Dual-core PCF-SPR ultrawide-range high refractive index sensor based on multi-field coupling synergistic effect

Abstract

This invention discloses a dual-core PCF-SPR ultrawide-range high refractive index sensor based on multi-field coupling synergistic effects. The invention uses a multi-layer air-hole array arranged in a triangular lattice as the cladding, with the air holes having a uniform diameter. Partial air holes are removed at symmetrical positions within the cladding to form a dual-core symmetrical structure. Symmetrical polishing is performed on both sides along the x-axis to construct an open external analyte channel. A large through-hole is introduced at the center of the optical fiber as a built-in sensing channel. Self-driven fluid communication between the built-in sensing channel and the external analyte channel is achieved through a vertical microchannel. A metal film is deposited on the side polished surfaces and the inner wall of the central large hole as a plasma excitation layer, forming a dual-channel SPP excitation interface. Through the dual-channel SPR excitation mechanism and dual-core modal coupling enhancement, the sensor increases the upper limit of refractive index detection to 1.66 and extends the detection range to 1.40–1.66.

Description

Technical Field

[0001] This invention belongs to the field of fiber optic sensing technology, specifically relating to a dual-core PCF-SPR ultrawide-range high refractive index sensor based on multi-field coupling synergistic effect. It is suitable for real-time detection of high refractive index, high sensitivity, and wide dynamic range refractive index sensing in fields such as biomedical detection, environmental monitoring, and organic solvent analysis. Background Technology

[0002] Surface plasmon resonance (SPR) technology, based on the collective oscillation characteristics of free electrons at the metal-dielectric interface, can produce subwavelength-level responses to changes in the refractive index (RI) of the interface, and has become a core technology for achieving ultrasensitive detection in the field of biochemical sensing.

[0003] Traditional SPR sensors typically employ prism coupling or planar waveguide structures (such as the Kretschmann configuration), but they suffer from inherent drawbacks such as large size, complex optical path calibration, and low evanescent field coupling efficiency, making it difficult to meet the requirements for miniaturization and high-throughput detection.

[0004] In recent years, photonic crystal fiber (PCF) has become a revolutionary carrier for SPR sensing due to its unique advantages: First, PCF can flexibly control the optical field mode and evanescent field distribution through structural parameters such as air hole arrangement and core size; second, its all-fiber integration characteristics can greatly simplify the complexity of sensing systems; and third, selective metallization (such as gold or silver nanocoatings) can precisely locate the SPR excitation region. Based on these characteristics, researchers have deposited metal thin films on the inner wall of the PCF air holes or the surface of the outer cladding, successfully developing a variety of PCF-SPR sensors and achieving high-sensitivity detection of low refractive index analytes (RI < 1.45).

[0005] However, with the expansion of biochemical detection scenarios, the demand for sensing high-refractive-index analytes (RI>1.45) has increased dramatically. Existing SPR-PCF sensors face significant technical bottlenecks in such applications: First, the intrinsic refractive index (n≈1.45) of silicon-based PCF materials forms an optical barrier. When the analyte RI approaches or exceeds 1.45, the evanescent field penetration depth decreases sharply, leading to a sharp decline in light-analyte coupling efficiency. Second, existing SPR excitation mechanisms suffer from problems such as a single excitation path and limited mode matching. Specifically: 1) Single-core, single-channel designs (relying solely on internal or external SPR) limit the upper limit of RI detection due to the fundamental mode cutoff effect; 2) While dual-core structures extend the detection range through external or internal channel SPR, a single excitation path still cannot overcome the sharp drop in sensitivity in the high-RI region; 3) Non-cooperative excitation in dual channels leads to overlapping resonance peaks, causing crosstalk in the detection signal.

