A helical taper metal probe and its method for liquid gradient refractive index measurement

Abstract

This invention discloses a spiral conical metal probe and its method for measuring the refractive index gradient in liquids. The spiral conical metal probe can be excited by white light to form a surface plasmon evanescent wave on the tip of the probe, thereby enabling refractive index detection of the environment. The spiral conical metal probe provided by this invention is simple to prepare and can be mass-produced. Furthermore, this probe can achieve high spatial resolution (micrometer-level) refractive index detection in liquids, exhibiting a highly sensitive detection time response during the convection process of water and ethanol solutions. This makes it suitable for measuring the refractive index gradient in liquids and detecting the refractive index distribution in liquids, showing broad application prospects in the field of liquid refractive index detection.

Description

Technical Field

[0001] This invention belongs to the field of surface plasmon optics, specifically relating to a spiral conical metal probe and its method for measuring the refractive index of a liquid gradient. Background Technology

[0002] With economic development and social progress, people are paying increasing attention to health, which places higher demands on the detection technology of toxic and harmful biochemical molecules, as well as bacteria and viruses. Refractive index sensing, as a label-free optical biosensor, can observe molecular interactions in real time and directly, and has enormous application prospects in life sciences, clinical diagnostics, medicine, and food safety testing. Among numerous refractive index sensors, surface plasmon resonance (SPR)-based refractive index sensors have the characteristics of high sensitivity and fast response speed. SPR is formed when incident light resonates with the vibration of free electron gas clusters on the surface of a metal structure, manifesting as a strong resonance absorption or scattering peak in the spectrum. Essentially, SPR sensing is due to the shift of the SPR resonance peak caused by changes in the refractive index of the medium surrounding the sensing region. In 1968, Kretschmann et al. proposed a prism structure that effectively excited SPR. This structure has good stability and high sensitivity, laying a solid foundation for the application of SPR technology in the field of sensing. Since Nylander et al. first used the Kretschmann prism for gas sensing in 1982, it opened the door to its application in the sensing field. To this day, this structure remains a common design for commercial SPR sensors. However, the prism structure also has some drawbacks: large size and inflexible operation, which limits its practical application in remote measurement and point-to-point detection. Fiber optic SPR sensors, which combine SPR sensing with optical fiber, offer a feasible solution to this problem. Compared to prism structures, fiber optic SPR sensors have advantages such as small size, compact structure, resistance to electromagnetic interference, and long-distance transmission of optical signals. They can be used in confined spaces for real-time detection and remote measurement of minute targets, and have become an important direction in SPR sensing research. For fiber optic SPR sensors, specific structures need to be fabricated in the middle or end face of the fiber so that the light transmitted within the fiber core illuminates the surface of the metal layer to excite SPR. Currently, commonly used structures for fiber optic SPR sensors include unclad fiber, heterogeneous fiber, D-type fiber, U-shaped fiber, tapered fiber, and fiber gratings. These structures have all been experimentally verified to effectively excite SPR, achieve refractive index sensing, and be used for the detection of minute targets. However, these structures are all on the order of millimeters or even centimeters, making it impossible to measure the refractive index in the micrometer range point by point, and their spatial resolution is generally limited.

[0003] In June 2022, Maria S. Soares et al. proposed a gold-plated D-type fiber optic sensor based on SPR for detecting cortisol. This structure was fabricated by polishing the sides of a single-mode fiber and depositing a 50 nm thick gold film. Theoretical calculations and experimental results showed that the sensor could achieve high sensitivity in the refractive index range of 1.33 to 1.39. However, its sensing area length was 8 mm, which was insufficient for measuring refractive indices in the micrometer range (Maria S. Soares, Luís C.B. Silva, Miguel Vidal, Médéric Loyez, Margarida). Christophe Caucheteur, Marcelo EV Segatto, Florinda M. Costa, Cátia Sónia O. Pereira, Nuno F. Santos, and Carlos AF Marques, "Label-free plasmonic immunosensor for cortisol detection in a D-shaped optical fiber," Biomed. Opt. Express 13, 3259-3274 (2022)). Summary of the Invention

[0004] This invention proposes a spiral conical metal probe and a method for measuring the refractive index gradient of liquids. The spiral conical metal probe provided by this invention is easy to use, its preparation method is compatible with existing technologies, and its operating wavelength range can be changed by altering the probe structure.

