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Abstract
We present a method for Raman examination using a silver-nanoparticles (Ag-NPs) coated D-shaped fiber (DSF) internally excited via an in-fiber azimuthally polarized beam (APB) generated by an acoustically induced fiber grating. Simulation results show that an electric-field intensity enhancement factor can be effectively improved under APB excitation compared with the linear polarization beam (LPB) excitation, because the strong gap-mode is uniformly generated between two adjacent Ag NPs on the surface of the DSF planar side. Experimental results show that the Raman signal intensity of the methylene blue (MB) detected by DSF in the case of APB excitation is ∼4.5 times as strong as that of LPB excitation, and the Raman detection sensitivity is ∼10−9 M. The time stability of this method is also tested to be guaranteed.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
The lab-on-fiber technology aims to offer a multifunctional platform for both communication and sensing applications [1] with advantages of miniaturization, portability and low cost by integrating different nanostructures and materials with a single optical fiber due to the intrinsic properties of flexibility, compatibility, lightweight, and anti-interference. D-shaped fibers (DSF) is one of the most important tools to expand the functionality [2–5], including temperature sensing [6], Raman detection [2,4], gas sensing [7], etc., because of the planar side providing a substrate for materials integration and access to evanescent field from the fiber core [8]. Raman spectrum is called “fingerprint” of molecules [9], but possessing a disadvantage that the scattering cross-section is very small which results in low detection sensitivity. Through combining noble metal nanostructures with DSF, it provides a new platform for highly sensitive label-free detection based on surface enhanced Raman spectroscopy (SERS) [10–12], which has been widely used in fields of biology [13], chemistry [14], materials [15], food safety [16], etc., in virtue of the significant enhancement of intrinsic Raman scattering based on the electric field enhancement mechanism [17–19] and chemical enhancement mechanism [20–22], where the electric field enhancement mechanism dominates the light-matter interaction process [23].
The development of nanoscience demands for a higher detection sensitivity for SERS to analyze nano-constituents [24], defects [25], deformation [26], etc, where the key is optimization of SERS-active substrates [27–29] and illumination methods [30–32]. The SERS-active substrates has gone from the original colloids to the transparent-and-flexible chip for the in-situ detection and real-life apllications [33]. Considering the flexibility, transparency and anti-interference of DSF, it is promising to realize the highly-sensitive in-situ detection via DSF integrated with noble metal nanostructures. Noble metal nanoparticles are the most commonly used nanostructures applied in SERS, although some plasmon-free nanoparticles are also reported to possess the ability to enhance the Raman scattering [34]. Chemical deposition method is often applid to prepare noble metal nanoparticles and much efforts have been made to realize the controlling and tunability of the size and concentration [35,36]. Illumination methods notably influence the electric field enhancement mainly based on the polarization-dependent excitation of localized surface plasmon modes [37–39]. Azimuthal polarization beam (APB) is a kind of special illumination source for its tangential polarization distribution on the cross-section of the light beam, which has been adopted to successfully improve the SERS sensitivity [40,41]. Thus, it is promising to promote the lab-on fiber technology using a metallized DSF excited via APB with advantages of flexibility, transparency, anti-interference, and notable SERS sensitivity.
In this paper, we report on a method for Raman examination with high sensitivity by a metallized DSF internally excited via an in-fiber azimuthally polarized beam (APB) generated by an acoustically-induced fiber grating. Silver nanoparticles (Ag-NPs) are deposited on the flat plane of DSF, which was fabricated by side-polishing a part of fiber cladding to approach the fiber core. Simulation results demonstrate that the electric-field intensity can be effectively improved by APB excitation compared with LPB excitation, because the strong gap-mode is uniformly generated between two adjacent Ag NPs on the surface of the DSF planar side. Experimental tests show that the Raman signal intensity of the methylene blue (MB) detected by the DSF in the case of APB excitation is ∼4.5 times stronger than that of the linear polarization beam (LPB) excitation. The Raman detection sensitivity is 10−9 M under the APB illumination, indicating the notable SERS-activity of the APB-excited DSF configuration. It is verified that the time stability of this method is guaranteed.