[0006] The root cause of these problems lies in the fact that traditional SPR-PCF designs have not yet established a collaborative optimization model for multi-path SPR excitation mechanisms, and have not systematically addressed the optical impedance mismatch between high-RI analytes and silicon-based materials. Therefore, it is urgent to innovate sensor structures and excitation mechanisms to achieve a synergistic improvement in both high sensitivity and ease of operation, thereby increasing the detection limit and expanding the detection range. Summary of the Invention

[0007] To address the problems of existing PCF-SPR sensors, such as low detection limit, narrow detection range, low sensitivity, complex manufacturing process, and poor real-time performance in high refractive index applications, this invention proposes a dual-core PCF-SPR ultrawide-range high refractive index sensor based on multi-field coupling synergistic effect. This sensor breaks through the silicon dioxide refractive index barrier, achieving an ultrawide-range detection of 1.40–1.66 and accurate linear response, while significantly improving sensitivity and ease of operation.

[0008] To achieve the above objectives, the present invention provides the following technical solution:

[0009] A dual-core PCF-SPR ultrawide-range high refractive index sensor based on multi-field coupling synergistic effect uses a multi-layer air hole array arranged in a triangular lattice as a cladding. The air holes have a uniform diameter, and some air holes are removed at symmetrical positions of the cladding to form a dual-core symmetrical structure.

[0010] Symmetrical polishing is performed on both sides along the x-axis to construct an open external analyte channel; a through-type large air hole is introduced in the center of the optical fiber as an internal sensing channel; self-driven fluid communication between the internal sensing channel and the external analyte channel is achieved through a vertical microchannel; a metal film is deposited on the side polished surface and the inner wall of the central large hole as a plasma excitation layer to form a dual-channel SPP excitation interface.

[0011] The sensor enhances the detection range by using a dual-channel SPR excitation mechanism and dual-core modal coupling, raising the upper limit of refractive index detection to 1.66 and extending the detection range to 1.40–1.66.

[0012] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0013] 1. Ultra-high and ultra-wide refractive index detection range breaks through material limitations.

[0014] Traditional PCF-SPR sensors are limited by the refractive index of the silica substrate (approximately 1.45), and can only detect materials whose analyte refractive index is lower than that of the substrate. This invention, through a dual-channel SPR excitation mechanism (the synergistic effect of the side-polished outer surface and the central gold-plated air hole) and a dual-core modal coupling enhancement design, raises the upper limit of refractive index detection to 1.66 and extends the detection range to 1.40–1.66, covering low-refractive-index biomolecules (RI 1.40) and high-refractive-index organic compounds (RI > 1.55).

[0015] 2. Synergistic effect of multiphysics coupling improves sensitivity and resolution

[0016] This study is the first to utilize a multi-physics coupling synergistic effect, including phase matching between the SPR and PCF guided modes, mode coupling between the local SPP excited by the gold-plated central pore and the extended SPP in the outer polished area, and the modulation effect of the dual-core structure on mode coupling. These techniques significantly enhance the interaction strength between the evanescent field and the analyte, improving sensor performance. The sensor achieves a maximum wavelength sensitivity of 6150 nm / RIU and a maximum resolution of 1.63 × 10⁻⁶ Hz in the 1.40–1.66 nm range. -5 RIU, polynomial fitting coefficients R 2 =0.99978, exhibiting excellent linear response and prediction accuracy.

[0017] 3. Open-type vertical microchannels achieve a balance between efficient permeation and structural robustness.

[0018] The vertically open microchannel constructed by focused ion beam etching technology enables the simultaneous and rapid wetting of analytes into the central air hole and the side parabolic surface while ensuring the mechanical strength of the optical fiber. This solves the problems of low permeation efficiency and structural fragility caused by complex microfluidic channels in traditional closed PCF sensors, and provides a hardware foundation for real-time online detection.

[0019] 4. Manufacturing process compatibility and application scalability

[0020] Based on standard stacking and pulling processes and post-processing technologies (wheel polishing, CVD gold plating), mass production is possible. The sensor exhibits stable performance over a wide range of 1.40–1.66, making it suitable for high-refractive-index media commonly used in biochemical reactions (such as glycerol solutions and protein complexes), providing a robust solution for scenarios such as cancer biomarker detection and industrial solvent concentration monitoring. Attached Figure Description

[0021] Figure 1 This is a schematic diagram of the cross-sectional structure of the PCF-SPR sensor of the present invention.

