High-sensitivity reconfigurable sensor and tuning method

Abstract

The invention discloses a high-sensitivity reconfigurable sensor and a tuning method, the sensor is of a laminated structure, the uppermost layer is a sensor body, the middle layer is a dielectric substrate, and the lowermost layer is of a microstrip line structure; the sensor body is formed by combining an artificial local surface plasmon resonance structure and a circular coupling structure, and a tiny gap is formed between the artificial local surface plasmon resonance structure and the circular coupling structure. According to the sensor, the local field enhancement characteristic of the artificial surface plasmon is utilized, a sharp Fano resonance line shape is excited through multi-mode interference, and the sensing sensitivity is greatly improved; meanwhile, the PIN diode is introduced, dynamic reconstruction of a surface plasmon mode is achieved by changing the bias voltage of the PIN diode, and the sensor has the advantages of being high in sensitivity, high in resonant frequency and adjustable in sensing frequency.

Description

Technical Field

[0001] This invention relates to the field of sensor design, and more particularly to a high-sensitivity metal-insulator-metal (MIM) sensor that modulates Fano resonance using the on / off characteristics of a PIN diode. Background Technology

[0002] Surface plasmons (SPs) are classified into conducted surface plasmons (SPPs) and localized surface plasmons (LSPs). Conducted surface plasmons are collective oscillations formed by the coupling of free electron charge density waves and electromagnetic waves at the interface between a metal and an electrolyte, and they can propagate along the interface. However, when the size of nanoparticles on the metal surface is much smaller than the wavelength of light, the charge density waves are confined to the metal nanoparticles due to the limitations of the particle boundary conditions, forming localized surface plasmons. Unlike SPPs, LSPs cannot propagate along the interface but oscillate as standing waves within a very small region of the metal surface. LSPs can strongly localize electromagnetic waves near nanoparticles, exhibiting the characteristic of localized electric field enhancement, and have important applications in the field of sensing.

[0003] The conditions for generating surface plasmons only apply to the optical frequency band. For low-frequency bands such as microwaves or terahertz, natural surface plasmon modes do not exist. In 2004, Pendry et al. proposed the concept of artificial surface plasmons (SSPPs) and designed a periodic square-hole metallic structure, extending the optical-frequency range of surface plasmons to the microwave frequency band. Artificial localized surface plasmons are localized electromagnetic resonant structures, extremely sensitive to their surroundings, capable of confining electromagnetic fields within a subwavelength range, resulting in a significant field enhancement effect. They have important applications in surface-enhanced Raman scattering, biomedicine, and high-sensitivity biochemical detection.

[0004] The concept of Fano resonance originated in the field of quantum mechanics. In 1935, Italian-American physicist Ugo Fano discovered the asymmetry of the absorption curve of helium while studying its properties. In 1961, Fano, along with several other scholars, explained the asymmetric line shape using quantum mechanics and named this asymmetric line shape the Fano resonance. The Fano resonance is formed by the destructive interference between continuous states (superradiative resonance modes) and discrete states (narrow-band subradiative resonance modes). Due to its unique asymmetric resonant line shape and high sensitivity, it can be used to design high-performance optical, biochemical, and other sensors.

[0005] A PIN diode is a special type of semiconductor diode. Its structure consists of P-type, N-type, and intrinsic semiconductor layers. Under forward bias, charge carriers from the P-type and N-type regions are injected into the intrinsic region, reducing resistance and enabling conduction. Under reverse bias, the high resistance of the intrinsic layer prevents current flow, resulting in a cutoff state. The switching characteristics of PIN diodes have important applications in the reconfigurability of surface plasmon polariton (SSPP) devices, enabling dynamic reconfiguration of surface plasmon modes by changing the bias voltage.

