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Abstract
A waveguide structure with two air holes is used as surface-enhanced Raman scattering (SERS) probe. Raman scattering signal collected via waveguide coupled and free space ways is completely analyzed with optical Green function and quantum optics method. The parameters including averaged enhancement factor (AEF) and waveguide collection efficiency are discussed. The corresponding optical mode analysis is carried on with COMSOL solution. Prepared Ag nanoparticles and R6G (Rhodamine 6G) are filled into the air holes with vacuum pressure method. Experiments on different R6G concentrations, different fiber lengths, and different Ag densities are carried on to investigate the properties of the fiber SERS probe. Comparative discussion confirms a detection limit of 10−15 M, an enhancement factor of 108, and a sample of 8 cm length and volume ratio of 1:6 (Ag: R6G) having the best performance.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Surface-enhanced Raman scattering (SERS) has been widely used in many applications, such as chemical, biological, environmental, medical and optical detection [1,2]. Its enhancement mechanism is attributed to the electromagnetic enhancement (EM) and the chemical enhancement (CM). The former one is due to the localized surface plasmon resonance (LSPR) of the metal structure surface induced by the incident laser, up to 1014 [3]. The later one is due to the charge transfer between the metal surface and the analyte, much lower than the EM, almost 10∼102 [2]. Many researches have been carried on, in order to improve the performance of the metal structures. Many types of metal nano structures, such as metallic nanorings [4], nanoshells [5], nanorods [6], nanowires [7], nanostars [8], nanoclusters [9], and metallic composites [3] have been used as SERS substrates.
For requirements of a remote and convenient detection, considering the advantages of the waveguide structure, there have been many studies on waveguide-coupled SERS substrates [10–12]. Making use of the long interaction length of incident laser and the analyte, high Raman scattering signal collection efficiency, many types of fibers have been used as fiber SERS probes, such as photonic crystal fiber (PCF) [10], hollow core PCF [11], and soft polymer optical fiber [12]. Meanwhile, some waveguide structures, such as strip, ridge and slot waveguides have been improved as SERS substrates [13–16].
We know that longer waveguide leads to longer interaction length between the incident laser and the analyte, which is positive to enhance Raman scattering. While metal nanostructure has more absorption on the light, leading to light attenuation along the transportation length, which is passive to Raman enhancement. So, there are two questions. What is the optimized length of the waveguide? What is the effect of metallic nanostructures on the SERS properties in a waveguide-coupled system?
As for the pure waveguide structure, Dr. Helmy has reported a theoretical study on the slot waveguide used to enhance Raman scattering, showing that an integrated plasmonic slot waveguide can attain additional higher enhancement factor compared to optofluidic and planar dielectric waveguide structures [17]. As for metallic surface structure, Dr. Dezfouli proposed a self-consistent quantum optics approach to theoretically calculate enhanced Raman spectrum of the analyte detected by a shaped plasmonic system [18].
Based on these studies and problems mentioned above, there are two aims. One is to calculate Raman enhancement performances induced by different waveguide lengths, different densities of metallic nanostructure, and different concentrations of the analyte. The other is not only to complete the theoretical analysis on the waveguide-coupled SERS structure, but also to complement the corresponding experimental investigation. Herein, a fiber with two holes is as the waveguide structure, Ag nanoparticles are as plasmonic structure, and R6G (Rhodamine 6G) is as the analyte.
2. Theoretical analysis
2.1 Waveguide-coupled system and free space system
Two types of Raman collection system are shown in Fig. 1, a waveguide-coupled SERS system (Fig. 1(a)) and a free space system (Fig. 1(b)).
For a waveguide-coupled SERS system, Raman intensity is dependent on several factors: the WG structure (L, n0, n1, n2), the analyte position $({\vec{r}_0},{l_1})$, the analyte characteristics (Raman tensor, the scattering cross section σ, density ρ, etc.), the metallic distribution/density, and the collection methods (backward / forward). The parameters n0, n1 and n2 are the refractive indexes of environment condition, waveguide and cladding layer, respectively. In Fig. 1(a), the inside of the waveguide can be filled with the analyte, so the light-matter interaction volume can be expressed as Vm=π(Dm/2)2L, where Dm is the diameter of the WG. Note that, in our two-air hole fiber sample mentioned in section 3, the light interaction volume is determined by the size of the air hole.
For a free space system, the light-matter interaction volume within Gaussian beam waist can be expressed as V0=π(D0/2)2b, where D0 is the waist diameter, and b is the depth of focus. The total collection efficiency can be expressed as [18]
2.2 Waveguide-coupled effect
2.2.1 Pure waveguide system
For an infinitesimal waveguide segment dz in pure waveguide system (without metallic plasmonic), the conversion efficiency can be expressed as [19]
${\tilde{A}_{eff}}$ is the effective waveguide mode, defined as ${\tilde{A}_{eff}}\textrm{ = }\int_S {w(r,\omega )dr} /\max \{ w(r,\omega )\}$. Where w is the waveguide mode energy density in the cross section, it can be calculated by the commercial software COMSOL mode analysis solver.
