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By Yun Han, Siliu Tan, Maung Kyaw Khaing Oo, Denis Pristinski, Svetlana Sukhishvili,* and Henry Du*

The integration of microfluidics with photonics on a single platform using well-established planar device technology has led to the emergence of the exciting field of optofluidics.[1] As both a light guide and a liquid/gas transmission cell, photonic crystal fiber (PCF, also termed microstructured or holey fiber),[2–7] synergistically combines microfluidics and optics in a single fiber with unprecedented light path length not readily achievable using planar optofluidics. PCF, an inherent optofluics platform, offers excellent prospects for a multitude of scientific and technological applications.[8–14] The accessibility to the air channels of PCF has also opened up the possibility for functionalization of the channel surfaces (silica in nature) at the molecular and the nanometer scales, in particular to impart the functionality of surface-enhanced Raman scattering (SERS) in PCF for sensing and detection.[15–18] SERS, an ever advancing research field since its discovery in the 1970s,[19–21] has tremendous potential for label-free molecular identification at trace or even single-molecule levels due to up to 1014 increase in the Raman scattering cross-section of a molecule in the presence of Ag or Au nanostructures.[22–24] Seminal work has been reported on the development of 1D[25] and 2D[26,27] SERS substrates for a variety of sensing applications.[28] The use of 3D[29–32] geometry, i.e., substrates obtained by the deposition of noble nanoparticles onto porous silicon[29] or porous aluminum membrane[30–32] offered additional advantages of increased SERS intensity due to increased SERS probing area,[29] as well as the membrane waveguiding properties.[30–32] Specifically, several orders of magnitude higher SERS intensity, affording pico- or zeptogram-level detection of explosives, has been demonstrated with porous alumina membranes containing 60-mm-long nanochannels, as compared to a solid planar substrate.[30–32] SERS-active PCF optofluidics, as a special fiber optic SERS [*] Prof. H. Du, Y. Han, Dr. S. Tan, M. K. K. Oo Department of Chemical Engineering and Materials Science Stevens Institute of Technology Hoboken, NJ 07030 (USA) E-mail: hdu@stevens.edu Dr. D. Pristinski Polymers Division, NIST Gaithersburg, MD 20899 (USA) Prof. S. Sukhishvili Department of Chemistry, Chemical Biology, and Biomedical Engineering Stevens Institute of Technology Hoboken, NJ 07030 (USA) E-mail: ssukhish@stevens.edu

DOI: 10.1002/adma.200904192

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Towards Full-Length Accumulative Surface-Enhanced Raman Scattering-Active Photonic Crystal Fibers

platform, offers easy system integration for in situ flow-through detection, and, more importantly, much longer light interaction length with analyte, thus promising to open a new vista in chemical/biological sensing, medical diagnosis, and process monitoring, especially in geometrically confined or sampling volume-limited systems. Various attempts have been made over the last several years to integrate SERS with the PCF platform by incorporating Ag or Au nanostructures albeit inside a very limited segment (typically a few centimeters) of the fiber air channels. Examples include deposition of Ag particulates and thin films by chemical vapor deposition at high pressure[15] or coating of Ag and Au nanoparticles using colloidal solutions driven into the microscopic air channels via capillary force with backscattering as the typical data acquisition mode.[16,17] Building uniform SERS functionality throughout the length of the PCF while preserving its light guide characteristics has remained elusive as measured Raman intensity is a combination of the accumulative gain from Raman scattering and the continuous loss from nanoparticle-induced light attenuation over the path length. As a result, we have recommended and recently described in a brief study forward scattering as a more suitable detection mode to unambiguously assess the SERS-active nature of PCF.[18] To the best of our knowledge, this article is the first report of net accumulative SERS from the full-length Ag-nanoparticlefunctionalized PCFs. The finding has been enabled by a fine control of the coverage density of Ag nanoparticles and studies of a competitive interplay between SERS gain and light attenuation in the Raman intensity with PCFs of varying length. Using two different types of PCF, i.e., solid-core PCF and hollow-core PCF, we show that Raman gain in PCF prevails at relatively low nanoparticle coverage density (below 0.5 particle mm 2), allowing full benefit of accumulation of Raman intensity along the fiber length for robust SERS sensing and enhanced measurement sensitivity. Light attenuation dominates at higher nanoparticle coverage density, however, diminishing the path-length benefit. Shown in Figure 1 are cross-sectional scanning electron microscopy (SEM) images of solid-core PCF and hollow-core PCF used in this work. Also depicted in the figure is the light-guide process in the corresponding PCFs that contain immobilized Ag nanoparticles and are filled with aqueous solution throughout the cladding air channels for solid-core PCF and in the center air core only for hollow-core PCF. Note that light is guided via total internal reflectance in both cases. The presence of the aqueous solution in the cladding air channels does not fundamentally change the contrast of the higher index silica core and the lower index liquid-silica cladding in the solid-core PCF. The selective