2. Theoretical design
The transmission process of LPB and APB from the conventional optical fiber to the DSF are respectively simulated by using the three-dimensional finite-difference time domain (3D FDTD, Lumerical) method, as exhibited in Fig. 1. Figure 1(a) shows the model of DSF integrated with conventional optical fiber by seamless docking, where the diameter of the optical fiber core is d=9 µm, and the flat plane of the DSF is right tangent with the fiber core. As shown in Figs. 1(b)–1(g), the electric-field intensity characteristics of the DSF cross-sections on the x-y plane and y-z plane have been visualized in the case of LPB and APB illumination. It can be seen from Figs. 1(b)–1(d) that the electric-field intensity distribution of the illumination light field maintains relatively unvariable after transmitting from the conventional optical fiber to the DSF, which ensures the accurate LPB and APB illuminating the DSF. Comparing the evanescent field intensity near the flat plane of DSF shown in Figs. 1(b) and 1(c) with that shown in Fig. 1(d), it is obvious that the the evanescent field intensity under APB illumination is stronger than that of LPB illumination. On y-z plane, the evanescent field distribution is simulated and exhibited in Figs. 1(e)–1(g). The electric-field intensity can up to ∼0.6 with APB excitation while the maximun value of elextric field intensity being ∼0.4 in the case of LPB excitation, revealing a stronger evanescent field under APB illumination.
Fig. 1. (a) Sketch map of DSF integrated with the conventional optical fiber; (b-d) Electric field intensity distribution of the illumination light field after transmitting from the conventional optical fiber to the DSF with LPB (b, c) and APB (d) illumination, respectively; (e-g) Evanescent field intensity near the polished flat plane of DSF with LPB (e, f) and APB (g) illumination, respectively.
The interaction processes of Ag-NPs coated on the flat plane of the DSF excited via LPB and APB are respectively simulated by using 3D FDTD method. Figure 2(a) shows the sketch map of the metallized DSF, where the radius of Ag NPs on the surface of DSF planar side is r=40 nm, and the gap between two adjacent nanoparticles is g=2 nm. As shown in Figs. 2(b)–2(g), the electric-field enhancement characteristics are explored by simulating the cross sections of the metallized DSF i.e. x-y plane and x-z plane under LPB and APB excitation, respectively. When the polarization direction of LPB is parallel to center line of Ag-NPs, shown in Fig. 2(b), it can be seen from Figs. 2(b) and 2(c) that the gap mode can be effectively excited and the electric-field intensity enhancement factor is ∼30. In view of the other degenerated mode of LPB with polarization direction vertical to the center line of Ag-NPs along x axis, it can be seen from Figs. 2(d) and 2(e) that the surface mode and gap mode are generated, simutaneously, but the electric-field intensity enhancement factor is no more than ∼8, which is notably weaker than that of the excitation of LPB with polarization direction parallel to the center line of Ag-NPs along x axis. Thus, the enhancement effect is quite different at two orthogonal polarization direction. In the case of APB excitation, as shown in Figs. 2(f) and 2(g), the gap mode can be effectively excited and the electric-field intensity enhancement factor is ∼60, which is larger than that of the LPB excitation.
Fig. 2. (a) Sketch map of DSF integrated with the conventional optical fiber; Electric-field intensity distribution on x-y plane (b) and x-z plane (c) under LPB excitation when the polarization direction is parallel to center line of Ag-NPs; Electric-field intensity distribution on x-y plane (d) and x-z plane (e) under LPB excitation when the polarization direction is vertical to center line of Ag-NPs; Electric-field intensity distribution on x-y plane (f) and x-z plane (g) under APB excitation.
