A broadband tunable fiber-optic surface plasmon sensing probe

Abstract

The present invention belongs to the field of micro-nano photonics and optical fiber sensing technology, and proposes a broadband tunable optical fiber surface plasmon sensing probe, which uses optical glue to integrate a three-layer grating coupling structure onto the end face of the optical fiber. Under the action of grating diffraction, the incident light can excite surface plasmons on the inner and outer surfaces of the metal film respectively, so that two resonance peaks appear in the reflection spectrum. The drift of the two resonance peaks in the reflection spectrum can respectively realize the detection of biological molecules and ultrasound. The advantage of this sensor is that by changing the period of the nano grating, the positions of the two reflection resonance peaks can be continuously tuned in the visible light and near-infrared range, which breaks through the problem that the traditional optical fiber surface plasmon excitation wavelength is difficult to extend to the near-infrared wavelength, and has broad application prospects in biological detection, chemical analysis, and ultrasonic detection.

Description

Technical Field

[0001] The invention relates to the field of micro-nano photonics and optical fiber sensing technology, and in particular to a broadband tunable optical fiber surface plasmon sensing probe. Background Art

[0002] Compared with traditional detection technologies, surface plasmon resonance (SPR) detection technology has the advantages of high sensitivity, strong anti-interference ability, label-free, and real-time detection, and has broad application prospects in the fields of life sciences, medical testing, food testing, etc. However, the current commercial SPR detection systems are generally prism-type SPR sensors, which are generally large in size and complex in system, and do not meet people's pursuit of miniaturization and portability today. For details, see Hlubina, P, et al. Spectral interferometry-based surface plasmon resonance sensor. Fiber-optic SPR sensors have the advantages of miniaturization and remote sensing, and can overcome the above limitations and are widely studied. They are one of the detection technologies with the most development potential at this stage. For details, see Micheletto R, et al. Modeling and test of fiber-optics fast SPR sensor for biological investigation. Bosch ME, et al. Recent development in optical fiber biosensors. Yao Y, et al. Surface plasmon resonance biosensors and its application. In order to make the sensor adapt to detection in various environments, many methods of tuning the resonant wavelength in the spectrum have been proposed, such as changing the material, film thickness, shape, structure and size of the nanostructure.

[0003] At present, the research on fiber-type SPR sensors is mostly focused on the side of the fiber, and the detection of the external environment is achieved by exciting the gold film surface plasmons (SPs) on the side of the fiber, such as tilted fiber Bragg grating (TFBG), etc. For details, see Caucheteur C, et al. Ultrasensitive plasmonics sensing in air using optical fibre spectral combs. Wang R, et al. Operando monitoring of ion activities in aqueous batteries with plasmonic fiber-optic sensors. However, the resonance wavelength of traditional SPR fiber-optic sensors is difficult to tune continuously, and the angle of the incident light is required to be high. Generally, it can only realize functions such as biochemical detection in the visible light band. In addition, compared with the sensor integrated into the side wall of the fiber, the SPR device integrated at the end of the fiber can insert a small amount of sample and read the signal directly through dip-and-read, which greatly reduces the difficulty of operation. Therefore, it is of great significance to design a structure that can effectively and continuously tune the excitation wavelength of SPs on the end face of the fiber and break through the working band to the near-infrared or mid-infrared region.

[0004] The present invention combines the detection advantages of optical fiber type SPR sensors and the principle of grating diffraction to design a broadband tunable optical fiber surface plasmon sensing probe, whose resonant wavelength can be continuously tuned and can be used in a variety of different application scenarios. Summary of the invention

[0005] The present invention aims to propose a broadband tunable optical fiber surface plasmon sensing probe, which can be used for highly sensitive biochemical detection and ultrasonic detection, and is a new type of SPR optical fiber sensor structure. The invention uses the diffraction effect of the grating to make the vertically incident light beam produce an inclination and excite the surface plasmon between the metal film and the object to be detected, thereby realizing the detection of the refractive index change of the object to be detected. This technology can achieve the regulation of the resonant wavelength in a wide spectrum, and has the advantages of high sensitivity and fast response time, and has broad application prospects in the fields of biochemical detection.