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filling of the center air channel with the aqueous solution has turned an otherwise bandgap hollow-core PCF to an index-guided liquid-core PCF. SERS is generated by evanescent field of the propagating light in the silica core for solid-core PCF and by direct excitation of the propagating light in the liquid core for the solution-filled hollow-core PCF. To realize SERS-active PCF while preserving the light-guide property of the fiber, an important step is uniform incorporation of Ag nanostructures inside the air channels along the entire fiber length with controlled surface coverage density. Similar to work by others,[30,31] we are using polyelectrolyte-mediated surface immobilization of noble nanoparticles. However, in contrast to previous work where Au nanoparticles were attached to 60-mm-long nanocanal walls of porous alumina,[30,31] we focus Figure 1. Cross-sectional SEM images of solid-core PCF and hollow-core on controlling nanoparticle density in a wide range, and achieving PCF and schematic illustrations of light guiding in the corresponding low and uniform coverage density of Ag nanoparticles liquid-filled structures with immobilized Ag nanoparticles. Scale bar in the SEM image: 10 mm. inside much longer air channels of PCFs. To mediate nanoparticle attachment, polyallylamine hydrochloride (PAH) was first allowed to adsorb at the surface of the air channels, served as anchoring site for Ag nanoparticles. Ag nanoparticles 35  5 nm in size with zeta potential of approximately 35 mV were synthesized using a modified Lee and Meisel method.[33] Surface functionalization of the fiber air channels and immobilization of Ag nanoparticles (Fig. 2A) were carried out in a custom-built pressure cell maintaining a 200 psi pressure differential between the entrance end (submerged in a solution) and the exit end (exposed to the laboratory ambient) of the PCF. In hollow-core PCF, only the hollow core underwent the various treatment steps by selectively sealing the cladding air channels at both distal ends of the fiber using a fusion splicer. The PCFs typically 25–30 cm in length were first filled with PAH solutions at pH 4–7 for polymer self-assembly on the air channels, then with Ag colloidal solution at pH 5.6. Intense rinsing with Milli-Q water was applied after each deposition step to remove unbound PAH or Ag nanoparticles from the channels. The negative surface charge of silica, whose charge density increases with pH >3, affords control over the amount of adsorbed polycation and thus the availability of binding sites for Ag nanoparticles.[34] Ag nanoparticle coverage density strongly correlates with the solution pH used during PAH adsorption step, with more nanoparticles immobilized in the air channels at higher pH values of the PAH assembly (Fig. 2B). We were specifically Figure 2. A) Molecular- and nanometer-scale modification leading to controlled immobilization interested in exploring the lower limit of Ag of Ag nanoparticles in the air channels of the PCFs. B) Effect of the deposition pH of PAH solution nanoparticles (0.1–5 particle mm 2) achieved (2 mg mL 1, deposited for 20 min) on nanoparticle coverage density. Ag nanoparticles were at PAH deposition pH <6.2 as we revealed attached to the PAH-modified air channels using 1012 particle mL 1 solution for 30 min. Error bars indicate standard deviation from six measurements taken at various sections along the fiber solid evidence that higher nanoparticle length. C,D,E,F) SEMs of immobilized Ag nanoparticles in the cladding air channels of solid-core coverage density effectively terminates the waveguide property of PCF. SEM microscopy PCF and hollow-core PCF, respectively.