[0022] Figure 2This is a graph showing the relationship between the loss of the PCF-SPR sensor of the present invention and the refractive index (1.40–1.66) of the solution to be tested.

[0023] Figure 3 This is a graph showing the relationship between the resonant wavelength of the PCF-SPR sensor of the present invention and the refractive index (1.40–1.66) of the solution to be tested. Detailed Implementation

[0024] The technical solutions in the embodiments of the present invention will be described in detail below with reference to the accompanying drawings. It should be understood that the described embodiments are only some embodiments of the present invention, and not all embodiments. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are within the scope of protection of the present invention.

[0025] This application provides a dual-core PCF-SPR ultrawide-range high refractive index sensor based on multi-field coupling synergistic effects. The sensor uses a three-layer array of air holes arranged in a triangular lattice as the cladding, with all air holes maintaining a uniform diameter. By precisely removing two air holes at the y-axis symmetrical position of the second layer, a dual-core symmetrical structure is formed.

[0026] Symmetrical polishing was performed on both sides of the PCF along the x-axis to construct an open external analyte channel; at the same time, a through-hole large air hole was introduced in the center of the optical fiber as an internal sensing channel, and a vertical microchannel was fabricated using micro-nano fabrication technology to achieve self-driven fluid communication between the inner and outer channels.

[0027] A gold film is deposited on the side surface and the inner wall of the central large hole as a plasma excitation layer. The entire sensor is based on a high-purity quartz glass substrate, realizing the synergistic effect of dual-channel surface plasma resonance and dual-core coupling to improve detection performance.

[0028] Furthermore, the PCF features a large air hole at its center forming an inner channel, while two symmetrical outer surfaces are polished to form two outer channels. This innovative design of the inner and outer analyte channels achieves performance breakthroughs through the following mechanisms: First, a three-dimensional coupling system is constructed through the central channel and the double-sided gold-plated polished surfaces—the analyte simultaneously contacts the radial gold layer of the central hole and the axial gold layer of the polished surface, forming a sandwich-like SPR excitation structure. Combined with the strong evanescent field hybridization effect generated by the dual solid core guiding modes through the channels, multi-dimensional sensitivity enhancement is achieved. Second, based on the composite resonant cavity formed by the open boundary of the central channel, the constraints of traditional photonic crystal fiber media are broken, allowing the surface plasmon resonance wavelength to continuously redshift with the refractive index of the analyte, thus broadening the refractive index detection range. An excessively large central air hole radius R will reduce manufacturing process tolerance, while an excessively small radius will weaken the central local field SPR effect and reduce sensor sensitivity. The central large air hole radius R is 1.25–1.35 μm.

[0029] Furthermore, an excessively large polishing depth L can lead to a deterioration in the dual-core coupling efficiency, resulting in asymmetric mode field leakage. It can also degrade the optical performance of the PCF, causing a surge in leakage loss and multimode interference. Conversely, an insufficient polishing depth L can lead to a decrease in the evanescent field intensity on the gold film surface and failure of the dual-channel synergy. It can also reduce the manufacturing tolerance of the PCF-SPR sensor. A polishing depth L of 5.9–6.0 μm is recommended.

[0030] Preferably, after comprehensively considering the manufacturing tolerances of the process and the overall performance evaluation of the sensor, the structural parameters R of the PCF in this embodiment are finally set to 1.30 μm and L to 6.00 μm.

[0031] Furthermore, the PCF features a vertically opening microchannel on the side of the central air vent. This channel allows for spontaneous analyte permeation via capillary action, avoiding the need for repeated refilling required in traditional closed channels and enabling real-time sensing. The channel thickness design must balance hydrodynamic resistance with optical mode constraints. s =0.20μm ensures rapid wetting while avoiding excessive transmission loss.