[0006] In the current research and application of artificial surface plasmon sensors, there are still technical bottlenecks that urgently need to be addressed. On the one hand, limited by sensing principles and structural design, traditional artificial surface plasmon sensors cannot simultaneously meet the dual requirements of high sensitivity and wide detection range. These sensors typically achieve high sensitivity to minute changes in the analyte based on specific resonant modes or structures, but this also leads to their ineffective response when detecting larger ranges of parameter changes due to the inherent limitations of resonant characteristics. On the other hand, existing sensors mostly adopt fixed structural designs, making it difficult to flexibly adjust their operating characteristics according to different detection scenarios and task requirements, greatly limiting their application scope and adaptability. To achieve sensor reconfigurability, systematic innovation is urgently needed in material selection, structural design, and control mechanisms to develop novel artificial surface plasmon sensor structures that can dynamically adjust sensing characteristics under different environments and detection tasks. Summary of the Invention

[0007] This invention addresses the shortcomings of existing sensors in terms of sensitivity and frequency tuning flexibility by proposing a highly sensitive reconfigurable sensor and tuning method. This method can adjust the sensing frequency while maintaining high sensing sensitivity and resonance intensity, thus meeting the sensing requirements of complex scenarios.

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

[0009] A highly sensitive reconfigurable sensor employs a three-layer stacked design, consisting of, from top to bottom: a sensor body 1, a dielectric substrate 4, and a microstrip line structure 5. The sensor body 1 is composed of an artificial localized surface plasmon resonance structure 2 and a circular coupling structure 3. The microstrip line structure 5 is a symmetrical structure, composed of a cuboid and a cylinder, with a length of wl, a width of ws, and a radius of wr.

[0010] The artificial localized surface plasmon resonance structure 2 is a periodically grooved metal ring structure, with three parts etched out: a metal ring 22, an outer branch 21, and an inner branch 23. The number of periods is 20. There are two gaps 2 (24) with a width g2 between 0.05 mm and 0.27 mm in the middle of the ring. The circular coupling structure 3 is a periodic coupling branch 31 combined with a circular hole metal plate 32. The number of periods is 20. There is a gap 1 (11) between the artificial localized surface plasmon resonance structure 2 and the circular coupling structure 3. The width g of the gap 1 (11) is between 0.01 mm and 0.5 mm.

[0011] Furthermore, the outer radius of the metal ring 22 is r, and the width is d2; all the branch structures are fan-shaped, the length of the outer branch 21 is h1, the length of the inner branch 23 is h2, the length of the coupling branch 31 is h3, and the angle of all the branch structures is θ.

[0012] The sensor body 1 and the microstrip line structure 5 are made of a single material or a composite material of copper, tin, gold, silver, aluminum, magnesium or titanium, and the thickness t of the material is between 0.018 mm and 0.1 mm; the dielectric substrate 4 is made of F4BM high-frequency board with a relative permittivity of 2.45 and a loss tangent of 0.0012; the thickness d of the dielectric substrate is between 0.1 mm and 2 mm.

[0013] The gaps 2 (24) are reserved positions for diodes and capacitors, respectively; the diodes are SMP1322 plastic-encapsulated PIN diodes; the capacitance of the capacitors is between 2.0pF and 3.0pF.

[0014] According to specific embodiments provided by the present invention, the present invention discloses the following technical effects:

[0015] This invention utilizes the interference between the discrete state generated by the SLSP resonant structure and the continuous state of the circular coupling structure to excite a sharp Fano resonance peak at a single frequency point, significantly improving the quality factor (Q value) and resonance intensity, thereby achieving high-sensitivity sensing of the dielectric environment.

[0016] Furthermore, this invention integrates a PIN diode in slot 2 (24) and achieves dynamic tuning of the sensing frequency by adjusting the resonant mode of the SLSP structure through external voltage. This design combines the functions of large-range coarse measurement and small-range fine measurement, enabling a single sensor to adapt to complex application scenarios.

[0017] Furthermore, the isolation capacitor installed in slit 2 (24) can block DC voltage while transmitting AC signals. By adjusting the capacitor value, not only can the intensity of the Fano resonance be flexibly controlled to enhance the sensor's detection capability, but dynamic tuning can also be achieved to adjust the sensor's operating frequency.