[0006] The technical solution of the present invention:

[0007] A broadband tunable optical fiber surface plasmon sensing probe, whose resonance wavelength is tunable and has two resonance modes, the two resonance modes respectively corresponding to two application scenarios, namely, high-sensitivity label-free biochemical detection and high-sensitivity ultrasonic detection;

[0008] The broadband tunable fiber surface plasmon sensing probe includes an optical fiber and a grating coupling structure; the grating coupling structure is fixed to the end face of the optical fiber by optical glue, and the detection of the object to be measured is achieved by exciting the SPs in the grating coupling structure. The designed structure is mainly composed of three layers, from bottom to top, grating, dielectric and metal film, in which the grating is fixed in contact with the optical glue, and the metal film is in contact with the solution to be measured;

[0009] The light beam is emitted from the core of the optical fiber, enters the optical glue and contacts the grating. Based on the principle of grating diffraction, the short-wavelength incident light passes through the grating and is obliquely incident into the dielectric at the first-order diffraction angle, while the long-wavelength incident light forms an evanescent wave at the interface between the grating and the dielectric.

[0010] When the wave vector matching condition is met, the short-wavelength incident light excites the surface plasmons at the contact surface between the metal film and the solution to be tested, which is the first resonance mode of the broadband tunable fiber optic surface plasmon sensing probe; the long-wavelength incident light excites the surface plasmons at the contact surface between the metal film and the dielectric, which is the second resonance mode of the broadband tunable fiber optic surface plasmon sensing probe; in the first resonance mode, different refractive indices of the solutions to be tested produce different short-wavelength resonance peaks, thereby realizing the detection of the object to be tested; in the second resonance mode, the refractive index of the optical glue changes due to the action of ultrasound, and the long-wavelength resonance peak drifts, thereby realizing the detection of ultrasound.

[0011] The light beam is incident vertically onto the grating.

[0012] The period of the grating is determined according to the target resonance wavelength. In addition, using incident light in a certain wavelength range can make the light beam produce only the first-order diffraction angle, ignoring the influence of higher-order diffraction. Based on the principle of grating diffraction, the position of the resonance peak can be continuously tuned from the visible light band to the near-infrared region.

[0013] The wave vector matching condition is specifically that when the beams all satisfy When , the broadband tunable fiber surface plasmon sensing probe only has the first resonance mode; when all beams satisfy When the broadband tunable fiber surface plasmon sensing probe has only the second resonance mode, The wavelength of When the wavelength is , the two resonance modes exist simultaneously; where λ is the wavelength of the light beam, p is the period of the grating, and n2 is the refractive index of the dielectric.

[0014] The optical fiber is selected from single-mode optical fiber or multi-mode optical fiber.

[0015] The grating is made by micro-nano processing technology or template transfer method; the grating material is metal; the grating period determines the working band of the broadband tunable optical fiber surface plasmon sensing probe; when a specific resonant wavelength is required, the corresponding grating period is calculated by combining the grating diffraction formula and the wave vector matching equation.

[0016] The resonance mode extends from visible light to near infrared and mid-infrared bands.

[0017] After passing through the grating, the light beam is incident obliquely into the dielectric at a first-order diffraction angle, generating an evanescent wave at the interface between the dielectric and the metal film. The evanescent wave has a certain penetrating ability and can pass through relatively thin metal films. When no surface plasmon resonance occurs, the evanescent wave is almost completely reflected back. When the wave vector of the evanescent wave is equal to the wave vector of the surface plasmon at the interface between the metal film and the object to be measured, the SPR phenomenon occurs, and the energy of the evanescent wave is transferred to the SPs, which reduces the reflectivity at the resonance wavelength and causes a resonance peak to appear in the reflection spectrum. When the refractive index of the object to be measured increases, the resonance peak will be red-shifted, thereby realizing the detection of the change in the refractive index of the object to be measured. This is the first mode of the sensor, and its resonance wavelength is shorter.

[0018] In addition to the resonance peak generated by the first mode, there is also a resonance peak with a longer wavelength in the reflection spectrum, which is caused by the second mode. When the grating period is constant, the larger the wavelength, the larger the corresponding first-order diffraction angle. The first-order diffraction angle corresponding to the short-wavelength incident light is less than 90 degrees, and it is obliquely incident into the dielectric at the first-order diffraction angle, which is the first mode. The first-order diffraction angle corresponding to the long-wavelength incident light is greater than or equal to 90 degrees, which causes the incident light to generate an evanescent wave at the interface between the grating and the dielectric. When the wave vector matching condition is met, the evanescent wave excites the SPs on the inner surface of the metal film, resulting in the appearance of a long-wavelength resonance peak. Therefore, there are two obvious resonance peaks in the reflection spectrum. In addition, optical adhesive is a flexible material, and its refractive index will change due to the action of ultrasound, causing the long-wavelength resonance peak to drift. As the refractive index of optical adhesive increases, the long-wavelength resonance peak will undergo a red shift, thereby realizing the detection of ultrasound.