[0032] Furthermore, gold, with its excellent chemical stability in aqueous environments and superior resonance peak shift characteristics, has become the preferred material, and its thickness directly affects sensor performance. In the PCF-SPR sensor structure design of this embodiment, an excessively large gold layer increases the difficulty of electric field penetration, weakening the peak value of the limiting loss while enhancing damping loss and reducing sensitivity. Conversely, an excessively thin gold layer causes the full width at half maximum (FWHM) of the SPR resonance peak to gradually widen, which is detrimental to improving sensor resolution. Therefore, optimizing the gold layer thickness is necessary to achieve coordinated control of sensitivity and resolution. The gold film thickness m is 25–40 nm.

[0033] Preferably, considering the tolerance of the gold film deposition process and the overall performance indicators, the thickness m of the PCF gold film in this embodiment is finally selected as 30nm.

[0034] Furthermore, the cladding has three layers of air pores arranged in a triangular lattice, with a spacing Λ of 1.98–2.02 μm between the air pores. All air pores are of the same size, with a diameter D of 1.18–1.22 μm. Preferably, the diameter D of the air pores in the cladding is 1.20 μm, and the spacing Λ between the air pores is 2.00 μm.

[0035] Figure 1 The image shows a cross-sectional view of a dual-core PCF-SPR ultrawide-range high refractive index sensor based on multi-field coupling synergistic effect, including: a cladding air hole array 1, with air holes arranged in a triangular lattice, three layers in total, air hole diameter D = 1.20 μm, and spacing Λ between air holes = 2.00 μm.

[0036] Two air holes were removed at symmetrical positions along the y-axis of the second layer to form a dual-core symmetrical structure. Symmetrical polishing was performed on both sides of the PCF along the x-axis to construct an open external analyte channel, with a polishing depth L = 6.00 μm. Simultaneously, a large through-hole was introduced at the center of the fiber as a built-in sensing channel, with a radius R of 1.30 μm. A vertical microchannel was fabricated using micro / nano fabrication techniques to achieve self-driven fluid communication between the inner and outer channels. The width t of the vertical microchannel is... s =0.20μm.

[0037] A gold film is deposited on the side surface and the inner wall of the central large hole as a plasma excitation layer 3, with a gold film thickness of m = 30 nm. The entire sensor is based on a high-purity quartz glass substrate 2.

[0038] The above describes the structure and parameters of this optical fiber. The working principle of this optical fiber will be explained below:

[0039] When the PCF-SPR sensor is immersed in analyte solution 4, the analyte simultaneously wets the outer side of the polished cladding surface and the interior of the central channel. When light waves pass through the dual-core fiber, the polished symmetrical gold-plated cladding surface and the gold layer on the inner wall of the central air core channel form a dual SPP excitation interface. The analyte induces phase-matched coupling between the fiber core guided mode and the plasma wave on the gold film surface, resulting in a large amount of optical energy being transferred to the metal surface through the evanescent field, forming a resonance peak loss spectrum. The resonance peak loss is wavelength-dependent.

[0040] When the refractive index of the analyte changes, the SPP propagation constant and the dual-core energy coupling efficiency also change, which breaks the original phase matching condition and causes a resonant wavelength shift (redshift / blueshift). By observing the change in the resonant wavelength of this PCF-SPR sensor, the refractive index value of the analyte can be detected, thus achieving the purpose of sensing.

[0041] Verification example:

[0042] The sensing model of this embodiment is established using the finite element method. By simulating the sensing conditions described above using a computer, the effective refractive index of the PCF's fundamental mode can be calculated. The transmission loss of the optical fiber can then be determined from the imaginary part of the effective refractive index.

[0043]

[0044] In the above formula, λ represents the incident light wavelength, in μm; Im(n eff ) represents the imaginary part of the effective refractive index.

[0045] When the refractive index of the analyte changes from 1.40 to 1.66 in steps of 0.02, loss spectra for different analyte refractive indices will be obtained, such as... Figure 2As shown, each curve has a peak, and the wavelength corresponding to the peak is the resonance wavelength. The resonance wavelength will shift depending on the refractive index of the analyte.