[0018] The working principle of this invention is as follows:

[0019] The Fano resonance generated by the sensor can be analyzed using multimode interference coupled-mode theory (MICMT). For a single-input, single-output sensor, its spectral response can be considered as the sum of field effects generated by the superposition of independent fields of degenerate modes coupled from different resonance modes. The corresponding MICMT equations are as follows:

[0020]

[0021]

[0022]

[0023]

[0024] Where n represents different resonant modes, A n and ω n These are the normalized field amplitude and resonant frequency, respectively. n τ represents the decay time of the internal loss of the nth resonant mode in the sensor. ni and τ no S represents the coupling decay time between the nth order resonant mode and the input and output ports, respectively. n,i+ and S n,o+ S represents the normalized amplitude of the nth resonant mode at the input port and the output port, respectively. i± and S o± These represent the field amplitudes at the input and output ports, respectively. A positive sign indicates energy inflow, and a negative sign indicates energy outflow. κ ni and κ no θ represents the coupling coefficients between the nth resonant mode and the input and output ports, respectively, with the asterisk representing the corresponding complex conjugate term. ni and θ no The coupling phases φ between the nth resonant mode and the input and output ports are described respectively. n γ is the phase difference between the input and output ports in the nth resonant mode. ni and γ no These represent the normalization coefficients of the input and output ports, respectively. and The phases propagating between the input / output ports and the coupling position in the nth resonant mode are described respectively.

[0025] Because the sensor structure is symmetrical and no energy is input to the output port, we have: τ ni =τ no denoted as τ nc θni =θ no And let This represents the total phase difference of the nth resonant mode.

[0026] Furthermore, for most resonant modes, there is γ ni =γ no ≈1. From this, the complex amplitude transmission coefficient t of the entire structure can be derived, with the expression:

[0027]

[0028] Where t0 represents the resonance effect outside the frequency band of interest, and is a relatively small constant. The transmission coefficient on a logarithmic scale can be calculated from the complex amplitude transmission coefficient, as expressed by:

[0029]

[0030] In Example 1, the theory of multimode interference coupling will be explained with a specific case. Attached Figure Description

[0031] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the services required in the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0032] Figure 1 This is a schematic diagram of the reconfigurable sensor disclosed in this invention.

[0033] Figure 2 This is a schematic diagram of the structure of the sensor body in the reconfigurable sensor disclosed in this invention.

[0034] Figure 3 This is a schematic diagram of the geometric parameters of the sensor body in the reconfigurable sensor disclosed in this invention.

[0035] Figure 4 This is a schematic diagram of the cross-sectional geometric parameters of the reconfigurable sensor disclosed in this invention.

[0036] Figure 5 This is a schematic diagram of the geometric parameters of the microstrip line structure in the reconfigurable sensor disclosed in this invention.

[0037] Figure 6 The image shows the transmission coefficient curve of the reconfigurable sensor disclosed in this invention under PIN diode switching conditions, as well as the transmission coefficient results fitted using MICMT.

[0038] Figure 7This is a graph showing the transmission coefficient of alcohol at different concentrations when the PIN diode is turned on.

[0039] Figure 8 This is a graph showing the transmission coefficient of alcohol at different concentrations when the PIN diode is turned off.

[0040] Figure 9 This is a graph showing the transmission coefficient of different high-frequency boards when the PIN diode is turned on.

[0041] Figure 10 This is a graph showing the transmission coefficient of different high-frequency boards when the PIN diode is turned off. Detailed Implementation

[0042] To make the objectives, technical solutions, and advantages of this invention clearer, specific embodiments of this invention will be described below in conjunction with the accompanying drawings.

[0043] This invention discloses a highly sensitive reconfigurable sensor, such as... Figure 1 As shown, the sensor has a three-layer stacked structure. Figure 1 (a) is a top view. Figure 1 (b) is a side view. The top layer is the sensor body 1, the middle layer is the dielectric substrate 4, and the bottom layer is the microstrip line structure 5;

[0044] like Figure 2 As shown, the sensor body 1 is composed of an artificial local surface plasmon resonance structure 2 and a circular coupling structure 3. A gap 1 (11) is provided between the artificial local surface plasmon resonance structure 2 and the circular coupling structure 3. The gap 1 is part of the sensing area. The width g of the gap 1 is between 0.01 mm and 0.5 mm, which can be flexibly adjusted according to the actual processing accuracy.

