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Abstract
A proposal toward the enhancement in the sensitivity of a fiber-based surface plasma resonance (SPR) refractive index (RI) sensor is explored experimentally using a Bessel-like beam as the input source. We splice a section of single-mode fiber and a section of multimode fiber to construct the Bessel-like beam, which contains a series of concentric rings for the consistency of the resonance angle configuration to improve the performance of the SPR sensor. We fabricate a dual-truncated-cone (DTC) structure of the fiber to excite and receive the SPR signals. The larger the number of concentric rings, the higher the sensitivity. The number of concentric ring is determined by the length of the multimode fiber. When the grinding angle of the DTC-sensing probe is 15° and the length of the multimode fiber is 500 µm, the maximum testing average sensitivity is 6908.3 nm/RIU, which is more sensitive than the previous SPR sensor introduced by the Gaussian beam as the input source in multimode fibers.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
The detection of biomolecules plays a vital role in pharmacological research and early diagnosis. However, the fact that biomolecules are often present at very low concentrations, and the sample also contains a variety of other molecules, making their identification a strenuous task. As a surface-sensitive analytical method, surface plasmon resonance (SPR) presents the most advanced label-free, high-sensitivity, and real-time biomolecule detection capability, and has become an important tool in the fields of medicine and biology [1–5]. To excite SPR at a metal-dielectric interface, various configurations have been proposed, including prism coupling [6,7], optical fiber coupling [8,9], waveguide coupling [10,11], and grating coupling [12]. Among them, the optical fiber-based SPR sensor has attracted large researches for its remarkable advantages, such as chemical inertness, miniaturization, flexibility in integration, and low dose detection [13]. With the field of biomedicine drawing more and more towards microcosmic schemes, this requires the SPR sensor to have higher sensitivity and specificity. Therefore, improving the sensitivity of the sensor has been the focus of SPR sensing technology research.
Current methods for improving the sensitivity of fiber-based SPR sensors are broadly divided into two categories: sensitive material coating and structural optimization. In term of sensitive material coating, recent researches have shown that many materials can be used to enhance the sensitivity of the SPR sensor due to their enhanced SPR signal characteristics, including graphene [14], transition sulfides [15], and oxides [16]. For structural optimization of the sensor, the use of special optical fibers and the design of optical fiber sensing probes with different geometries have been proven to effectively increase the sensitivity of the SPR sensor. Some typical examples include D-type fiber [17], tapered fiber [18], and heterogeneous core fiber [19]. However, the traditional fiber-based SPR sensors are always based on the transmission of Gaussian beam in multimode fibers. The laser propagating in the multimode fiber performs multiple modes, which propagate along multiple directions when they reach the surface of the fiber (see the right sketch diagram of Fig. 1(c)), which increase the preparation difficulty of the fiber probe with high sensitivity. It is difficult to control the transmission angle of the multimode fiber, and the resolution is also limited due to the inherent modal noise.
Fig. 1. (a) Sketch diagram of the SPR sensor illuminated with a Bessel-like beam. (b) Schematic diagram of Bessel-like beam generation based on the SMF-MMF structure. (c) Comparison between Bessel-like beam and Gaussian beam transmitted in the optical fiber, lengthwise section.
Recently, beam shaping has become a tantalizing theme and generates unusual light patterns like Bessel, Laguerre-Gaussian, Vortex, and Airy beam. Such non-Gaussian beams have triggered many types of researches in optical micromanipulations and non-linear optics [20,21]. Bessel beam, which has the diffraction-free and self-healing properties [22], spreads its energy equally across the concentric rings [23], being suitable for SPR sensing. Compared to the Gaussian beam transmitted in multimode fiber with large numbers of modes, Bessel-like beam compresses these modes into separate energy rings that propagate along the fiber main-axis direction, which helps to improve the excitation efficiency and sensitivity of SPR due to the consistency of the resonance angle (see the left sketch diagram of Fig. 1(c)). There will be more overlap between SP modes and the surrounding sample, which implies an enhanced sensitivity and better sensing resolution of the proposed SPR sensor than the conventional sensor using a Gaussian beam in multimode fibers. So far as we know, none of the past literary works has described such kind of optical fiber-based SPR sensor where the Bessel-like beam is exercised as an input source.