[0005] The purpose of this invention is to provide a spiral conical metal probe for fiber optic refractive index sensing systems, which can realize refractive index sensing in liquids and be applied to liquid gradient refractive index measurement.

[0006] To achieve the above objectives, the present invention adopts the following technical solution.

[0007] A spiral conical metal probe based on an optical fiber end face includes a photoresist spiral conical dielectric layer and a thin metal coating. The photoresist spiral conical dielectric layer includes a conical base structure and a semi-elliptical spiral grating structure on the surface of the cone.

[0008] The above-mentioned method for measuring the refractive index gradient of a liquid using a spiral conical metal probe includes the following steps:

[0009] (1) Strip the coating layer from the coupling end fiber of the 1×2 single-mode fiber coupler, cut the fiber end face flat with a fiber cleaver, and fix it on a fiber clamp with a single-axis displacement stage. Fix a glass slide at the front end of the clamp, adjust the displacement stage so that the fiber end face is close to the glass slide, drop photoresist on the edge of the glass slide and guide it to the fiber end face. The distance between the fiber end face and the glass slide is controlled above the height of the spiral cone structure to ensure sufficient space to prepare the structure, while making the fiber end face completely immersed in the photoresist.

[0010] (2) Fix the fiber optic clamp in the laser direct writing platform, and control the laser direct writing platform by writing a program through the computer system. First, print a conical structure on the end face of the fiber using the galvanometer mode, and then print a semi-elliptical spiral grating structure along the surface of the cone using the piezoelectric mode. After that, remove the fiber from the clamp, immerse it in the developer to dissolve the unreacted photoresist, and then move the fiber to the isopropanol solution to dissolve the developer. After taking it out, the isopropanol solution on the surface of the fiber can evaporate naturally at room temperature.

[0011] (3) Transfer the optical fiber to the magnetron sputtering coating machine, fix the optical fiber so that its end face is facing up, and perform coating on the structural surface of the optical fiber end face. The coating target is gold. Set the coating power and coating time. After the coating is completed, the spiral cone metal probe is obtained.

[0012] (4) Fix the spiral tapered metal probe, use a white laser as the light source, inject the light source into one branch port of the fiber coupler through an objective lens, the incident light is transmitted to the coupling end through the fiber coupler, and illuminates the spiral tapered metal probe on the end face of the fiber. The reflected light is transmitted in the opposite direction to the fiber coupler and enters another branch port for collection by the spectrometer. This completes the construction of the fiber optic sensing system and allows for the next measurement.

[0013] (5) Place the spiral cone metal probe in a convection pipe of two solutions with different refractive indices, collect and measure the refractive index at its location during the convection process, and obtain the curve of refractive index change over time by data processing. The time interval between steep changes in the curve is the response time.

[0014] (6) Fix the spiral cone metal probe on a displacement platform, control the probe to scan in the liquid with gradient refractive index distribution and collect data at the same time. The refractive index distribution in the liquid can be measured by data processing.

[0015] Furthermore, in step (2), the optical fiber is removed from the fixture and first immersed in the developer to dissolve the unreacted photoresist for a period of not less than 20 minutes to ensure that the unreacted photoresist is completely dissolved.

[0016] Furthermore, in step (2), the optical fiber is moved into an isopropanol solution to dissolve the developer, and the immersion time is not less than 3 minutes to ensure that the developer adhering to the probe surface is completely dissolved.

[0017] Furthermore, in step (3), the coating power is set to 50W-100W and the coating time is 50 seconds-115 seconds, so that the coating thickness reaches the required thickness, and the thickness is preferably controlled between 80nm-100nm.