[0045] The artificial localized surface plasmon resonance structure 2 is a periodically grooved metal ring structure, etched with three parts: a metal ring 22, an outer branch 21, and an inner branch 23, with a period number of 20. For example... Figure 3 As shown, the outer radius of the metal ring 22 is r, and the width is d2. Two gaps 2 (24) with a width g2 between 0.05mm and 0.27mm are opened in the middle of the ring. The width can be flexibly adjusted according to the actual processing accuracy. The gaps 2 are reserved positions for diodes and capacitors, respectively. The diodes are plastic-encapsulated PIN diodes of model SMP1322; the capacitance is between 2.0pF and 3.0pF. The circular coupling structure 3 is a periodically grooved metal hollow structure, which consists of a circular hole metal plate 32 and coupling branches 31, with a period number of 20. All branch structures are fan-shaped. The length of the outer branch 21 is h1, the length of the inner branch 23 is h2, the length of the coupling branch 31 is h3, and the angle of all branch structures is θ.

[0046] like Figure 5 As shown, the lower microstrip structure 5 is a symmetrical structure, composed of a combination of cuboid and cylindrical structures. The length of the microstrip line is wl, the width is ws, and the radius is wr.

[0047] The dielectric substrate 4 is fabricated using an F4BM high-frequency board with a relative permittivity of 2.45 and a loss tangent of 0.0012. The sensor body 1 and the microstrip line structure 5 are made of a single material or a composite material of copper, tin, gold, silver, aluminum, magnesium, or titanium. Figure 4 As shown, the thickness d of the dielectric substrate is between 0.1 mm and 2 mm, and the thickness t of the sensor body 1 and the microstrip structure 5 is between 0.018 mm and 0.1 mm. The thickness can be adjusted according to the actual size requirements.

[0048] Example 1:

[0049] The structure of the reconfigurable sensor in this embodiment is as follows: Figure 1-5 As shown, the geometric parameters are: l = 50 mm, g = 0.1 mm, r = 5 mm, d2 = 3 mm, g2 = 0.2 mm, h1 = 2 mm, h2 = 1 mm, h3 = 0.217 mm, θ = 10.2°, wl = 19 mm, ws = 2 mm, wr = 5 mm. The sensor is excited by a 50 Ω microstrip line. The dielectric substrate 4 is a 0.5 mm thick F4BM high-frequency board with a relative permittivity of 2.45 and a loss tangent of 0.0012. The sensor body 1 and the microstrip line structure 5 are made of 0.018 mm thick copper foil.

[0050] To obtain the transmission coefficient of the sensor, a simulation was performed using the finite element method (FEM), and the results are as follows: Figure 6 As shown, the solid line represents the simulation results when the PIN diode is on, and the dashed line represents the simulation results when the PIN diode is off. It can be seen that the resonant frequency is 1.263 GHz when on, and 2.386 GHz when off, achieving frequency adjustment via external voltage. Simultaneously, the resonant strength is slightly higher when on (23 dB) and slightly lower when off (18 dB). However, the quality factor Q is larger when off; here, it is defined as... Where f0 is the resonant frequency, and Δf 3dB This represents the 3dB bandwidth of the resonant peak value. Calculations show that the Q-value is 10.8 when on and 18.6 when off.

[0051] For the specific structural parameters of Embodiment 1, the multimode interference coupled-mode theory was applied, and the parameters were obtained through multivariate nonlinear fitting. Three resonant modes were selected under both on and off conditions: the Fano resonance peak, the Fano resonance valley, and the Lorentz resonance. The parameters are shown in Table 1. The fitting results are as follows: Figure 6The circular and triangular markers in the diagram show that the transmission coefficient fitted by MICMT agrees well with the simulation results, thus confirming the feasibility of our theory.

[0052] Table 1: Parameters under on / off conditions of multimode interference coupled mode theory.

[0053] Example 2

[0054] To verify the sensor's sensing performance in solutions, simulations were conducted using alcohol solutions of different concentrations. The simulation results for diode conduction are as follows: Figure 7 As shown, the simulation results for diode turn-off are as follows: Figure 8 As shown. Sensitivity is defined here. Where Δf is the varying resonant frequency and Δn is the varying refractive index. Calculations show that the maximum sensitivity under conduction mode is 12 MHz / RIU, and the sensitivity remains almost constant with increasing concentration; the maximum sensitivity under deactivation mode is 49 MHz / RIU, and the sensitivity decreases slightly with increasing concentration.