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Importantly, for planar substrates[37] with the same Ag nanoparticle coverage density, no SERS could be observed for a much higher R6G concentration of 10 5 M. The effect of PCF length on SERS intensity was evaluated in fiber cut-back experiments. PCFs with Ag nanoparticles immobilized to various particle coverage densities were filled with 10 6 M R6G. The dependence of the measured SERS R6G peak intensity at 1351 cm 1 on the fiber length of solid-core PCF and hollow-core PCF is shown in Fig. 3C and Fig. 3D. In both cases, SERS intensity increased with the fiber length at the lower end of the Ag nanoparticle coverage (below 0.5 particle mm 2), indicating a net signal accumulation along the optical path. This demonstrates a new feature enabled by the full-length functionalized PCF, i.e., an increase in sensitivity in SERS sensing via a facile increase in the PCF length. Note that the Raman signal declined with the fiber length at higher nanoparticle densities. The pattern is a result of the interplay between accumulative Figure 3. A,B) SERS spectra of various R6G solutions using 20 cm long PCFs coated with Ag Raman signal gain and the progressive nanoparticles: A) solid-core PCF, 0.1 particle mm 2; and B) hollow-core PCF, 0.2 particle mm 2. scattering and absorption loss of both C,D) SERS intensity of 10 6 M R6G as a function of fiber length for solid-core PCF (C) and the excitation light intensity and the Raman hollow-core PCF (D). Raman excitation wavelength was 632.8 nm; the laser power 5 mW; and signal intensity as the path length increases. acquisition time 60 s. The Raman gain prevails at lower nanoparticle coverage densities where light attenuation is low, whereas at higher nanoparticle coverage densities the loss overwhelms the Raman gain over the fiber images of various segments of the PCF showed similar coverage length. Importantly, substantial gain in Raman sensitivity can be of discrete Ag nanoparticles (Fig. 2C–F). These results indicate achieved using longer PCF with very low nanoparticle densities that our experimental approach allows a high degree of control in (such as 0.1–0.2 particle mm 2). immobilization of Ag nanoparticles inside the microscopic air channels along the entire PCF length. Our SERS spectral measurements in the forward propagating The PCFs with immobilized Ag nanoparticles were filled with geometry have conclusively shown both the SERS-active and Rhodamine 6G (R6G) solution in Milli-Q water at 200 psi waveguiding features of the PCF when Ag nanoparticles are pressure differential. Raman measurements were performed controllably immobilized inside the air channels. They do not using a forward propagating transmission geometry at an allow, however, the assessment as to how the waveguide core excitation wavelength of 632.8 nm with a custom-built Raman (fundamental core mode) and the cladding structure (cladding setup.[18] Note that the use of 532-nm excitation wavelength, mode) contribute to the overall Raman intensity. We demonstrate here that hyperspectral imaging at the distal end of the PCF offers generally preferable for SERS measurements using individual Ag an excellent means of mapping the intensity distribution of a nanoparticles, was not pursued because of severe interference specific Raman line when overlaying with the cross-sectional from the fluorescence of R6G. Figure 3A and 3B compare the microstructure of the PCF. The hyperspectral Raman imaging SERS spectra of R6G solutions using a 20-cm-long solid-core PCF was performed on 20-cm-long SERS-active solid-core PCF and and hollow-core PCF containing immobilized Ag nanoparticles of hollow-core PCF, both coated with 0.5 particle mm 2 Ag particle coverage densities of 0.1–0.2 particle mm 2. In the case 1 of solid-core PCF, dominant peaks at 400–1200 cm (with the nanoparticles, also using forward-propagating Raman signal at the distal end of the PCF. A relatively high 10 5 M R6G main peak at 485 cm 1 assigned to the Si-O bending vibration) [14] originate from direct laser excitation of the silica core. In the concentration was used to allow hyperspectral Raman imaging as the employment of a line filter substantially suppresses the case of hollow-core PCF, however, Raman spectra were Raman signal intensity. Shown in the far left of Fig. 4A and dominated by intense water peak around 3381 cm 1 caused by Fig. 4B are Raman spectra obtained respectively from solid-core laser excitation of the liquid core. The silica bands present are PCF and hollow-core PCF with the cladding air channels (for weak as expected. With both PCF types, peaks at 1307, 1351, 1509, solid-core PCF) or the hollow core (for hollow-core PCF) filled 1570, and 1645 cm 1 are associated with characteristic aromatic with 10 5 M R6G solution. The right three images in Figure 4A C C stretching vibrations of R6G molecule.[35,36] The R6G 7 vibrational features were evident even at 10 M and became and 4B show Raman intensity distribution of silica (485 cm 1), more distinct and intense at higher R6G concentrations. R6G (1351 cm 1), and water (3381 cm 1) in solid-core PCF and