[0018] This invention discloses a fiber optic SPR sensor with a spiral cone structure. Compared to a conical structure, the spiral cone structure effectively breaks the symmetry of the shape due to the spiral, giving it polarization-insensitive characteristics. Surface plasmons (SPPs) can be excited at the cone tip using linearly polarized light. Previous research has theoretically calculated and experimentally verified that polarization-insensitive gold spiral cone structures can effectively excite SPPs at the cone tip and be used for fluorescence enhancement. Therefore, this structure is feasible for SPR sensing. According to the SPR matching condition, the wavelength of light exciting the SPPs is highly sensitive to changes in the external refractive index. Using white laser irradiation, a surface plasmon evanescent wave is formed on the cone tip surface. By detecting the reflection spectrum, the absorption wavelength can be measured, thus achieving refractive index sensing. Since the size of this structure is on the micrometer scale, it can achieve refractive index detection in the micrometer range, possessing strong spatial resolution. It can be used to measure the refractive index distribution in non-uniform liquids and has broad application prospects in the field of fiber optic refractive index sensing.

[0019] The present invention can achieve the following beneficial effects:

[0020] (1) The spiral cone metal probe structure of the present invention can realize the detection of refractive index in the range of 1.33 to 1.40 by detecting the SPR absorption wavelength in the reflection spectrum. And it can realize the detection of refractive index in the micrometer range.

[0021] (2) The spiral cone metal probe structure of the present invention has a fast time response capability, with a response time of less than 100ms in the experiment of water and ethanol solution convection.

[0022] (3) The spiral cone metal probe structure of the present invention has excellent spatial resolution and can measure the refractive index distribution of liquids with gradient concentration distribution.

[0023] (4) The preparation method of the spiral cone metal probe provided by the present invention is simple and can be mass-produced. Attached Figure Description

[0024] The accompanying drawings, which are included to provide a further understanding of the invention and form part of this invention, illustrate exemplary embodiments of the invention and are used to explain the invention, but do not constitute an undue limitation of the invention. In the drawings:

[0025] Figure 1This is a schematic diagram illustrating the preparation method and structure of the spiral conical metal probe of the present invention.

[0026] Figure 2 The graph shows the change in refractive index of a point inside a pipe over time, measured by the spiral conical metal probe of the present invention in a convection experiment with 80% ethanol solution and water.

[0027] Figure 3 This is a schematic diagram of the scanning method of the spiral conical metal probe of the present invention for measuring the refractive index gradient of a liquid.

[0028] Figure 4 The image shows the gradient refractive index distribution obtained by scanning at the interface between water and 50% glycerol solution using the spiral conical metal probe of this invention.

[0029] Figure 5 The image shows the gradient refractive index distribution obtained by scanning at the interface between 80% ethanol solution and water using the spiral conical metal probe of this invention. Detailed Implementation

[0030] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions of this invention will be clearly and completely described below in conjunction with specific embodiments and corresponding drawings. Obviously, the described embodiments are only a part of the embodiments of this invention, and not all of them. Based on the embodiments of this invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this invention.

[0031] The technical solutions provided by the various embodiments of the present invention will be described in detail below with reference to the accompanying drawings.

[0032] Example 1

[0033] The preparation steps of a spiral cone metal probe described in this example are as follows:

[0034] (1) Strip the coating layer from the coupling end fiber of a 1×2 single-mode fiber coupler, cut the fiber end face flat with a fiber cleaver, and fix it on a fiber clamp with a single-axis displacement stage. Fix a glass slide at the front end of the clamp, adjust the displacement stage so that the fiber end face is close to the glass slide, drop photoresist on the edge of the glass slide and guide it to the fiber end face. The distance between the fiber end face and the glass slide should be controlled above the height of the spiral cone structure to ensure sufficient space for fabrication of the structure, while ensuring that the fiber end face is completely immersed in the photoresist.

