1. Introduction

In 2018, Arthur Ashkin received a Nobel Prize for his discovery on trapping dielectric microparticles using a focused laser beam known as optical tweezing [1]. The phenomenon of optical tweezing was used in developing optical tweezers (OTs), which became a powerful tool in physics as well as in biology. Optical tweezing has been used to trap dielectric particles, metallic particles, and bacteria [1–3]. A rough metallic surface is required to develop a surface-enhanced raman spectroscopy (SERS) substrate. The optical tweezing process has been used to tweeze nanomaterials on the surface of a tapered fiber, which can be used as a SERS substrate [4–6]. Further, a number of research articles reported on the development of SERS substrate, using self-assembled nanoparticles, aggregated GNRs, evanescent waves and electrostatic attraction [7–10]. Additionally, the optical tweezing process was used to develop optical fiber tip for field-enhanced second harmonic generation [11].

In the present investigation, GNRs were used to develop a plasmonic structure on the surface of a tapered fiber, which can be used as a SERS substrate. The use of GNRs was advantageous as the anisotropic shape of the GNRs (aspect ratio = 6.4; length = 64 nm and diameter = 10 nm) gave two localized surface plasmon resonance (LSPR) frequencies (transverse at 532 nm and longitudinal at 1064 nm), which can be excited by two different lasers. In the experiment, the GNRs on the tapered fiber were excited using a laser close to the transverse LSPR wavelength (532 nm), which enhanced the interaction of the chemicals adsorbed to the GNRs, and increased the electric field at the dielectric and metallic interface [12,13].

It was also discussed how the GNR distribution varies with both the length of the tapered fiber and the tweezing conditions. The effect of single or multiple tweezing using either the same wavelengths or different wavelengths on the GNR distribution has been explored. The preliminary experimental results based on the plasmonic structure were presented. Further, we presented an optical image of the fiber probe.

2. Experimental results and discussions

The tapered fiber was manufactured using a multimode step-index fiber of core and cladding diameters 110 µm, and 125 µm, respectively by a dynamic etching process [6,14,15]. The fields in the tapered region interacted with the GNRs in the external medium during the tweezing process. The gradient and scattering forces both occurred; however, the GNRs deposited on the tapered fiber surface when the gradient force dominated over the scattering force [16]. The magnitude of these forces depends on how many modes are supported by the fiber and how those modes interfere in the external medium. Thus, the interference of modes decides the distribution of the GNRs on the tapered fiber surface. The large core diameter multimode fiber assisted in developing a long tapered length with the core exposed to the surrounding medium, which increased the evanescent fields, and thus a large surface area of the tapered fiber could be covered with GNRs.

Figure 1 shows the distribution of the GNRs on the tapered fiber of length 1.69 mm manufactured by a dynamic etching process when tweezed with a 1064 nm laser of 8.5 mW power. It is clear from the figure that the GNRs distributed in the form of rings with varying periodicity along the tapered region. It is important to note that the number of modes supported by the fiber at different diameters was not the same. Thus, the resultant field at a particular location, which determined the gradient and scattering forces, was different. Further, the insets of Fig. 1 show the variation in the density of the GNR rings in three different segments of the tapered fiber. It has been observed that at lower and higher diameters, the density of the rings is lower than in the middle region.

 

Fig. 1. SEM images showing the distribution and density of GNR rings in three different segments (a) near the tip (∼100 nm – 16 µm), (b) middle region (16 µm – 30 µm), and (c) end region (30 µm – 45 µm) of the tapered fiber.

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Figure 2 shows the experimental setup for the tweezing process. We used a He-Ne laser to obtain a 632 nm wavelength. Light from the laser transmitted through a microscope objective, and the untapered end of the fiber was placed at the focus of the objective. A fiber optic positioner was used to couple the laser light efficiently to the untapered end of the fiber. Further, a pigtailed semiconductor diode laser was used to obtain a 1064 nm wavelength, and a fiber connector was used to couple light at the untapered end of the fiber. The tapered end of the fiber was mounted to fiber holder fitted with a translation stage, and then lowered into the GNR solution.

 

Fig. 2. Experimental setup for tweezing GNRs with a He-Ne Laser.