[0055] It should be noted that the volume of the solution has a relatively small impact on the transmission coefficient when the circuit is on, but a significant impact when it is off. This can also be seen from the degree to which the addition of the solution disrupts the linearity. Therefore, under the on-condition, the sensor has a higher resonance intensity and Q value, but lower sensitivity, making it suitable for sensing large-scale and large-volume solutions. Under the off-condition, the sensitivity is higher, making it suitable for small-volume solutions and precise measurements.

[0056] Example 3

[0057] To verify the sensor's sensing performance on high-frequency boards, simulations were conducted using high-frequency dielectric boards of different materials. The simulation results for diode conduction are as follows: Figure 9 As shown, the simulation results for diode turn-off are as follows: Figure 10 As shown, the high-frequency board has relatively low dielectric loss, resulting in minimal disruption to the transmission coefficient linearity. With increasing material dielectric, its resonant peak exhibits a significant redshift, allowing sensing to occur in two different frequency bands during conduction and turn-off, while maintaining high sensitivity. Calculations show that the highest sensitivity under conduction conditions is 276 MHz / RIU, and the highest sensitivity under turn-off conditions is 2854 MHz / RIU.

[0058] Table 2: Electromagnetic characteristic parameters of the high-frequency board used in the simulation.

Claims

1. A highly sensitive reconfigurable sensor, characterized in that, The sensor has a three-layer stacked structure, with the uppermost layer being the sensor body (1), the middle layer being the dielectric substrate (4), and the lowermost layer being the microstrip line structure (5). The sensor body (1) is composed of an artificial local surface plasmon resonance structure (2) and a circular coupling structure (3). A gap 1 (11) is provided between the artificial local surface plasmon resonance structure (2) and the circular coupling structure (3), and the gap 1 is part of the sensing area.

2. The high-sensitivity reconfigurable sensor according to claim 1, characterized in that, The width g of the slit 1 (11) is between 0.01 mm and 0.5 mm; the artificial local surface plasmon resonance structure (2) is a periodically grooved metal ring structure, which is etched with three parts: a metal ring (22), an outer branch (21) and an inner branch (23), with a period number of 20. There are two slits 2 (24) with a width g2 between 0.05 mm and 0.27 mm in the middle of the ring; the circular coupling structure (3) is formed by combining a periodic coupling branch (31) structure with a circular hole metal plate (32), with a period number of 20.

3. The high-sensitivity reconfigurable sensor according to claim 2, characterized in that, The outer radius of the metal ring (22) is r, and the width is d2; all the branch structures are fan-shaped, the length of the outer branch (21) is h1, the length of the inner branch (23) is h2, the length of the coupling branch (31) is h3, and the angle of all the branch structures is θ.

4. The high-sensitivity reconfigurable sensor according to claim 1, characterized in that, The microstrip structure (5) is a symmetrical structure, which is composed of a combination of cuboid and cylindrical structures. The length of the microstrip line is wl, the width is ws, and the radius is wr.

5. The high-sensitivity reconfigurable sensor according to claim 1, characterized in that, The dielectric substrate (4) is prepared using an F4BM high-frequency board with a relative permittivity of 2.45 and a loss tangent of 0.0012; the thickness d of the dielectric substrate is between 0.1 mm and 2 mm.

6. The high-sensitivity reconfigurable sensor according to claim 1, characterized in that, The sensor body (1) and the microstrip line structure (5) are made of a single material or a composite material of copper, tin, gold, silver, aluminum, magnesium or titanium, and the thickness t of the material is between 0.018 mm and 0.1 mm.

7. The high-sensitivity reconfigurable sensor according to claim 2, characterized in that, The artificial local surface plasmon resonance structure (2) also includes a PIN diode and a capacitor; the PIN diode and the capacitor are disposed in the gap 2 (24), the diode is a plastic-encapsulated PIN diode of model SMP1322; the capacitance value of the capacitor is between 2.0pF and 3.0pF.

8. The tuning method for the high-sensitivity reconfigurable sensor according to claims 1-7, characterized in that, By adjusting the external voltage of the PIN diode, the electromagnetic coupling state of the artificial local surface plasmon resonance structure (2) is changed, and the operating frequency band and sensing frequency of the sensor are finally adjusted.

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