[0035] (2) Fix the fiber optic clamp in the laser direct writing platform, and control the laser direct writing platform by writing a program through the computer system. First, print a conical structure on the end face of the fiber using the galvanometer mode, and then print a semi-elliptical spiral grating structure along the surface of the cone using the piezoelectric mode. After that, remove the fiber from the clamp, immerse it in the developer for 20 minutes to dissolve the unreacted photoresist, and then move the fiber to the isopropanol solution for 3 minutes to dissolve the developer. After taking it out, the isopropanol solution on the surface of the fiber can evaporate naturally at room temperature.

[0036] (3) Transfer the optical fiber to the magnetron sputtering coating machine, fix the optical fiber with its end face facing upward, and perform coating on the structural surface of the optical fiber end face. The coating target is gold, and the coating power is set to 50W and the coating time is 115s to achieve a coating thickness of 90nm. After the coating is completed, a spiral tapered metal probe based on the optical fiber end face is obtained.

[0037] Figure 1 a) in the diagram is a schematic diagram of the method for preparing the photoresist spiral cone dielectric layer in the above-mentioned spiral cone metal probe. Figure 1 b) in the figure is a schematic diagram of the coating. In this example, a 1×2 visible light single-mode fiber coupler was used to fabricate a spiral conical metal probe with the following structural parameters: the bottom diameter D of the conical structure is 10 μm, the height H is 13.7 μm, the thread period is 650 nm, and the width is 570 nm.

[0038] Example 2

[0039] The specific working method for measuring the time response of a spiral tapered metal probe described in this example is as follows:

[0040] The spiral-tipped metal probe prepared in Example 1 was fixed in place. Using a white laser as the light source, white light was injected into one port of the fiber coupler through an objective lens. The fiber end position was adjusted to achieve a coupling efficiency of 10%. The white laser light was transmitted through the fiber coupler to the coupling end, illuminating the spiral-tipped metal probe on the fiber end face. The reflected light was transmitted back to the fiber coupler and entered another port for collection by a spectrometer. The SPR wavelength was obtained by analyzing the reflection spectrum, thus measuring the refractive index. Next, the spiral-tipped metal probe was placed in a solution of water and 80% ethanol (n... 水 =1.333, n 80％乙醇 =1.356) In a convection pipe, the curve of the refractive index changing with time at its location is shown below. Figure 2 As shown. By Figure 2It can be seen that the refractive index change of the liquid at the location of the fiber optic probe is: 1.333→1.356→1.333→1.356, corresponding to the liquid change from water to 80% ethanol solution to water to 80% ethanol solution. The transition from water to 80% ethanol solution involves a sharp change, and the time interval of this sharp change is the response time of the fiber optic probe. Measurements show that the response time of the spiral tapered metal probe in the water-ethanol solution is ~100ms, ~40ms, and ~60ms, all within 100ms, with an average of 67ms, indicating its fast response characteristics.

[0041] Example 3

[0042] The specific working method of the spiral conical metal probe used in this example for measuring the refractive index of liquids is as follows:

[0043] The spiral tapered metal probe prepared in Example 1 was fixed on a displacement platform. Using a white laser as the light source, the white light was injected into one port of the fiber coupler through an objective lens. The position of the fiber end was adjusted to achieve a coupling efficiency of 10%. The white laser light was transmitted through the fiber coupler to the coupling end, illuminating the spiral tapered metal probe on the fiber end face. The reflected light was transmitted back to the fiber coupler and entered another port for collection by a spectrometer. The SPR wavelength was obtained by analyzing the reflection spectrum, which is the measured refractive index.