[0019] When the wavelength is constant, the first-order diffraction angle is inversely proportional to the period of the grating, and the first-order diffraction angle is related to the resonant wavelength, so the period of the grating can determine the resonant wavelength. The specific formula is as follows.

[0020] pn2sin(θ)=mλ (1)

[0021] k0n2sin(θ)=k spp (2)

[0022] Formula (1) is the grating diffraction formula, where p is the period of the grating, θ is the diffraction angle, n2 is the refractive index of the dielectric, λ is the wavelength of the incident light, and m is the diffraction order. When the minimum wavelength of the broadband incident light satisfies When , we only need to consider the first-order diffraction. Formula (2) is the wave vector matching equation, k spp and k0 represent the surface plasmon wave vector and the incident light wave vector respectively. Combining formula (1) and formula (2), we can get:

[0023]

[0024] where ε m represents the dielectric constant of the metal, ε d Represents the dielectric constant of the solution to be tested. As shown in formula (3), a specific resonant wavelength corresponds to a specific grating period. By processing a grating coupling structure with a specific period and fixing it to the end face of the optical fiber through optical glue, the resonance peak can be continuously tuned in the sensor.

[0025] Analysis of the variables in formula (3) shows that the resonance wavelength of the first mode is independent of the refractive index of the optical glue. In addition, the resonance wavelength of the second mode is also independent of the refractive index of the solution to be tested. Therefore, these two modes can coexist without affecting each other. By analyzing the structure of the numerical calculation, we can see that: when the refractive index of the object to be tested changes, the short-wavelength resonance peak in the reflection spectrum will drift, but the long-wavelength resonance peak will remain stable; when the dielectric or optical refractive index changes, the short-wavelength resonance peak in the reflection spectrum will remain stable, and the long-wavelength resonance peak will drift. This is consistent with the theoretical analysis results, indicating that the sensor has the advantage of multifunctional detection.

[0026] Compared with the prior art, the present invention has the following advantages: first, by combining the grating coupling structure with the optical fiber, the resonant wavelength of the sensor can be continuously tuned by controlling the period of the grating; second, there are two application scenarios in the sensor, the interference between them is very small and it can be used for high-sensitivity biochemical detection and high-sensitivity ultrasonic detection at the same time; third, the sensor can expand the working wavelength from visible light to near-infrared and mid-infrared bands; fourth, the sensitivity of the sensor has a good polarization angle tolerance. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 The structure diagram of a broadband tunable optical fiber surface plasmon sensing probe;

[0028] Figure 2 It is the principle diagram of the first mode, which is used to detect the solution to be tested;

[0029] Figure 3 is the schematic diagram of the second mode, which is used to detect ultrasonic waves;

[0030] Figure 4 is the reflection spectrum when the refractive index of the solution to be tested changes and there is no ultrasonic wave. At this time, the refractive index of the UV optical glue remains unchanged;

[0031] Figure 5 It is the reflection spectrum when ultrasonic wave exists and the refractive index of the solution to be tested remains unchanged. At this time, the refractive index of the UV optical glue changes. DETAILED DESCRIPTION

[0032] In order to enable those skilled in the art to better understand the solution of the present application, the technical solution in the embodiments of the present application will be clearly and completely described below in conjunction with the drawings in the embodiments of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, not all of the embodiments. Based on the embodiments in the present application, all other embodiments obtained by ordinary technicians in this field without creative work should fall within the scope of protection of the present application.

[0033] In this example, gold film is selected as the outermost metal film, SiO2 is used as the dielectric, gold is used as the grating material, and UV curing glue is used to fix the grating structure to the end face of the optical fiber. As shown in formula (3), the position of the short-wavelength resonance peak corresponds to a specific grating period, and the resonance wavelength is selected by setting a specific grating period. Taking the sensor with a resonance wavelength of 1550nm as an example, by analyzing formula (3), the period of the grating is calculated to be 1153nm.

