieve spectral signal enhancement at the characteristic absorption peak of the target molecule within a certain range of incident angles.
[0032] The core of this application is to enhance mid-infrared absorption by utilizing the quantum interference effect of Fano resonance. Fano resonance is generated by the coupling interference of the discrete state (localized waveguide mode supported by the MgO layer) and the continuous state (radiation mode formed by the MgF2 / SiO2 layer). Figure 7 As shown in the figure, when the incident light is irradiated at a specific angle (65°-66.2°), the two modes produce asymmetric linear resonance under phase matching conditions. Affected by the parameters of the enhanced structure of this application, the mid-infrared band enhancement range of this application is 2800nm-4000nm. By optimizing the thickness parameters of each layer, such as Figure 8 As shown in the figure, an average of 5.7×10 5 The electric field strength in V / m (enhancement factor |E / E0| 2 ≈10 3At the same time, the enhanced structure of the present application has the ability to dynamically tune the resonant wavelength: through 0.1° level angle fine-tuning, the precise displacement of the resonant wavelength can be achieved to adapt to different molecular fingerprint regions.
[0033] The following is based on the attached Figure 3-5 The preparation process of the multilayer thin film structure in this application and its experimental testing are further explained.
[0034] Example 1: Preparation of multilayer film structure Preparation process as Figure 3 As shown, a MgF2 optical thin film was first prepared: 1.96g of magnesium acetate was dissolved in 80mL of methanol, and 0.3mL of 40% hydrofluoric acid was added dropwise. The mixture was stirred for 2 hours and aged for 24 hours to prepare a precursor sol. A first MgF2 layer was formed on a substrate using a two-step spin coating method, followed by gradient annealing at 200°C to eliminate internal stress. The MgO layer was then prepared: a complex sol was prepared using magnesium nitrate and citric acid in a 1:2 molar ratio. The pH was adjusted to 7 and the reaction was carried out in an 80°C water bath for 2 hours. A thin film was formed by high-speed spin coating and annealed at 300°C to obtain the MgO layer. The spin coating process was then repeated to form a second MgF2 thin film. Finally, a SiO2 sol was prepared by alkaline hydrolysis of tetraethyl orthosilicate. Thin films were also formed by spin coating and densified by annealing at 450°C. The prepared films were characterized by SEM and AFM to determine film formation and elemental distribution.
[0035] Example 2: Prism coupling and multilayer thin film structure integration The glass substrate used for spin coating must be ultrasonically cleaned three times with acetone, isopropyl alcohol, and deionized water. After the multi-layer thin film structure is spin-coated, NOA61 UV curing adhesive is used to align and bond the K9 prism to the glass substrate where the multi-layer thin film structure is located, and nitrogen is used to cover the entire prism surface; then the coated surface of the glass substrate is aligned parallel to the prism surface and pressed to ensure that bubbles are expelled; after the substrate and prism are bonded, the bonding surface is irradiated with ultraviolet light for 1 minute.
[0036] Example 3: Building an experimental platform for angle scanning and debugging of the Fano structure Build as Figure 4The mid-infrared test platform shown uses a silicon nitride broadband light source (2-20μm) in conjunction with an Omni-λ monochromator (resolution 1nm) to scan and output incident light of different wavelengths. After the incident light interacts with the sample in the mid-infrared absorption enhancement structure based on Fano resonance, part of the light signal is absorbed, while the remaining light signal carries the spectral information of the sample. After the light signal is reflected by the mid-infrared absorption enhancement structure based on Fano resonance, it is received by the MCT detector and converted into an electrical signal. Finally, the signal processed by the phase-locked amplifier is transmitted to a computer (PC) to record and analyze the experimental data. Figure 5 As shown, the Fano resonance peak position was observed to shift from a blue wavelength of 3150 nm to a gray wavelength of 2810 nm with increasing incident angles within the range of 65.9°-66.2°, demonstrating that the spectral properties of the Fano resonance-based mid-infrared absorption enhancement structure in the experiment are strongly dependent on the incident angle. By varying the incident angle, the position of the Fano resonance peak can be precisely aligned with the characteristic absorption peak of the target molecule. When the resonance peak coincides with the absorption peak of the molecule, the strong localized electric field generated on the surface of the Fano structure significantly enhances the vibrational excitation efficiency of the molecule, thereby strengthening its absorption signal.