[0044] Based on the property of a refractive index gradient distribution formed by the compositional gradient due to interdiffusion between two liquids, this example uses water and a 50% glycerol solution to prepare the liquid sample with the desired gradient refractive index distribution. The cylindrical container holding the liquid has a diameter of 2 cm, with a diameter ratio of 160:1 (area ratio of 25600:1) to the fiber optic cladding and a diameter ratio of 2000:1 (area ratio of 4000000:1) to the spiral conical metal probe. Therefore, the disturbance to the liquid during the probe's downward scanning process can be ignored. A gradient refractive index liquid is formed using a combination of glycerol solution and water. Due to the higher density of the glycerol solution, 50% glycerol solution and water are slowly poured into the container sequentially. After standing for several minutes, a stable stratification phenomenon forms between the two liquids. Then, a displacement platform is used to control the spiral conical metal probe to... Figure 3 The scanning method shown is in water and 50% glycerol solution (n 水 =1.333, n 50％甘油 At the boundary layer (1.398), data was scanned from top to bottom, with data collected every 15.2 μm (the step distance of the electronically controlled displacement platform). After data processing and fitting, the results were obtained as follows: Figure 4 The refractive index spatial distribution curve is shown. (From...) Figure 4It can be seen that the refractive index distribution measured by the spiral conical metal probe ranges from n=1.333 for water to n=1.398 for 50% glycerol solution from top to bottom, which is consistent with the actual liquid distribution. Importantly, the probe scan also measured the refractive index distribution within the boundary layer.

[0045] Example 4

[0046] The specific working method of the spiral conical metal probe used in this example for measuring the refractive index of liquids is as follows:

[0047] The spiral tapered metal probe prepared in Example 1 was fixed on a displacement platform. Using a white laser as the light source, the white light was injected into one port of the fiber coupler through an objective lens. The position of the fiber end was adjusted to achieve a coupling efficiency of 10%. The white laser light was transmitted through the fiber coupler to the coupling end, illuminating the spiral tapered metal probe on the fiber end face. The reflected light was transmitted back to the fiber coupler and entered another port for collection by a spectrometer. The SPR wavelength was obtained by analyzing the reflection spectrum, which is the measured refractive index.

[0048] Based on the property that a refractive index gradient distribution is formed at the interface between two liquids due to the interdiffusion of compositional gradients, this example uses water and an 80% ethanol solution to prepare the liquid sample with the desired gradient refractive index distribution. In the experiment, the diameter of the cylindrical container holding the liquid was 2 cm, with a diameter ratio of 160:1 to the fiber optic cladding (area ratio of 25600:1) and a diameter ratio of 2000:1 to the spiral conical metal probe (area ratio of 4000000:1). Therefore, the disturbance to the liquid during the probe's top-down scanning process can be ignored. A liquid with a refractive index gradient distribution was formed using a combination of 80% ethanol solution and water. Due to the higher density of water, water and 80% ethanol solution were slowly poured into the container sequentially. After standing for several minutes, a stable stratification phenomenon formed between the two liquids. Then, a displacement platform was used to control the spiral conical metal probe... Figure 3 The scanning method shown is in 80% ethanol solution and water (n 80％乙醇 =1.356, n 水 At the boundary layer (=1.333), data was scanned from top to bottom, with data collected every 15.2 μm (the step distance of the electronically controlled displacement platform). After data processing and fitting, the results were obtained as follows: Figure 5 The refractive index spatial distribution curve is shown. (From...) Figure 5 It can be seen that the refractive index distribution measured by the spiral conical metal probe changes from n=1.356 for 80% ethanol solution to n=1.333 for water from top to bottom, which is consistent with the actual liquid distribution. Importantly, the probe scan also measured the refractive index distribution within the boundary layer.

[0049] In the above embodiments, by Figure 2It can be seen that the spiral cone metal probe of the present invention has a fast time response capability, with a time response of less than 100 ms in the experiment of water and ethanol solution convection. Figure 4 and Figure 5 It can be seen that the spiral cone metal probe of the present invention has excellent spatial resolution and can realize the detection of refractive index distribution in liquids with gradient concentration distribution.