In this work, the fiber-based SPR sensor with high sensitivity is proposed and investigated by employing a Bessel-like beam as the input source. We produce the Bessel-like beam by splicing a section of SMF and a section of MMF (see Fig. 1(b)). The core diameter of SMF is 8.2 µm, and the core diameter of MMF is 105 µm. Combining with the optical fiber grinding technology, SPR is excited on the reflective surface coated with gold film. The one-to-one relationship between the surrounding refractive index and resonance wavelength has been explained and verified numerically for various lengths of MMFs under a certain grinding angle. And the average testing sensitivity is as high as 6908.3 nm/RIU with the sample refractive index range of 1.333-1.393. The proposed SPR sensor shows a great sensing sensitivity than that of the previous SPR sensor where the Gaussian beam is used as the input source, and has the merits of low cost and easy fabrication.
2. Principle and experiment
2.1 Design and fabrication of fiber probe
Figure 1(a) provides the sketch diagram of the SPR sensor illuminated with a Bessel-like beam. All structures in this work are fabricated by using the single-mode fiber (SMF, 8.2/125 µm) and the step-index multimode fiber (MMF, 105/125 µm). A commercial refractive index profiler (S14, Photon Kinetics) is employed to obtain the RI profile of the SMF and MMF. For the SMF, the core RI and the cladding RI are 1.4661 and 1.457, respectively. For the MMF, the core RI and the cladding RI are 1.4446 and 1.4277, respectively. The SMF-MMF structure is used for the generation of a Bessel-like beam, as shown in Fig. 1(b). The MMFs are polished as a circular truncated cone shape and spliced together for the satisfaction of total light reflection. We deposit the gold film on the grinding side-surfaces and excite the surface plasmon resonance in the gold film region by using the Bessel-like beam. Compared with the Gaussian beam, only part of the transmitted light meets the resonance angle, the Bessel-like beam transmitted in the fiber has a smaller divergence angle after being totally reflected by the grinding side-surface, which contributes more modes that can be used to excite SPR and helps to improve the excitation efficiency of SPR, as shown in Fig. 1(c).
The output field of the Bessel-like beam is related to the transmission distance of the light in the MMF, thus, it is necessary for us to control the length of MMF precisely. As shown in Fig. 2, a precision fiber cleaver for controlling the length of the MMF is used, which allows us to move the position of the fiber fusion point under the microscope. Firstly, a section of MMF is spliced to a section of SMF with a commercial fusion splicer (FSM-100P+, Fujikura). Secondly, we fix the fiber fusion point between SMF and MMF with a fiber clamp and align it with the fiber cleaver by moving the fiber under the microscope. Then, we move the fiber along the axial direction with the length of L, and cut the other part of the MMF. The length of MMF is controlled by a metrics micrometer with an accuracy of 10 µm.
Fig. 2. (a) Aligning the fusion point with the fiber cleaver under the microscope. (b) Moving the fiber along the axis direction with the length of L.
In order to satisfy the Kretschmann configuration and bring the incident angle up to the resonance angle, the dual-truncated-cone (DTC) structure is employed for exciting and receiving SPR signals [24]. Figure 3(a) shows the sketch diagram of the fiber polishing process. The SMF-MMF structure that generates the Bessel-like beam and a section of multimode fiber are grinded into two truncated cones with same grinding depth. The polishing angle α is defined as the angle between the rotational axis of the fiber and the abrasive disk plane. Here, we control the polishing angle α=12°. A special program for splicing two grinding fibers coaxially is developed using the optical fusion splicer, which allows us to reduce insertion losses due to the splicing between the two truncated cones. Then, we place the DTC fiber in the vacuum chamber of the plasma sputtering apparatus (JS-1600, HTCY) to deposit a gold film on the fiber grinding side-surfaces to configure the sensing zone, as shown in Fig. 3(b). The thickness of the gold film is controlled by adjusting the sputtering time and current. A three-dimensional topography analyzer (New View7200, Zygo) is employed to evaluate the gold coating quality and thickness [25]. Figure 3(c) shows the sensing zone with a uniform gold film under the scanning electron microscope. Here, the gold film with a thickness of 50 nm is used for the preparation of SPR sensors due to its optimal SPR performance.
Fig. 3. (a) Sketch diagram of the fiber polishing process. (b) Image of the dual-truncated-cone structure under the 200x magnification. (c) SEM image of the gold-coated sensing zone.
