Leaky Mode Resonance of Polyimide Waveguide Couples Metal

Article pubs.acs.org/JPCC

Leaky Mode Resonance of Polyimide Waveguide Couples Metal Plasmon Resonance for Surface-Enhanced Raman Scattering Shuai Wang,† Zhiyong Wu,†,‡ Lei Chen,†,‡ Yuejiao Gu,†,§ Hailong Wang,† Shuping Xu,*,† and Weiqing Xu† †

State Key Laboratory of Supramolecular Structure and Materials, Institute of Theoretical Chemistry and ‡College of Physics, Jilin University, Changchun 130012, P. R. China S Supporting Information *

ABSTRACT: In the studies of surface-enhanced Raman scattering (SERS), it is considered to be a key point to couple the surface plasmons of metallic nanomaterials and structures to resonate, which can assist higher SERS signal enhancement. This paper is to explore a strategy for plasmon resonating based on the leaky mode resonance (LMR) of a polyimide (PI) optical waveguide (OWG), for the purpose of achieving the highly sensitive evanescent field-excited SERS. PI was chosen to build the waveguide layer due to its merits of exhibiting small extinction coefficients in the natural light frequency, low cost, high flexibility, easy fabrication, and almost no Raman spectral interference. The OWG configuration guarantees a high harvesting efficiency for the incident light. Ag nanoparticles were assembled on top of the OWG layer as plasmonic antennas to provide a large scattering cross section based on the coupling of the LMR and metal plasmon resonance (MPR), which supports highly efficient SERS radiation and being conducive to the far-field collection. The LMR-MPR coupling can facilitate stronger local electromagnetic field around the side surfaces of the Ag nanoparticles, which is favorable to the adsorption of analytes. The PI OWG-coupled MPR structure can realize the integration of SERS excitation light paths and elements, which is not only a valuable SERS enhancement configuration but also a promising technique for the surface and thin film analysis. couple the MPR for higher SERS gains.15−21 In an OWG, the light can be reflected for multiple times in the OWG layer. When the thickness and refractive index (n) of the OWG well match, the resonance of light will occur (called modes) and the incident electric field will be amplified in the dielectric layer. Owing to the evanescent field generated in the adjacent medium spreading in dielectric medium layer as a constructive interference wave, a leaky mode resonance (LMR) will result and its refractive index sensitivity has been proved to be higher than that of a conventional surface plasmon resonance (SPR) device.22 Therefore, if the LMR can be employed to couple MPR, we can expect to achieve a higher local electric field.18−22 Correspondingly, this design can also work for improving SERS due to the improved electric field enhancement. The SERS signal of the probe on the metal surface will be enhanced by coupling the evanescent field with the MPR. According to this SERS enhancement strategy, a more than 108 enhancement factor of SERS could be acquired.18 Andrews17 modified a planar waveguide with two-dimensionally aggregated Ag nanoparticles and let a TE wave propagated in the waveguide couple the MPR of Ag through the evanescent field. By using

1. INTRODUCTION Surface-enhanced Raman scattering (SERS) spectroscopy featuring insights of nano-optics and nanoplasmonics has been widely used in nanotechnology, biochemistry, environmental science, and many other analytical fields.1−6 The detection sensitivity of SERS becomes especially attractive because it has a comparable sensitivity with fluorescence and realizes single molecule detections.7−11 To achieve a high SERS sensitivity, the key is to couple surface plasmon resonance (SPR) on the metal surface according to the electromagnetic enhancement mechanism of SERS. The greatest coupling of SPs, i.e., metal plasmon resonance (MPR), can guarantee the excitation and emission of SERS signal with high efficiency.12−14 Previous studies proposed many SP coupling strategies for SERS mainly based on the optimization of geometries of metal nanoparticles (e.g., size and shape) with a desired laser wavelength,12 which is also termed as localized surface plasmon resonance (LSPR). In these strategies, the scattering and absorption cross sections of the metal nanoparticles dictate their abilities of light radiation and harvesting in nanoscale, which are in turn limited by their geometries. Can we further reinforce this light harvesting effect? Many optical devices designed for the MPR coupling address this issue. Optical waveguide (OWG), which is most commonly used in integrated optics and devices, can also be adopted to © 2015 American Chemical Society