Claims

1. A method for measuring the refractive index gradient of a liquid using a spiral conical metal probe, characterized in that, The spiral conical metal probe comprises a photoresist spiral conical dielectric layer and a thin metal coating. The photoresist spiral conical dielectric layer includes a conical base structure and a semi-elliptical spiral grating structure on the conical surface. The structural parameters of the spiral conical metal probe are: a base diameter D of 10 μm, a height H of 13.7 μm, a thread period of 650 nm, and a width of 570 nm. This spiral conical metal probe is excited by white light to form a surface plasmonic evanescent wave on the conical tip surface, thereby achieving refractive index detection of the environment. The method for measuring the refractive index gradient of a liquid includes the following steps: (1) Strip the coating layer from the coupling end fiber of the 1×2 single-mode fiber coupler, cut the fiber end face flat with a fiber cleaver, and fix it on a fiber clamp with a single-axis displacement stage. Fix a glass slide at the front end of the clamp, adjust the displacement stage so that the fiber end face is close to the glass slide, and drop photoresist on the edge of the glass slide to guide it to the fiber end face so that the fiber end face is completely immersed in the photoresist. (2) Fix the fiber optic clamp in the laser direct writing platform, and control the laser direct writing platform by writing a program through the computer system. First, print a conical structure on the end face of the fiber using the galvanometer mode, and then print a semi-elliptical spiral grating structure along the surface of the cone using the piezoelectric mode. After that, remove the fiber from the clamp, immerse it in the developer to dissolve the unreacted photoresist, and then move the fiber to the isopropanol solution to dissolve the developer. After taking it out, the isopropanol solution on the surface of the fiber can evaporate naturally at room temperature. (3) Transfer the optical fiber to the magnetron sputtering coating machine, fix the optical fiber so that its end face is facing up, perform coating on the structural surface of the optical fiber end face, set the coating power and coating time, and the spiral conical metal probe is obtained after the coating is completed. (4) Fix the spiral tapered metal probe, inject the light source into one branch port of the fiber coupler through an objective lens, the incident light is transmitted to the coupling end through the fiber coupler, and illuminates the spiral tapered metal probe on the end face of the fiber. The reflected light is transmitted in the opposite direction to the fiber coupler and enters another branch port for collection by the spectrometer. This completes the construction of the fiber optic sensing system and allows for the next measurement. (5) Place the spiral cone metal probe in a convection pipe of two solutions with different refractive indices and collect and measure the refractive index at its location during the convection process. (6) Fix the spiral cone metal probe on a displacement platform, control the probe to scan in the liquid with gradient refractive index distribution and collect data at the same time. The refractive index distribution in the liquid can be measured by data processing.

2. The method for measuring the refractive index gradient of a liquid using a spiral conical metal probe according to claim 1, characterized in that, In step (1), the distance between the fiber end face and the glass slide is controlled above the height of the spiral cone structure to ensure sufficient space for fabrication of the structure, while ensuring that the fiber end face is completely immersed in the photoresist.

3. The method for measuring the refractive index gradient of a liquid using a spiral conical metal probe according to claim 1, characterized in that, In step (2), the optical fiber is removed from the fixture and first immersed in the developer to dissolve the unreacted photoresist. The immersion time is not less than 20 minutes to ensure that the unreacted photoresist is completely dissolved.

4. The method for measuring the refractive index gradient of a liquid using a spiral conical metal probe according to claim 1, characterized in that, In step (2), the optical fiber is moved into the isopropanol solution to dissolve the developer, and the immersion time is not less than 3 minutes to ensure that the developer adhering to the probe surface is completely dissolved.

5. The method for measuring the refractive index gradient of a liquid using a spiral conical metal probe according to claim 1, characterized in that, In step (3), the coating power is set to 50W-100W.

6. The method for measuring the refractive index gradient of a liquid using a spiral conical metal probe according to claim 1, characterized in that, In step (3), the coating time is set to 50-115 seconds.

7. The method for measuring the refractive index gradient of a liquid using a spiral conical metal probe according to claim 1, characterized in that, In step (3), the coating target is gold.

8. The method for measuring the refractive index gradient of a liquid using a spiral conical metal probe according to claim 1, characterized in that, In step (4), a white laser is used as the light source.

9. The method for measuring the refractive index gradient of a liquid using a spiral conical metal probe according to any one of claims 1-8, characterized in that, In step (5), the collected refractive index is processed to obtain the curve of refractive index change over time, and the time interval of the curve change is the response time.

Images