2.2 Sensing principle
The structure of the proposed sensor is depicted in Fig. 4. The sensing region of the proposed sensor consists of two grinding multimode fibers, the gold film, and a dielectric layer whose RI needs to be tested. Similar to the prism-based SPR sensor, when the p-polarized light excites SPR on the surface of the metal film, the reflectance R of the light intensity can be calculated based on the three-layer Fresnel equation [26]:
For the proposed sensing scheme, the generation of the Bessel-like beam can be explained as follows: When the SMF and MMF are spliced coaxially, the fundamental LP01 mode of the SMF is coupled to the MMF, where LP0m modes (m is the radial index) are excited. The fields of the LP0m modes in the MMF core can be represented as the apertured Bessel function ${J_0}({{k_{z,fm}}r} )$ with transverse wave vectors:
where k is the wave vector, nf is the refractive index of the fiber core, βfm are the propagation constants of the LP0m modes, and r is the radial coordinate and smaller than the core radius of the MMF Rf.Because each LP0m mode propagates along the waveguide independently with its respective propagation constant, the field at the output facet of the MMF is the superposition of Bessel-like fields and can be expressed as [23]:
When the input Gaussian mode from SMF strikes at the MMF section, the input Gaussian beam will be divided into various higher-order modes inside the MMF and these modes have different propagation constants due to various effective refractive indices. That means these modes will travel at various speeds inside the MMF, which caused the profile of the light field distribution at the output facet of the MMF to change along with the length of the propagating distance. Unlike a Gaussian beam, as the input Bessel-like beam has multiple annular rings around its maxima, so the ability to compress the resonance angle is related to the length of the MMF, which is reflected in the improvement of the sensitivity of the SPR sensor.
2.3 Experimental setup
The experimental setup of the SPR sensor with the DTC structure is shown in Fig. 5. The laser from the supercontinuum source (SuperK compact, NKT Photonics) is launched into the left SMF-MMF structure to generate the Bessel-like beam. Then the SPR is excited on the grinding sensing region, and the transmitted beams are received by an optical spectrum analyzer (AQ6373, Yokokawa, wavelength resolution 0.02 nm.). We place the DTC-sensing probe in the sealed liquid chamber, and a programmable micro-injecting pumper (LSP01-1A, Longer Pump) is used to inject the glycerin-aqueous solution with different refractive indices into the chamber, and the waste liquid is received by a waste bucket. The glycerin-aqueous solution refractive index is measured and calibrated by the Abbe refractometer (GDA-2S, Gold).
3. Results and discussion
Before the experiment, we prepare the glycerin-aqueous solution with a refractive index ranging from 1.333 to 1.393, and the interval is 0.01. The measurements are carried out at room temperature, and all the results are normalized using the transmission spectrum of the sensing probe in the air.
To study the influence of Bessel-like beam on SPR, four different lengths (L=500, 700, 900, and 1500 µm) of MMFs are used in the experiment. Figure 6 shows the testing results of the SPR spectra with different MMF lengths under the grinding angle of 12°, it can be seen that the resonance dip becomes shallower, wider and shifts to the longer wavelength with the increasing refractive index. When the MMF length is 500 µm, the SPR resonance dip changes from 661.9 nm to 984.6 nm, and the average testing sensitivity is calculated as 5378.3 nm/RIU; when the MMF length is 700 µm, the SPR resonance dip changes from 663.7 nm to 950.5 nm, and the average testing sensitivity is calculated as 4780 nm/RIU; when the MMF length is 900 µm, the SPR resonance dip changes from 659.1 nm to 932.9 nm, and the average testing sensitivity is calculated as 4563.3 nm/RIU; when the MMF length is 1500 µm, the SPR resonance dip changes from 658.5 nm to 855.9 nm, and the average testing sensitivity is calculated as 3290 nm/RIU. The experimental results indicate that the sensitivity is affected by the MMF length. As shown in Fig. 6, the number of energy rings on the fiber end face gradually increases as the MMF length decreases, and the rings become narrower. This allows more transmitted light to interact with the gold film at the same angle, and improves the excitation efficiency of SPR by compressing the resonance angle, and the sensitivity of the sensor is improved which benefits from more overlap between SP modes and the surrounding sample. Totally, the more the number of energy rings, the stronger the compression capability of the resonance angle.
Fig. 6. Testing results of the SPR spectra with the MMF length of (a) 500 µm; (b) 700 µm; (c) 900 µm; (d) 1500 µm under the grinding angle of 12°.
Figure 7 shows the relationship between the refractive index and the resonance wavelength of the four sensors under the grinding angle of 12°. The redshift in resonance dip is observed, however, the shift of the resonance dip is not linear, the higher the refractive index, the higher the sensitivity. The sensors with MMF lengths of 700 µm and 900 µm have similar sensitivity, which is due to the measurement error caused by the flat resonance valley of the former when the refractive index is 1.393.