[0046] The calculated resonance wavelengths for refractive indices of 1.40, 1.42, 1.44, 1.46, 1.48, 1.50, 1.52, 1.54, 1.56, 1.58, 1.60, 1.62, 1.64, and 1.66 were 1414 nm, 1438 nm, 1480 nm, 1545 nm, 1635 nm, 1744 nm, 1865 nm, 1988 nm, 2110 nm, 2228 nm, 2341 nm, 2459 nm, 2567 nm, and 2660 nm, respectively. Based on these data, the relationship curve between the resonance wavelength and the refractive index of the analyte solution was obtained, as shown below. Figure 3 As shown. The fitting formula after polynomial fitting is:

[0047] λ=283609.68-554020.03na+359232.64na 2 -76773.10na 3

[0048] Here, λ represents the resonance wavelength in nm, na represents the refractive index of the analyte solution in RIU, and R0 is the polynomial fitting coefficient. 2 = 0.99978. The sensitivity of the PCF-SPR sensor in this example is calculated using the following formula:

[0049]

[0050] Here, Δλ represents the change in resonant wavelength, and Δna represents the change in the refractive index of the analyte. Therefore, the maximum sensitivity of the PCF-SPR sensor can be calculated to be 6150 nm / RIU. The resolution of the PCF-SPR sensor in this example can be obtained using the following formula:

[0051]

[0052] Here, Δλ min The minimum wavelength resolution representing the spectrum is set to 0.1 nm. Therefore, the maximum resolution of the PCF-SPR sensor can be calculated to be 1.63 × 10⁻⁶. -5 RIU.

[0053] In summary, this sensor employs a dual-core symmetrical side-projection structure in conjunction with a central air hole vertical microchannel design. Gold plating on the inner and outer surfaces enables bidirectional SPR excitation and bidirectional dual-core coupling modulation. This overcomes the detection bottleneck of traditional sensors limited by the refractive index of the silicon dioxide substrate, extending the analyte refractive index (RI) detection range to 1.40-1.66, achieving a maximum sensitivity of 6150 nm / RIU, and a resolution better than 1.63 × 10⁻⁶. -5 RIU. Through dual-core polarization mode coupling optimization and polynomial fitting algorithm, the resonant wavelength of the sensor exhibits a high-precision correlation with the RI variation over an ultrawide refractive index range (1.40-1.66). 2 >0.99978), and supports real-time dynamic detection. It solves the sensing problem of high refractive index biochemical samples (such as concentrated DNA solutions, high-concentration biomarkers, organic solvents, etc.), and features controllable manufacturing process, ultra-wide detection range, high sensitivity and excellent response linearity.

[0054] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A dual-core PCF-SPR ultrawide-range high refractive index sensor based on multi-field coupling synergistic effect, characterized in that: A multilayer air hole array arranged in a triangular lattice is used as the cladding, wherein the air holes have a uniform diameter, and some air holes are removed at symmetrical positions of the cladding to form a double-core symmetrical structure. Symmetrical polishing is performed on both sides along the x-axis to construct an open external analyte channel; a through-hole large air hole is introduced in the center of the optical fiber as an internal sensing channel; self-driven fluid communication between the internal sensing channel and the external analyte channel is achieved through a microchannel perpendicular to the x-axis, with one end of the microchannel connected to the internal sensing channel and the other end connected to the external analyte channel; a metal film is deposited on the side polished surface and the inner wall of the central large hole as a plasma excitation layer to form a dual-channel SPP excitation interface; The large air hole has a radius of 1.30 μm, a polishing depth of 6.00 μm, a microchannel width of 0.20 μm, and a gold film thickness of 30 nm. The air holes are arranged in a triangular lattice pattern, with a total of three layers. The air hole diameter is 1.20 μm, and the spacing between the air holes is 2.00 μm. Two air holes are precisely removed at the y-axis symmetrical position in the second layer to form the dual-core symmetrical structure. The sensor enhances the detection range by using a dual-channel SPR excitation mechanism and dual-core modal coupling, raising the upper limit of refractive index detection to 1.66 and extending the detection range to 1.40–1.66.

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