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Figure 3(a) shows the number of modes supported by the tapered fiber at various diameters. The number of modes at a specific diameter and interference of their fields in the surrounding medium determines the resultant field and thus decides the distribution of GNR rings (dense or light) along the length of the tapered fiber. Figure 3(b) demonstrates the fact that the resultant field determines the GNR distribution at different locations. For example, no GNRs are present at some locations (diameter 17.9 µm), but it can be seen that when the diameter changes slightly, GNRs are once again present (at 19.9 µm and 20.9 µm). By comparing Figs. 3(a) and 3(b), it is clear that because the fiber supports a different number of modes at different diameters, it is the number of modes present at a particular location and the resultant interference of those modes, which decides the distribution of GNRs. The amount of light transmitted to the tapered end, and thus the ability to tweeze, depends on the coupling of the laser at the untapered end.

 

Fig. 3. (a) The number of modes supported by the fiber vs. the diameter of the fiber at different locations of the tapered fiber, and (b) distribution of GNR rings between 17.9 µm and 20.9 µm diameter range, when tweezed with a 1064 nm laser of 8.5 mW power [18].

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Further, not all modes are equally excited by the light coupled to the untapered end. The modes with strong evanescent fields play an important role in the interference process. Good coupling is required to achieve optimum GNRs distribution. The tapered fiber (Refractive index = 1.4578) was dipped into the GNRs solution (Fig. 2), a lower index medium (Refractive index = 1.33) which acts as a cladding, surrounded the tapered fiber. As such, the core-cladding index difference was large, and the weakly guiding approximation was not valid. Thus, the vector wave equation was solved to find the number of modes [17,18].

To manipulate the GNR distribution structure on the tapered fiber surface, GNRs were tweezed twice consecutively with a 1064 nm laser (Fig. 4(a)). GNR rings with smaller periodicity compared to the single tweezing case were observed. Although the authors were able to obtain more GNRs distributed along the length, the shape of the GNRs were destroyed in some places. This is due to the excessive heat produced due to the absorption of the 1064 nm laser by the GNRs deposited on the tapered fiber after the first tweezing. A lower input laser power during the second tweezing process can eliminate the damage. However, GNRs should be intact to take advantage of their excitation at the LSPR frequency.

 

Fig. 4. (a) SEM image showing dense and light ring structure of GNRs on the tapered fiber when tweezed twice consecutively with a 1064 nm laser, and (b) SEM image showing the ring structure of GNRs at a smaller diameter when tweezed twice consecutively with 1064 nm and 632 nm lasers.

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Tweezing was repeated with two different wavelengths, 1064 nm and 632 nm (Fig. 4(b)) consecutively and results were compared with the distribution of GNRs when either the 1064 nm or 632 nm wavelength laser alone was used for tweezing. The periodic structure was observed to begin at a smaller diameter (∽ 4 µm), whereas in the case of single tweezing with a 1064 nm laser, consistent and dense rings were observed to begin at a larger diameter (∽ 16 µm). Also, it was explained that the periodic structure of GNRs on the tapered fibers due to tweezing by the 1064 nm laser act as a waveguide for the 632 nm light to propagate without leaking and thus allowed more power to propagate through the fiber closer to the tip. Therefore, the modes at smaller diameters have enough power to produce a gradient force to pull the GNRs on the fiber surface. Further, the interference pattern at the smaller diameter is different from that at the larger diameter, as the number of modes supported by smaller-diameter fiber is lower compared to at the larger diameter. Thus, we obtained different periodicity in the distribution. As the larger portion of the tapered fiber is covered with GNRs, it can act as an excellent substrate for the enhancement of the Raman signal.

The tapered fiber was investigated by SEM and it was found that the surface of the tapered fiber was rough, resembling potholes. For tweezing, 250 µl of colloidal GNR solution was placed on a microscope coverslip, the tapered fiber was dipped into the solution, and laser light was coupled at the untapered end of the fiber (Fig. 2). During the tweezing process, GNRs filled those holes, and the rods were held in place by Van der Waals force after the laser was turned off. The tapered fiber was removed from the fiber holder when the solution dried. When the tapered fiber was put back into the solution for a second time for tweezing, it was expected that some of the GNRs would go back to the solution, or the distribution of the GNRs would be disturbed. But the effect was not noticeable as the coupled laser also produced gradient and scattering forces. To further verify those results, the authors dipped a tapered fiber, after tweezing, into the GNR solution and left it for an hour. The SEM image does not show any significant change in the distribution of the GNRs. Thus, the insertion of the tapered fiber in the GNRs solution for double tweezing did not disturb the periodic structure of the GNRs on the tapered fiber.