We assume that scattered molecules are distributed uniformly within the waveguide with a density ρ. For emitters during a thickness dz orthogonal to the waveguide, the efficiency is given by [18]
When the input power is Pin, in the forward/backward propagating configuration, only half of the collected power (Pcol) is measured at the output facet with an output coupling efficiency ηD, Meanwhile, a pump laser is incident on the input facet of the waveguide with a coupling efficiency ηC. For a waveguide L, the conversion efficiency for backscattered Raman with analyte uniform density (ρ) is calculated bySo, the enhancement factor of waveguide system compared to free space system can be expressed as FWG=ηWG /ηFS.
2.2.2 Waveguide-coupled SERS system
For a waveguide-coupled SERS system, due to the influence of metallic structure, we pay attention to two parameters. One is the averaged enhancement factor (AEF) induced by the localized electro-field effect of the metallic structure [20]. The efficiency induced by metallic structure compared to free space system (ηSERS) can be expressed as ηSERS=ηFS${\cdot}$AEF. The other (ηWGC) is induced by optical mode field re-distribution within the waveguide. So, the total waveguide efficiency would be expressed as
Where the collection efficiency ηWGC induced by the waveguide, it also can be calculated by [18,21]So, the enhancement factor of a waveguide-coupled system compared to a free space system without metallic structure and with metallic structure is described as FWGC=ηWGC/ηFS, FWG&SERS=ηWG&SERS /ηFS, respectively.
3. Preparation and characterization
3.1 Preparation of filled fiber probe
Firstly, Ag sol is prepared with a traditional simple hydrothermal method [22], which can obtain uniform and stable Ag nanoparticles (AgNPs). The volume and concentration of AgNO3 used is 1 ml and 0.1 M/l. The volume and concentration of sodium citrate is 1 ml and 1 g/100 ml, respectively. The density of AgNPs solution after chemical reaction is calculated to be ∼3.143×1014 particles/l, with an estimated diameter of 25 nm. In order to remove remains, the Ag solution is centrifugated three times, and the density of prepared AgNPs solution for further use is ∼9.429×1014 particles/l. The quantity of AgNO3 leads to different size of Ag nanoparticles. R6G solutions with different concentrations (from 10−15 M to 10−8 M) are prepared. The mixture of R6G and Ag sol with different volume ratio is prepared. As for the mixture solution with volume ratio of 1:3 (Ag: R6G), the estimated AgNPs density is ∼2.357×1014 particles/l. In order to mix enough, the mixture was taken by an ultrasonic oscillation for 2 hours for further use.
We choose a typical fiber as the waveguide structure. It is made of pure silica with two circle air holes, a core diameter of 9 µm (cladding diameter of 125 µm), an air hole diameter of 30 µm, and a gap of 20 µm. In order to fill the solution into the holes, a syringe is used, shown in Fig. 2(a). There are three steps to prepare the fiber SERS probe. Firstly, the fiber is cut into 10∼20 cm segment with both ends cleaved carefully. One segment is inserted into the exit end of a syringe, and the exit end is sealed with flue. The other end of the fiber is dipped in the mixture solution. Secondly, the prepared mixture will be filled into the air holes with the help of the syringe needle (pressure setup). Thirdly, the fiber is cut into different lengths (4, 6, 8, 10 cm) waiting for Raman measurements, and the mixture would not flow out due to the function of surface tension.
Fig. 2. (a) Preparation of the fiber SERS probe, images of the fiber, the fiber filled with solution, and samples are also shown. (b) Raman measurement setup.
3.2 Raman measurement
Raman measurement setup is shown in Fig. 2(b), with a laser of 532 nm (power of 50 mW with 10% power filter) as the incident light, and a 10 × objective lens (numerical aperture of 0.25, work distance of 10.6 mm) with function of coupling pump light into the fiber probe and Raman spectrum collected lens. In order to reduce the heat, an integration time of 5 s is used. In order to guarantee well alignment, an optical microscope and a three-dimensional (3D) fiber holder (displacement accuracy of 0.1µm) is used. The fiber core center is aligned to the optical axis of the objective lens. For comparative analysis, all Raman spectral signals are averaged. Note that we prepared several (N1) fiber samples with the same length, Raman tests on one fiber sample are carried on several (N2) times, so the averaged result is coming from all tests (N1× N2).