Received: May 19, 2015 Revised: October 14, 2015 Published: October 14, 2015 24942

DOI: 10.1021/acs.jpcc.5b04791 J. Phys. Chem. C 2015, 119, 24942−24949

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The Journal of Physical Chemistry C Scheme 1. Schematic Diagram of the LMR-MPR Configuration for SERS

and dried in air. A 30 nm Ag was deposited on the slide by vacuum evaporation (Beijing Technol. Sci. Co. Ltd., China) to improve light reflection effect in the PI OWG layer. It should be noted that the thickness of Ag film should be in a proper range. A thick Ag film will increase the light energy loss, whereas a thinner one will cause weak light reflection in the OWG and decrease the intensity of the electromagnetic field within the OWG. So the optimized thickness 30 nm is rational for the present configuration.18 Next, PI (ZKPI-410, purchased from POME Sci-tech Co., Ltd. Beijing) was coated onto the Ag modified slide using a KW-4A spin coater (SIYOUYEN Electronic technology Co., Ltd. Shanghai). To accurately control the thickness of the PI film, we changed the concentration of the PI solution and optimized the spinning speed. More details about the control of PI film are provided in the Supporting Information. For example, here, to obtain a 450 nm PI OWG, we need to spin-coat the PI solution (solid content is 5% ∼ 6%) twice: 700 rpm for 10 s and additional 2000 rpm for 20 s. The PI film was dried in a 230 °C oven for 30 min for the purpose of hardening. The PI film as an OWG layer was characterized by the atomic force microscopy (AFM, SPA300, NSK Ltd. Japan) and UV−vis−NIR spectroscopy (Lambda 950, PerkinElmer America). The optical parameters of PI waveguide layer in our experiment were determined by the spectroscopic ellipsometry (Uvisel II, Horiba). A PI/Ag modified glass slide obtained by vacuum evaporation and spin coating was used for measurement. The ellipsometric spectra of the PI/Ag modified slide were acquired under the backside reflection mode with the single incidence angle of 70°. By fitting the data through the settings shown in the Supporting Information, a five-layer structure including air, PI, PI/Ag interface, Ag, and glass layers (Scheme S1, Supporting Information) was established along with the process searching, in which the PI layer was described by a dispersion formula of New Amorphous (short for “nam”) derived by Horiba Jobin-Yvon from Forouhi-Bloomer formulation. The resulting root-mean-square of 10.02 within the range of 1.5−5.5 eV exhibits accessible correspondence between the measured and fitted data. Additionally, the absolute value of every single number in correlation matrix less than or equal to 0.835 indicates an excellent independence between every two parameters. The n of PI is about 1.760 according to our fitting data (Supporting Information). 2.2. Immobilization of Ag Nanoparticles on PI Waveguide. Ag nanoparticles were immobilized on the PI

this structure, they obtained the SERS signal of the probe adsorbed on Ag surface. Xu18 and Qi20 employed the SiO2 and Ta2O5 waveguides for coupling the MPR of Ag nanoparticles to achieve ultrasensitive SERS signal. The strongest enhancement of electric field could be generated on the two sides of Ag nanoparticles, which is favorable for the detection of large molecules. Li and Sun21 made the incident light propagate along a one-dimensional MoO3 stripped dielectric waveguide for 7.3 μm, to remotely excite the LSPR of Ag nanoparticles for SERS measurements. They also studied the chemical enhancement effect of charge transfer between analytes and the MoO3 stripe based on this waveguide SERS structure.21 In this paper, we employed a PI OWG to couple the LSPR of Ag nanoparticles for SERS. PI film has a high refractive index and is a kind of transparent or nearly transparent materials. As a kind of polymeric optical waveguide material, it attracts us because of its several significant advantages. First, PI has excellent physical strength and elongation,23 which makes it convenient for long-term preservation. Second, PI has high thermal stability,24 which prevents it from thermal deformation during heating. Third, the chemical property of PI is extremely stable, which guarantees the consistency of PI films in selfassembly. Fourth, PI exhibits small extinction coefficients in natural light frequency region and it is an ideal waveguide material due to its low propagating loss. Fifth, PI possesses low cost and high flexibility.25 Hence it is easier to fabricate PI devices relative to other waveguide materials, e.g., SiO2 and Ta2O5. Most importantly, PI gives weak Raman activity, which allows for a low background in SERS spectra, and almost no strong spectral interference is observed in the further SERS detections (Figure S1, Supporting Information). Above PI OWG, Ag nanoparticles are assembled as plasmonic antennas to produce an adequately big scattering cross section (Scheme 1). We adopted the LMR-MPR coupling to excite the SERS signal of probes absorbed on Ag nanoparticles. This configuration features the merits of both excitation and emission. The enhanced light harvesting from OWG is conducive to Raman signal excitation. Also, the Ag nanoparticles improve the scattering cross section, which benefits subsequent far-field SERS measurements.