Fig. 7. The relationship between the refractive index and the resonance wavelength when the MMF lengths are 500 µm, 700 µm, 900 µm, and 1500 µm, respectively.
We also compare four sensing probes with the DTC structure grinding different angle ranges, which is 6°-15° with the interval is 3° (see Figs. 8 and 9). Here, the MMF with a length of 500 µm is used due to its highest sensitivity in the above tests. According to the results, for the same MMF length and surrounding refractive index range of 1.333-1.393, the larger the grinding angle, the higher the sensitivity. When the grinding angle is 6°, the SPR resonance dip changes from 588.5 nm to 762.2 nm and the average testing sensitivity is 2895 nm/RIU; when the grinding angle is 9°, the SPR resonance dip changes from 598.4 nm to 809.6 nm, and the average testing sensitivity is 3520 nm/RIU; when the grinding angle is 15°, the SPR resonance dip changes from 648.4 nm to 1062.9 nm and the average testing sensitivity is 6908.3 nm/RIU. As the grinding angle increases, the incident angle of light on the gold-analyte surface decreases, which causes the resonance wavelength to shift towards a longer wavelength and increases sensitivity. The reasons are as follows: Wave vector kew of an evanescent wave decreases with a decrease of the incident angle of the light. Wave vector ksp of the surface plasmon decreases with an increase of the wavelength of the light [27]. For exciting SPR, both of these wave vectors, kew and ksp should be equal. Thence, wave vector ksp with a longer wavelength is excited by light with a smaller incident angle. The changes in wave vector ksp that result from changes in the analyte are enhanced by longer wavelength. Therefore, we can conclude that a smaller incident angle (or a larger grinding angle) is beneficial to increase sensitivity. The sensor with the grinding angle of 15° has smaller resonance wavelengths than that of the sensor with the grinding angle of 12° in the refractive index range of 1.333-1.343, but it has higher sensitivity in the refractive index range of 1.333-1.393 (see Fig. 9). In our opinion, the main reason is the measurement error introduced by the dual-truncated-cone structure. Although the resonance wavelength has an error near the initial refractive index, the results show a good agreement overall.
Fig. 8. Testing results of the SPR spectra with the grinding angle of (a) 6°; (b) 9°; (c) 12°; (d) 15° under the MMF length of 500 µm.
Fig. 9. The relationship between the refractive index and the resonance wavelength when the grinding angles are 6°, 9°, 12°, and 15°, respectively.
To test the repeatability of the sensing device, we carried out the measurement using the same sensing probe with a MMF length of 500 µm and a grinding angle of 12° after a time interval of over one week. The results show that the performance of the sensor is stable throughout one week with less than 0.97% fluctuations from the initial test results, and the sensor demonstrates good repeatability (see Fig. 10).
The detection limit (DL) of the sensor can be expressed as δλDR/S, where δλDR is the spectral resolution of the optical spectrum analyzer and equal to 0.02 nm in the experiment, S is the sensitivity. The lowest DL of 2.9 × 10−6 RIU is achieved when the MMF length is 500 µm and the grinding angle is 15° in this experiment. The detection limit of the refractive index sensor is on the order of 10−6 RIU. Table 1 shows the performance comparison of the proposed SPR sensor with other previously reported SPR sensors. Compared with the SPR sensor with special fibers [29,30,32] or special structures [28,31], the proposed sensor has a higher sensitivity and a lower detection limit. Meanwhile, the proposed sensor has the merits of low cost and easy fabrication.
Table 1. Comparison of various fiber optic SPR sensors
4. Conclusion
In this study, we propose and demonstrate a fiber SPR sensor illuminated with a Bessel-like beam. The DTC structure is employed for the excitation and reception of SPR signals. We experimentally study the influence of Bessel-like beam on SPR, the results show that Bessel-like beam has the ability to compress the resonance angle, which improves the sensitivity of the sensor. The sensitivity of the sensor increases with the number of energy rings on the fiber end face, and achieves 6908.3 nm/RIU when the MMF length is 500 µm under the grinding angle of 15°, and the detection limit is on the order of 10−6 RIU. It can find wide applications in chemical and biological fields for the measurement of refractive index or other measurements in small doses of analytes, such as substance analysis or medical diagnosis.
Funding
National Key Research and Development Program of China (2018YFC1503703); National Natural Science Foundation of China (61775047, 61975039); Natural Science Foundation of Heilongjiang Province (YQ2020F011); Fundamental Research Funds for Harbin Engineering University of China; 111 Project (B13015).
Disclosures
The authors declare no conflicts of interest.
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