4. Results and analysis
4.1 Comparative analysis on different samples
The fiber samples with a length of 8 cm are used. The concentration of pure R6G solution used to fill into the fiber and the glass tube is 10−3 M. While R6G concentration used in the mixture is 10−9 M. The volume ratio in the mixture is 1 (Ag sol):3 (R6G).
Raman signal of the fiber itself (data E) is collected (Fig. 3(a)), as the background information. The corresponding Raman signals of R6G are shown in Fig. 3(a), data A for the mixture in the fiber, data B for the mixture in the glass tube, data C for pure R6G in the fiber, data D for pure R6G in the glass tube, respectively. In Fig. 3(b), there are the enlarged signals of data C and data D.
Fig. 3. (a) Raman intensities of different samples, data A: fiber with mixture solution (R6G of 10−9 M), data B: glass tube with mixture (R6G of 10−9 M), data C: fiber with R6G (10−3 M), data D: glass tube with R6G (10−3 M), data E: fiber itself signals. For seeing clearly, enlarged signals are shown, the corresponding enlargement factor is listed. (b) The detailed signals of data C and data D in (a), in order to see more clearly. The mode distributions of waveguides calculated by COMSOL are shown at the top of each figure.
As for pure R6G solution, the enhancement factor FWG induced by the fiber (data C/D) is almost 5.36 at ∼1364 cm−1. As for the mixture, the enhancement factor FWG induced by the fiber (data A/B) is almost 21.06, 24.35, 19.87, at ∼610, ∼773, and ∼1364 cm−1, respectively.
Herein, based on data B and data D, the calculated AEF induced by AgNPs is 5.66 × 106. Taking consideration of the waveguide and Ag effects, the calculated total enhancement factor FWG&SERS is 1.45 × 108 at ∼1364 cm−1.
As for comparative analysis, we calculate the theoretical efficiency of a free space system. Theoretical density ρ of R6G is 6×1019 molecules/cm3 for concentration of 10−9 mol/L. σ is 6.7, 7.6, 18×10−28 cm2/sr/molecule at∼610, 773, 1364 cm−1[20]. αR and αP is 0.176/cm, respectively [23]. Based on Eq. (1), D0, b, θ, calculated $\eta _D^\prime $ and $\eta _0^\prime $ is 1.3×10−6 m, 7.96×10−5 m, 0.2527 rad, 0.0159 and 6.66×10−14, respectively. So, the calculated theoretical value of ηFS is 7.82×10−13.
For a waveguide system, the coupling efficiency ηC and ηD is estimated to 1. The optical modal field results are shown in Figs. 3(a) and 3(b) with COMSOL multiphysics, ignoring the coupling loss and detecting loss. The corresponding calculated value of η0 is 2.08∼2.85×10−5 (waveguide structure) and 4.23×10−4 (waveguide-coupled SERS structure). So, theoretical ηWG and ηWG&SERS is 1.82×10−12 and 4.06×10−11 with a fiber length of 8 cm, respectively.
Based on Eqs. (3)–(7), the calculated theoretical enhancement factor is about 4.15∼5.87 (FWG, experimental value: 5.36) for pure R6G solution, 29.02 (FWGC, experimental value: 25.58) for the mixture at ∼1364 cm−1, respectively; the calculated total enhancement factor is 1.64 × 108 (FWG&SERS, experimental value: 1.45 × 108). The experimental enhancement factor is slightly smaller than the theoretical value because of the inevitable actual coupling loss and detecting loss.
4.2 Detection limit
The mixture with different R6G concentration from 10−15 to 10−8 M is filled into the fibers. The volume ratio in the mixture is still 1 (Ag sol):3 (R6G). Raman signals are shown in Fig. 4(a). In order to see clearly, some signals of lower concentration (10−15 to 10−11 M) are enlarged, and the enlarged factor is listed beside the corresponding spectrum line. The characteristic peaks of 10−15 M R6G are still observed at ∼610, 773, 1187, 1312, 1364, 1509, 1575, and 1652 cm−1, attributing to the C-C-C ring in-plane, out-of-plane, C-H in-plane bending vibration, and to symmetric models of C-C in-plane stretching vibrations [7].
Fig. 4. (a) The averaged Raman intensities of R6G with different concentrations from 10−15 M to 10−8 M. (b) Averaged Raman signals of 10−9 M R6G. The intensity (c) at ∼610 cm−1 and (d) at ∼773 cm−1 changes with the concentration. The linear fitting is shown.
The averaged Raman intensity of 10−9 M R6G is illustrated in Fig. 4(b). We noticed the variation of the Raman intensity. There could be two factors. One is the different enhancements induced by a non-uniformity distribution of Ag nanoparticles, and a random distribution of R6G molecules caused by the Brownian motion. The handmade processes during the measurement is also an inevitable factor. So, the averaged values are fully used to reduce the impacts of two factors.