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Figure 4. SERS spectra of 10 5 M R6G and hyperspectral Raman images of silica, R6G, and water from solid-core PCF (A) and hollow-core PCF (B) with immobilized Ag nanoparticles of 0.5 particle mm 2 in coverage density. The measurements were done using forward-propagating geometry at an excitation wavelength of 632.8 nm, a power of 5 mW, and an acquisition time varying from 0.2 s for strong signal intensity to 20 s for weaker one.

hollow-core PCF, respectively. For solid-core PCF (Fig. 4A), the propagating core mode contributed primarily to the measured Raman intensity of both silica (by direct excitation) and R6G (via evanescent field interaction), with effective confinement of the SERS intensity of R6G to the light-guide core. Weak contributions by the triple-air-channel junctions (each junction is like a silica light guide surrounded by three air channels) of both silica and R6G are also seen in the hyperspectral images in the cladding region (middle two images in Fig. 4A). No water Raman image could be acquired from solid-core PCF. In contrast, Raman imaging of hollow-core PCF yielded unusual dashed ring-like Raman intensity distribution of silica surrounding the liquid core (left image in Fig. 4B), a clear evidence that the triple air channel junctions between the core and two adjacent air channels allowed light guiding, i.e., forward propagating of silica signal. The Raman distributions of R6G and water (right two images in Fig. 4B) were also confined in the liquid core, suggesting dominant core mode contribution to the measured Raman intensities. These results provide strong evidence that the liquid core is a robust waveguide, and that the SERS signal of R6G can be effectively coupled and forward-propagated along the liquid core. The Raman image of the water line appears to be larger in diameter than that of R6G due to the dispersion property of light propagation.[38,39] In summary, we have shown that the full-length SERS-active PCF optofluidic platform can be endowed the capacity of accumulation of Raman signal along the fiber length, demonstrating a novel way of enhancing SERS measurement sensitivity by a simple increase in the fiber length. We have shown the competitive interplay between SERS gain and light attenuation as the optical path length increases for a PCF containing immobilized Ag nanoparticles, with low particle coverage density being essential for a net accumulative Raman gain along the fiber

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length. Key to achieving the SERS-active PCF optofluidic platform lies in the high degree of control of nanoparticle coverage density via polyelectrolyte-based surface modification technique which can be applied to PCF far longer than the 20 plus cm fiber length we explored in this study. Of particular interest and as a future direction is the introduction of sparsely distributed nanoparticle clusters (e.g., dimers and trimers) as SERS hot spots inside the air channels of PCFs for further increase of detection sensitivity down to the single-molecule level. SERS-active PCF optofluidic platform is inherently easy for system integration, robust in light coupling and harvesting, and unparalleled in optical path length for label-free and sensitive identification. Its potential applications include fundamental studies of chemical, biological, and catalytic interactions in geometrically confined systems; chemical and biological sensing and detection; and in situ process monitoring. Specific examples include detection of contaminants in water for environmental protection, diagnosis of body fluids for healthcare, and in-line measurements of trace chemicals for quality control.