To investigate the effect of the tapered fiber length and cone angle on the distribution of GNRs, when tweezing with a single wavelength (1064 nm), two samples were prepared with lengths 1.49 mm and 0.804 mm. In both cases, a periodic distribution of the GNRs was obtained. As can be seen in Fig. 5, (a) segment of approximately 54 µm has been studied for each fiber length. This segment began at a 13 µm (approximately) diameter in each case to compare the number of modes in that specific portion of the probe. Further, in Fig. 5(a) (length = 1.49 mm), the number of modes supported by the fiber at the beginning (13 µm diameter) and end (8.60 µm diameter) of the 54 µm region are 42 and 28, respectively. The number of modes leaking out of the fiber along this section were thus 42-28 = 14. In Fig. 5(b), (length = 0.804 mm) the number of modes supported by the fiber at the beginning (13 µm diameter) and end (3.42 µm diameter) of the 54 µm region are 42 and 10, respectively, and the number of modes leaking out of the fiber along this section were thus 42-10 = 32. So, when the length of the tapered fiber is short and the cone angle is large, the number of modes leaking out of the fiber is quite large (i.e. 32), which means high power is present in the surrounding medium (GNR solution), which can destroy the shape of the GNRs. From experimental data, it has been observed that GNRs were not destroyed when the length of tapered fiber was greater than 1.0 mm. Also, by controlling the coupling of laser light at the untapered end (Fig. 2), we can keep the GNR's shape intact. In addition, it was experimentally observed that by decreasing the tapered length, GNR rings can be found at diameters of less than 1 µm, similar to the result seen when tweezing a longer tapered fiber multiple time. Both of these different techniques fulfill the goal of covering the maximum area of tapered fiber with GNR rings, which minimizes the probability of signal leakage near the tip and makes it a more attractive SERS substrate.

 

Fig. 5. SEM images of two different samples with GNR distribution tweezed by a 1064 nm laser of 8.5 mW power for (a) Taper length = 1.49 mm (b) Taper length = 0.804 mm.

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We investigated the plasmonic structure optically. To study the distribution of the GNRs on the tapered fiber a He-Ne laser (632 nm) was coupled to the untapered end of the fiber. The tapered end was placed under the microscope objective. Figure 6(a) was taken using a camera placed at the eyepiece of the microscope, and the distribution of GNRs was visible. The same sample was tested with SEM, and the image confirms the distribution of GNRs (Fig. 6(b)).

 

Fig. 6. Images taken using (a) camera and (b) SEM of the same tapered fiber coated with GNRs when tweezed with a single wavelength (1064 nm) laser.

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Figure 7 shows the Raman spectra taken with and without GNRs on the surface of the fiber probe. Two peaks were visible when the fiber probe with GNRs on its surface was used to obtain the Raman spectrum [19]. The graphite powder was sprinkled, which adsorbed on the surface of the tapered fiber covered with GNRs, and without GNRs. In both cases the amount of graphite on the fiber probe was insignificant due to the extremely small probe size. The black spectrum was obtained with 1 s exposure, where the red spectrum is obtained with 5 s exposure when the graphite powder was sprinkled on a bare fiber probe. Thus, the presence of GNRs increased the intensity of the Raman scattered light.

 

Fig. 7. Comparing the Raman Spectra of graphite (collected with a Renishaw Raman Spectrometer) produced by a 532 nm laser with power 5 mW, with GNRs on the surface of the probe (1 s exposure), and without GNRs on the surface of the probe (Red – with 5 s exposure).

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The experiment was repeated with R6G. A mixture of 0.2550 mg of R6G and 230 µl of GNR solution was prepared before tweezing with a 1064 nm laser. Figures 8(a) and 8(b) compare the results with and without GNRs present on the probe. Figures 8(a) and 8(b) were obtained with 1 s exposure. In Fig. 8(a), only a few peaks are visible, however in Fig. 8(b) all peaks reported in the literature are visible. Thus, a significant enhancement was observed when GNRs were present, because the number of peaks in the case of both R6G and Graphite are more numerous and distinct, and overlap with previously characterized spectra [20]. Thus, the presence of GNRs enhanced the Raman signal for both Graphite and R6G.

 

Fig. 8. Raman Spectrum (collected with a Renishaw Raman Spectrometer) using (a) Fiber + R6G and (b) Fiber + GNRs+ R6G.

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3. Conclusion

In conclusion, the advantages of the developed probe are as follows: (i) the fiber probe is compact and cost-effective to manufacture; (ii) the GNR distribution structure can be manipulated by using different tweezing wavelengths or probe length, and finally (iii) the Raman signal can be collected from the untapered end of the fiber. Overall, the fiber probe covered with GNRs in a periodic structure may be an excellent SERS Substrate for the detection of chemicals at lower concentration.