3. Experimental results and discussion
3.1 Preparation of metallized DSF
A DSF was fabricated by side-polishing a part of fiber cladding using mechanical polishing method, so as to form a D shape on the cross-section of the optical fiber. The fiber used here is a step-index few-mode fiber (FMF), the core radius and cladding radius of which is ρco=4.5 µm and ρcl=62.5 µm, respectively, in visible band. Figure 3(a) shows the optical microscope image of the D-shaped cross-section. Note that the distance between the cambered surface and the polished planar side is ∼67 µm. When the fiber core is illuminated by LPB, the optical microscope image of D-shaped cross-section is exhibited in Fig. 3(b). The length of the polished planar-surface can up to 5 mm, offering a significantly large region to deposit SERS-active materials, which cannot reach by end-tip fiber probes [42]. Subsquently, the chemical deposition method was adopted to deposit the Ag nanostructures onto the surface of DSF planar side. The silver ammonia solution (15 mg/ml AgNO3 solution and a moderate amount of ammonia water) was mixed with potassium sodium tartrate (75 mg/ml) with a volume ratio of 1:1 and reacted for 30 min to diposit the silver nanostructures. The nanoscale surface topography of the metallized DSF depends on both the chemical deposition process and the polishing process because the polishing process significantly alters the surface roughness, which further influences nanoscale ion exchange transformations and seeded-growth process of the chemical reaction [43]. Figures 3(c) and 3(d) exhibits the surface topography of the metallized DSF characterized by the scanning electron microscope (SEM). The inset in Fig. 3(c) shows the configuration of the Ag-NPs deposited DSF. Figure 3(c) is enlargement of area in the inset marked by solid yellow wireframe. Figure 3(d) is an enlargement of area in Fig. 3(c) marked by the dotted yellow wireframe, which demonstrates that the radius of the Ag NPs is ∼40 nm, the standard deviation of which is caculated to be 12.65 nm. The inset in Fig. 3(d) is SEM image from the top-down view of planar side of DSF, which exhibits that NPs are located close to each other, where the scale of most of the nano-gaps between two adjacent NPs is estimated to no more than 2 nm and the cooresponding standard deviation is 1.46 nm.
Fig. 3. (a) Optical microscope image of the D-shaped cross-section; (b) Optical microscope image of D-shaped cross-section when the fiber core is illuminated by LPB; (c) SEM image of the metallized flat plane of DSF corresponding to the area marked by solid yellow wireframe in the inset. The inset of (c) is the configuration of the Ag-NPs deposited DSF; (d) Partial enlargement of the area marked by dotted yellow wireframe in (c). The inset of (d) is the SEM image from the top-down view of planar side of metallized DSF.
3.2 APB excited DSF for SERS
The configuration of APB excited DSF was adopted to detect Raman spectroscopy. A fiber with DSF segment was integrated with a segment of conventional optical fiber by an automated fiber welding machine. The experimental setup for Raman detection based on the APB excited DSF configuration is shown in Fig. 4(a). A long working-distance micro-objective (MO 100×) is used to collect the Raman signal. A fiber adapter (FA) and a segment of multi-mode fiber (MMF) are adopted to collect, transmit and couple the Raman signal into a Raman spectrometer (RS). A home-made APB generator is adopted to directly generate APB in an FMF based on the acoustically-induced fiber grating (AIFG) [44–46]. Figure 4(b) and Figs. 4(b1)–4(b4) depict the transverse mode intensity distribution of APB with azimuthal polarization output from the fiber flat end and the corresponding polarization examination results, respectively, which indicates high mode purity of the generated in-fiber APB. Figure 4(c) and Figs. 4(c1)–4(c4) show the transverse mode intensity distribution of APB with azimuthal polarization from the DSF and the polarization examination results, respectively. Note that the tangential polarization distribution remains unchanged after integrating with the D-shaped fiber by fusion welding method, which is consistent with the simulation results exhibited in Fig. 1(d).
Fig. 4. (a) Experimental setup of Raman detection using metallized DSF internally excited via APB. DSF: D-shaped fiber; MO: micro-objective; FA: fiber adapter; MMF: multi-mode fiber; RS: Raman spectrometer; (b) Transverse mode intensity distribution of APB with azimuthal polarization output from the flat fiber end and the polarization examination results; (c) Transverse mode intensity distribution of APB with azimuthal polarization from the optical fiber with D-shape segment and the polarization examination results.
As a widely accepted target analyte, the methylene blue (MB) moleculars were adsorbed on flat surface of the metallized DSF. The metallized DSF segment was slightly bending and approximately horizontally immersed in the MB dispersion liquid (MB molecules dispersed in distilled water) with specific concentration for about 10 min. The MB dispersion liquid was placed in a large-diameter petri dish to prevent the DSF from bending too much and breaking. Then the metallized DSF was taken out and natural drying for about 10 min in lab environment. Raman spectra of MB with concentrations 10−4 M (mol/l) are internally excited via LPB under different polarization direction, as shown in Fig. 5(a), and the corresponding histogram of Raman intensity at 1620 cm−1 detected via LPB with different polarization directions is shown in Fig. 5(b). Polarization direction dependent characteristic of this configuration is obviously illustrated in Fig. 5(b). Note that the maximum Raman intensity and the minimum Raman intensity are achieved at two orthogonal polarization direction (∼100° and 10°), which is significantly consistent with the simulation results shown in Figs. 2(b)–2(f).