Experimental PCFs: Solid-core PCF (model RB087) and hollow-core PCF (model HC19-1550-01) used in our investigation were obtained respectively from OFS Laboratories, Somerset, NJ, USA and Crystal-Fibers A/S, Birkerød, Denmark. Specifically, solid-core PCF contains a 2.5-mm silica core surrounded by an array of 126 cladding air channels 3.5 mm in diameter. Hollow-core PCF has a large hollow core of 20 mm in diameter surrounded by 282 cladding air channels with an average diameter of 3.5 mm. Synthesis of Ag Nanoparticles: Ag nanoparticles were produced by a UV-assisted citrated reduction based upon modified Lee and Meisel recipe [34]. sodium citrate (8 mg) in water (0.8 mL) was dropwise added into

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[1] [2] [3] [4] [5]

[6] [7] [8]

[9] [10] [11] [12] [13] [14] [15]

[16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27]

Acknowledgements We thank Dr. Dennis J. Trevor of OFS Laboratories for supplying solid-core PCF, Dr. Yinian Zhu and Dr. Rainer Martini of Stevens for their valuable discussions, and Mr. Vassili Belov for his help with the preparation of the manuscript. This work was supported by NSF under ECS-0404002 and by the US Army ARDEC under W15QKN-05-D-0011. Received: December 7, 2009 Published online:

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[28] [29] [30] [31] [32] [33]

D. Psaltis, S. R. Quake, C. Yang, Nature 2006, 442, 381. J. C. Knight, Nature 2003, 424, 847. P. St. J. Russell, Science 2003, 299, 358. L. Dong, H. A. McKay, L. Fu, Opt. Lett. 2008, 33, 2440. J. Limpert, T. Schreiber, S. Nolte, H. Zellmer, A. Tunnerman, R. Iliew, F. Lederer, J. Broeng, G. Vienne, A. Petersson, C. Jakobsen, Opt. Express 2003, 11, 818. J. M. Dudley, G. Genty, S. Coen, Rev. Mod. Phys. 2006, 78, 1135. F. Benabid, F. Couny, J. C. Knight, T. A. Birks, P. St, J. Russell, Nature 2005, 434, 488. P. J. A. Sazio, A. Amezcua-Correa, C. E. Finlayson, J. R. Hayes, T. J. Scheidemantel, N. F. Baril, B. R. Jackson, D. J. Won, F. Zhang, E. R. Margine, V. Gopalan, V. H. Crespi, J. V. Badding, Science 2006, 311, 1583. J. Villatoroa, V. Finazzi, V. P. Minkovich, V. Prunerib, G. Badenes, Appl. Phys. Lett. 2007, 91, 091109. T. M. Monro, W. Belardi, K. Furusawa, J. C. Baggett, N. G. R. Broderick, D. J. Richardson, Meas. Sci. Technol. 2001, 12, 854. F. Couny, F. Benabid, P. S. Light, Phys. Rev. Lett. 2007, 99, 143903. S. Smolka, M. Barth, O. Benson, Appl. Phys. Lett. 2007, 90, 111101. S. O. Konorov, C. J. Addison, H. G. Schulze, R. F. B. Turner, M. W. Blades, Opt. Lett. 2006, 31, 1911. D. Pristinski, H. Du, Opt. Lett. 2006, 31, 3246. A. Amezcua-Correa, J. Yang, C. E. Finlayson, A. C. Peacock, J. R. Hayes, P. J. A. Sazio, J. J. Baumberg, S. M. Howdle, Adv. Funct. Mater. 2007, 17, 2024. Y. Zhang, C. Shi, C. Gu, L. Seballos, J. Z. Zhang, Appl. Phys. Lett. 2007, 90, 193504. F. M. Cox, A. Argyros, M. C. J. Large, S. Kalluri, Opt. Express 2007, 15, 13675. M. K. Khaing Oo, Y. Han, R. Martini, S. Sukhishvili, H. Du, Opt. Lett. 2009, 34, 968. D. L. Jeanmaire, R. P. Van Duyne, J. Electroanal. Chem. 1977, 84, 1. M. Fleischman, P. J. Hendra, A. J. McQuillan, Chem. Phys. Lett. 1974, 26, 163. M. G. Albrecht, J. A. Creighton, J. Am. Chem. Soc. 1977, 99, 5215. S. Nie, S. R. Emory, Science 1997, 275, 1102. K. Kneipp, Y. Wang, H. Kneipp, L. T. Perelman, I. Itzkan, R. R. Dasari, M. S. Feld, Phys. Rev. Lett. 1997, 78, 1667. A. Campion, P. Kambhampati, Chem. Soc. Rev. 1998, 27, 241. L. Qin, S. Zhou, C. Xue, A. Atkinson, G. C. Schatz, C. A. Mirkin, Proc. Natl. Acad. Sci. USA 2006, 103, 13300. J. C. Hulteen, R. P. Van Duyne, J. Vac. Sci. Technol. A 1995, 13, 1553. S. J. Lee, A. R. Morrill, M. Moskovits, J. Am. Chem. Soc. 2006, 128, 2200. H. Ko, S. Singamaneni, V. V. Tsukruk, Small 2008, 4, 1576. S. Chan, S. Kwon, T. W. Koo, L. P. Lee, A. A. Berlin, Adv. Mater. 2003, 15, 1595. H. Ko, V. V. Tsukruk, Small 2008, 4, 1980. H. Ko, S. Chang, V. V. Tsukruk, ACS Nano 2009, 3, 181. S. Chang, H. Ko, S. Singamaneni, R. Gunawidjaja, V. V. Tsukruk, Anal. Chem. 2009, 81, 5740. P. C. Lee, D. Meisel, J. Phys. Chem. 1982, 86, 3391.