[0034] The specific process of designing a fiber optic sensor is mainly divided into three steps: the first step is to use a clean silicon wafer as the substrate, and use a magnetron sputtering coating device to coat 25nm of gold, 220nm of SiO2 and 10nm of gold on the silicon wafer in sequence; the second step is to use the focused ion beam technology (FIB) to etch a grating with a period of 1153nm, a width of 300nm and a depth of 10nm on the outer gold film; the third step is to apply a layer of ultraviolet curing glue on the end face of the optical fiber; the fourth step is to contact the end face of the optical fiber with the grating coupling structure, and cure the ultraviolet curing glue by irradiation with an ultraviolet lamp; the fifth step is to separate the optical fiber from the silicon wafer. Since the bonding force between gold and the silicon substrate is weak, the grating coupling structure on the silicon wafer is completely transferred to the end face of the optical fiber, and the preparation of the sensor is completed. When the wavelength of the incident light ranges from 1400nm to 2000nm, there are two modes in the sensor, thereby realizing the detection of the solution to be tested and ultrasound.

[0035] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. For those skilled in the art, the present invention may have various modifications and variations. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A broadband tunable optical fiber surface plasmon sensing probe, characterized in that: The resonance wavelength of the broadband tunable optical fiber surface plasmon sensing probe has two resonance modes, and the two resonance modes correspond to two different application scenarios, namely, highly sensitive label-free biochemical detection and highly sensitive ultrasonic detection. The broadband tunable fiber surface plasmon sensing probe includes an optical fiber and a grating coupling structure; the grating coupling structure is fixed to the end face of the optical fiber by optical glue; the grating coupling structure is divided into three layers, namely, a grating, a dielectric and a metal film; the grating is fixed in contact with the optical glue, and the metal film is in contact with the solution to be measured; The light beam is emitted from the core of the optical fiber, enters the optical glue and contacts the grating. Based on the principle of grating diffraction, the short-wavelength incident light passes through the grating and is obliquely incident into the dielectric at the first-order diffraction angle, generating an evanescent wave at the interface between the dielectric and the metal film. The long-wavelength incident light forms an evanescent wave at the interface between the grating and the dielectric. The specific wave vector matching condition is that when the beams all satisfy When , the broadband tunable fiber surface plasmon sensing probe only has the first resonance mode; when all beams satisfy When the broadband tunable fiber surface plasmon sensing probe has only the second resonance mode, The wavelength of When the wavelength is , the two resonance modes exist simultaneously; where λ is the wavelength of the beam, p is the period of the grating, and n2 is the refractive index of the dielectric; When the wave vector matching condition is met, the short-wavelength incident light excites the surface plasmons at the contact surface between the metal film and the solution to be tested, which is the first resonance mode of the broadband tunable fiber optic surface plasmon sensing probe; the long-wavelength incident light excites the surface plasmons at the contact surface between the metal film and the dielectric, which is the second resonance mode of the broadband tunable fiber optic surface plasmon sensing probe; in the first resonance mode, different refractive indices of the solutions to be tested produce different short-wavelength resonance peaks, thereby realizing the detection of the object to be tested; in the second resonance mode, the refractive index of the optical glue changes due to the action of ultrasound, and the long-wavelength resonance peak drifts, thereby realizing the detection of ultrasound.

2. The broadband tunable optical fiber surface plasmon sensing probe according to claim 1, characterized in that: The light beam is incident vertically onto the grating.

3. The broadband tunable optical fiber surface plasmon sensing probe according to claim 1 or 2, characterized in that: The period of the grating is determined according to the target resonance wavelength.

4. The broadband tunable optical fiber surface plasmon sensing probe according to claim 1, characterized in that: The optical fiber is selected from single-mode optical fiber or multi-mode optical fiber.

5. The broadband tunable optical fiber surface plasmon sensing probe according to claim 1, characterized in that: The grating is made by micro-nano processing technology or template transfer method; the grating material is metal; the grating period determines the working band of the broadband tunable optical fiber surface plasmon sensing probe; when a specific resonant wavelength is required, the corresponding grating period is given by combining the grating diffraction formula and the wave vector matching equation.

6. The broadband tunable optical fiber surface plasmon sensing probe according to claim 1, characterized in that: The resonance mode extends from visible light to near infrared and mid-infrared bands.

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