[0037] Example 4: Comparative analysis of the mid-infrared enhancement effect of Fano resonance-based mid-infrared absorption enhancement structure and conventional waveguide structure on AFP protein molecules In this embodiment, AFP protein is used as the target molecule, and the absorption enhancement effect of the mid-infrared absorption enhancement structure based on Fano resonance is used to improve its signal intensity in the infrared spectrum. At the same time, by designing a comparative experiment, the mid-infrared absorption enhancement effect of AFP protein is compared with that of an ordinary waveguide (single-layer thin film medium) structure, thereby verifying the superior performance of the Fano structure in biological molecule detection. After multiple experimental tests, the infrared absorption peak of AFP protein with a concentration of 5ng / mL is around 3000nm. After the AFP protein is modified on the mid-infrared absorption enhancement structure based on Fano resonance and the ordinary waveguide structure, the angular spectrum of the mid-infrared absorption enhancement structure based on Fano resonance and the ordinary waveguide structure is scanned to make the resonance peak position coincide with the absorption peak position of the target molecule, thereby enhancing infrared absorption. The final enhanced effect is as follows: Figure 6 As shown in the figure. Without any structure (Control), this represents the absorption of the AFP protein itself, which is weak and difficult to detect. (WG) represents the reflectivity curve for a conventional waveguide structure. Compared to the unstructured case, the waveguide structure exhibits a significant drop in reflectivity around 3000nm, indicating a greater enhancement of infrared absorption. (Fano) represents the reflectivity curve for a mid-infrared absorption-enhancing structure based on Fano resonance. The drop in reflectivity around 3000nm is much greater than that of a conventional waveguide structure, enhancing the characteristic absorption of AFP by 5-6 times, enabling trace detection of target molecules.
[0038] In summary, through theoretical simulation and experimental verification, we systematically studied the Fano resonance characteristics of the structure and its absorption enhancement performance in the mid-infrared band. This application utilizes the low-loss characteristics of all-dielectric materials and the sol-gel preparation process to realize an efficient, stable, and tunable molecular detection platform.
[0039] Finally, it should be noted that the above embodiments 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, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or make equivalent replacements for some or all of the technical features therein. These modifications or replacements do not deviate the essence of the corresponding technical solutions from the scope of the technical solutions of the embodiments of the present application.
Claims
1. A mid-infrared absorption enhancement structure based on Fano resonance, characterized in that: The invention comprises a prism, a substrate, a first MgF2 layer, a MgO layer, a second MgF2 layer and a SiO2 layer which are sequentially arranged from top to bottom, and generates a Fano resonance effect between the first MgF2-MgO-second MgF2-SiO2 with staggered refractive index distribution.
2. The mid-infrared absorption enhancement structure based on Fano resonance according to claim 1, characterized in that: The thickness of the first MgF2 layer is smaller than the incident wavelength.
3. The mid-infrared absorption enhancement structure based on Fano resonance according to claim 2, characterized in that: The thickness of the first MgF2 layer, the second MgF2 layer, and the SiO2 layer is in the micrometer order, and the thickness of the MgO layer is in the nanometer order.
4. The mid-infrared absorption enhancement structure based on Fano resonance according to claim 1, characterized in that: The prism is fixed to the substrate by UV curing adhesive, and the prism and the substrate are made of the same material.
5. The mid-infrared absorption enhancement structure based on Fano resonance according to any one of claims 1 to 4, characterized in that: When the incident angle of the incident light is within the range of 65°-66.2°, the mid-infrared enhancement band is 2800-4000nm.
6. The mid-infrared absorption enhancement structure based on Fano resonance according to claim 5, characterized in that: The SiO2 layer is placed on the object to be tested.
7. Use of a mid-infrared absorption enhancement structure based on Fano resonance according to any one of claims 1 to 6 to enhance molecular absorption in the mid-infrared band.
8. The use of a mid-infrared absorption enhancement structure based on Fano resonance in enhancing molecular absorption in the mid-infrared band according to claim 7, characterized in that: By changing the incident angle of the incident light, the position of the Fano resonance peak is precisely aligned with the characteristic absorption peak of the target molecule on the analyte.
9. The method for preparing a mid-infrared absorption enhancement structure based on Fano resonance according to any one of claims 1 to 6, characterized in that: The following steps are involved: S1: Preparation of multilayer thin film structure: Preparation of MgF2 optical thin film, using a two-step spin coating method to form a first MgF2 layer on the substrate, annealing to prepare MgO thin film, high-speed spin coating to form an MgO layer on the first MgF2 layer, annealing, and then repeating the spin coating process to prepare a second layer of MgF2 thin film, and finally preparing SiO2 sol, also by spin coating to form a thin film, and annealing to achieve densification; S2: Prism coupling and multi-layer thin film structure integration: After spin coating the multi-layer thin film structure, use UV curing adhesive to align and bond the prism with the substrate where the multi-layer thin film structure is located, and use nitrogen to spread the UV curing adhesive over the entire prism surface; then align the coated surface of the substrate parallel to the prism surface and press to ensure that bubbles are expelled; after completing the bonding of the substrate and prism, use UV lamp to irradiate the bonding surface.
10. The method for preparing a mid-infrared absorption enhancement structure based on Fano resonance according to claim 9, characterized in that: The substrate was ultrasonically cleaned three times with acetone, isopropanol, and deionized water before spin coating.