Shown in Figs. 4(c) and 4(d), with the increase of concentration, Raman intensity increases orderly. The R2 of linear fitting is 99.6% and 98.1% for the data at ∼610 and ∼773 cm−1, respectively, exhibiting a good linear relationship, which is consistent with Eqs. (5) and (7).
4.3 Effects of different lengths
Fiber samples with different lengths are prepared to investigate the effect of the fiber length. We prepared fibers with length of 4, 6, 8, and 10 cm. The pure solution (R6G of 10−3 M) and the mixture (R6G of 10−9 M) is filled into the fibers. The normalized Raman signals are shown in Figs. 5(a) and 5(b). For a comparative analysis, the intensity of fiber sample with 10 cm at ∼1364 cm−1 is used to normalize other data.
Fig. 5. Normalized Raman signals of fiber samples filled with (a) pure R6G solution and (b) the mixture. The theoretical and experimental enhancement factors of different fiber lengths, filled with (c) pure R6G solution and (d) the mixture.
Based on Eq. (5), the theoretical FWG is shown in Fig. 5(c) in red region. The averaged experimental enhancement factor FWG is 3.48, 5.02, 5.36, 7.27 for fiber lengths of 4, 6, 8, 10 cm, at ∼1364 cm−1. The experimental enhancement factor FWG increases linearly with the length of the fiber in general, which shows a good match with the theoretical calculation.
Based on Eq. (7), the theoretical FWGC at 610, 773 and 1364 cm−1 is shown in Fig. 5(d) in dashed line. The experimentally averaged enhancement factor FWGC is 5.57, 12.83, 21.06, and 35.45 for fiber lengths of 4, 6, 8, and 10 cm, at ∼610 cm−1.
4.4 Effects of Ag with different densities
In order to investigate the effects of different densities of Ag, the fiber samples are filled with different mixtures with volumes ratioes (Ag: R6G) of 1:3, 1:6, 1:9, 1:12 and 1:15. Herein, R6G concentration is 10−9 M. As for comparison, the corresponding mixtures are also detected in glass tube. Raman signals are shown in Figs. 6(a) and 6(b). Raman intensities at ∼610, ∼771 and 1361 cm−1 are shown in Fig. 6(c).
Fig. 6. Raman signals when the mixture filled in (a) glass tube, (b) the fiber; (c) Raman signals at ∼610, ∼771 and 1361 cm−1, “Fiber” and “W” means the mixture filled in the fiber and without the fiber (in glass tube); (d) absorbance of different mixtures.
The volume ratio 1:6 has the largest Raman intensity. As we mentioned above, there are two effects induced by Ag nanoparticles. One is to improve the enhancement by its LSPR. The other is to reduce the light power attributed to metal absorption and transportation loss. The former one is a positive effect to Raman enhancement; while the later one is a passive effect. So, the samples with volume ratio of 1:9, 1:12 and 1:15 have lower Raman enhancement, due to less Ag LSPR. The sample with volume ratio of 1:3 also has lower Raman enhancement, because of larger absorption of more AgNPs. In order to confirm our analysis, we measure the absorbance of different mixtures, shown in Fig. 6(d), the mixture with volume ratio of 1:3 has larger absorbance than that of others.
4.5 Repeatable property
In order to investigate the repeatability of our fiber SERS probes, firstly, the mixtures are sucked out after Raman measurement using the pressure setup. Then the sodium borohydride solution is cyclically filled and sucked out three times, in order to clean the organic molecules. Thirdly, deionized water is also filled and sucked out three times, to clean the remains. The Raman signals tested first time (before cleaning) and after cleaning are shown in Fig. 7. The variation of two measurements is almost within 0.75% to 18.7% at Raman peaks.
5. Conclusions
In conclusion, our SERS probes show a good performance, with R6G limit detection of 10−15 M, EF of 108, and repeatable property. The best volume ratio of Ag to R6G in our samples is 1:6. As for waveguide-coupled SERS structures, the interaction length of the pump light and the analyte, an enhancement effect induced by metallic LSPR, and transportation loss induced by the waveguide coupling and metallic absorption should be simultaneously taken into consideration. In this paper, we only discuss one waveguide structure, with the development of nanofabrication technique, other strip, ridge and slot waveguide-coupled SERS structures will be analyzed in near future.
Funding
National Natural Science Foundation of China (61875024); Chongqing Outstanding Youth Fund (cstc2019jcyjjqX0018); Fundamental Research Funds for the Central Universities (CQU2018CDHB1A07).
Acknowledgements
We would like to thank Prof. Dr. Gong Xiangnan at Analytical and Testing Centre of Chongqing University for his help in Raman measurement.
Disclosures
The authors declare no conflicts of interest.
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