Fig. 5. (a) Raman spectra of MB (10−4 M) detected by the metallized DSF internally excited via LPB with different polarization direction. Excitation power is 6 mW, and the integration time is 10 s; (b) The corresponding histogram of Raman intensity at 1620 cm−1 detected via LPB with different polarization directions as shown in (a).
Figure 6(a) shows the experimental results of Raman intensity under the APB and LPB excitation, where the polarization direction of LPB is at the maximum value direction. It reveals that the Raman spectrum intensity under APB excitation is ∼4.5 times stronger than that of LPB excitation. The comparison result is consistent with the theoretical analysis. Considering the difference between the experiment and simulation model that the size of the prepared nanogaps fluctuates around 2 nm with standard deviation of 1.46 nm, while the gap size in the simulation model keeping 2 nm, the larger nanogaps can also contributes to the SERS enhancement, but the gaps no more than 2 nm dominate the process for the huge enhancement compared with the larger gaps [47,48]. The Raman spectra of MB with lower concentrations (10−6, 10−7, 10−8 and 10−9 M) detected by the metallized DSF excited via APB are exhibited in Fig. 6(b). It can be seen that the detection sensitivity can reach 10−9 M under internal APB illumination with Raman characteristic peaks of MB being distinguished. However, the Raman spectrum of 10−9 M MB solution adsorbed on the flat of the metallized DSF under LPB excitation loses most of the Raman characteristic peak information even though the polarization direction located at the position that can reach maximum Raman intensity. The time stability of the metallized DSF excited via APB in fiber is examined. Figure 6(c) shows the Raman spectra of MB recorded at 10 min intervals for 60 min, and the corresponding histogram of the Raman intensity at the 1620 cm−1 band is shown in Fig. 6(d), which are unvariable after 60 min of preservation. The relative standard deviation (RSD) is calculated to be 9.7%, respectively, illustrating that the time stability is guaranteed.
Fig. 6. (a) Raman spectra of MB with 10−5 M concentration excited via APB (red curve) and LPB (black curve). Integration time is 10 s, and the excitation power is 6 mW; (b) Raman spectra of MB with different concentrations (10−6, 10−7, 10−8, and 10−9 M) adsorbed on the DSF plane excited via APB and Raman spectrum of 10−9 M MB solution adsorbed on the DSF plane (black curve) excited via LPB. Integration time is 10 s, and the excitation power is 6 mW; (c) Raman spectra of MB (10−5 M) detected by the metallized DSF with APB excitation as a function of storage time during 60 min in lab environment. Integration time is 30 s, and the excitation power is 6 mW; (d) The corresponding histogram of Raman intensity at 1620 cm−1 as shown in (c).
4. Conclusion
In conclusion, a method for Raman detection with high sensitivity was presented adopting a metallized DSF internally excited via in-fiber APB generated by a home-made generating system based on the acoustic-optical coupling technology. Simulation results demonstrate that the electric field intensity can be effectively improved by APB excitation compared with LPB excitation, because the strong gap-mode is uniformly generated between two adjacent Ag NPs on the surface of the DSF planar side. The Raman intensity of MB is tested by the DSF based SERS-active substrate. In the case of LPB exitation, the maximum Raman intensity and the minimum Raman intensity are achieved at two orthogonal polarization direction (∼100° and 10°). When in-fiber APB was adopted as the excitation source, the Raman intensity is ∼4.5 times stronger than that of LPB excitation with the optimal polarization direction. The detection sensitivity is 10−9 M under the APB illumination, indicating considerable SERS-activity of the APB-excited DSF configuration. Moreover, the time stability are also examined to be guaranteed. This method of combining the metallized DSF and APB excitation is promising to expand the applications in remote, label-free, and in-situ detection.
Funding
National Natural Science Foundation of China (11974282, 61675169, 61675171, 91950207); Fundamental Research Funds for Central Universities (310201911cx026, 3102019JC008).
Disclosures
The authors declare no conflicts of interest.
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