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AgNO3 solution (1 mM, 40 mL) in a glass beaker (50 mL) under continuous stirring and then the mixture transferred under UV lamp (UV Flood Curing System, Cure Zone 2 (CON-TROL-CURE, Chicago, IL). A water bath was used to keep the AgNO3 and sodium citrate mixture solution below water bath level to avoid locally increased temperature induced by strong light absorption of glass beaker. The mixture solution was kept under UV light with continuous stirring for 4 hr. The water bath was changed every half an hour during the nanoparticle synthesis with a temperature range of 25–40 8C. The z-potential of produced Ag colloids measured by means of dynamic light scattering using Zetasizer Nano Series (Malvern Instruments) was 35 mV. Particle size of the Ag colloids characterized both by transmission electron microscopy (TEM, Philips CM20) and by Zetasizer was 35 nm  5 nm. UV–visible absorption spectrum of Ag nanoparticle solution obtained from the Synergy HT Multi-Detection Microplate Reader (BioTek Instruments) showed an absorbance peak at 408 nm. Immobilization of Ag Nanoparticles in PCFs and on Planar Substrates: The PCF with both distal ends freshly cut (or sealed in the case of hollow-core PCF) was filled with PAH (weight-average molecular weight of 70000 g mol 1, 2 mg mL 1) in water at certain solution pH under a pressure differential of 200 psi. The applied pressure was then quickly released to leave the PAH solution inside the fiber for a deposition time of 20 min. After thorough rinsing the PAH-modified fiber by continuous flowing Milli-Q water at the same pH as used in PAH solution, the unbound and/or weakly absorbed PAH molecules were completely removed from the fiber channels. In a similar way, the Ag collodial solution (1012 particles mL) was filled into the fiber and sit for 30 min to allow for Ag nanoparticles immobilize. Finally, the fiber was copiously rinsed by continuous flowing of Milli-Q water to remove free Ag nanoparticles. Custom-made glass cells were prepared using a previously developed method [37] for immobilization of Ag nanoparticles on planar substrates. The glass cells were filled with PAH (2 mg mL 1) in water, pH adjusted to 4.5, for 20 min, and thoroughly rinsed with Milli-Q water at pH 4.5. Finally, they were filled with the Ag colloidal solution for 30 min, then rinsed with Milli-Q water, and kept full with Milli-Q water for future SERS measurements. SEM measurements revealed a coverage density of 0.2 particle mm 2 by immobilized Ag nanoparticles on the bottom of the cell. SERS Measurements and Raman Imaging: All SERS and Raman image measurements were conducted in a forward propagating geometry using the PCFs with immobilized Ag nanoparticles. A home-built Raman spectrometer in conjunction with high-resolution hyperspectral Raman imaging system was used in the study.[18] For comparison, SERS measurements on planar substrates were carried out using the same system as that for fiber measurements but reconfigured for planner geometry.

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Towards Full-Length Accumulative Surface-EnhancedRaman Scattering-Active Photonic Crystal Fibers  

Towards Full-Length Accumulative Surface-Enhanced Raman Scattering-Active Photonic Crystal Fibers

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