2. EXPERIMENTAL SECTION 2.1. PI Film Preparation. A glass slide (BK7, n = 1.516) with the size of 25 × 25 × 1 mm3 was cleaned with deionized water, ethanol, acetone, ethanol, and deionized water in order 24943


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Figure 1. Photo (A), AFM image (B), and UV−vis−NIR spectrum (C) of a PI film with a thickness of 300 nm on a BK7 glass slide.

(numerical aperture (NA) = 0.18) and a polarizer to form an spolarized parallel light. Then the s-polarized light passed through Lens 2 (NA = 0.15) and focused on the bottom surface of the semicylindrical prism (BK7, n = 1.516 at 532 nm) that attaches to a PI waveguide/Ag film/glass slide with an indexmatching oil. This PI waveguide/Ag film/glass slide should be cut for better fitting the size of the semicylindrical prism. The incident laser was reflected on the interface of the prism/Ag film, and the reflected light passed through Lens 3 then was detected by a photodiode installed in two arms of a two-arm goniometer. The photodiode connects with a signal amplifier and signals could be read by a data acquisition card (PCI-MIO16XE-50, National Instruments Co). The angle-dependent reflection spectra were recorded when two arms of the goniometer were rotated in the opposite direction synchronously with a resolution of 0.002°. 2.5. SERS Measurement. 4-Mercaptobenzoic acid (4MBA) was used as a Raman probe to evaluate the SERS enhancement activity in the LMR-MPR coupling configuration. The Ag nanoparticles modified PI waveguide was immersed in a 4-MBA aqueous solution for 20 min and then dried in room temperature. The PI waveguide modified glass slide with 4MBA and Ag nanoparticles was cut into a proper size and then attached to the bottom of a semicylindrical prism by a refractive index matching fluid (n = 1.515, Shanghai Specimen and Model Factory). SERS spectra at different incident angles were measured by the above-mentioned self-developed microspectrometer.28 A 532 nm s-polarized laser for TE mode (if we want to excite the TM modes, the polarizer needs to be rotated by 90° to obtain a p-polarized laser) was used to excite SERS. The power of the laser is 1.5 mW. The incident angle is tuned via one arm of a goniometer. The SERS spectra are collected from the bottom side of the prism (Scheme 1). To focus the detection area that is as same as the excitation spot, an inverted microscope with a 20× objective lens (NA = 0.35, focal length = 20.5 mm) and an imaging camera is employed. Also, the SERS signal is collected under the prism through a switchable channel with a monochromator (iHR320, Jobin-Yvon Co.) and a CCD (Synapse, Jobin-Yvon Co.). The integration time of the CCD was 15 s. Rayleigh scattering is filtered by an edge filter (Semrock Inc.).

layer (with the thickness of 300 nm) by the electrostatic selfassembly method. The waveguide substrate was immersed in a poly(sodium-p-styrenesulfonate) (PSS) aqueous solution (1.0 mg/mL) for 20 min and then cleaned with deionized water and dried with a stream of N2 gas. Then the PSS-modified PI film was immersed in a poly(diallyldimethylammonium chloride) (PDDA) aqueous solution (0.5%) for 20 min and then cleaned and dried in the same method. According to the measured thickness of the multilayered PSS/PDDA over a glass slide, we confirm that one PSS/PDDA bilayer has a thickness of 2.1 nm (Figure S2, Supporting Information). Finally, the PSS/PDDA modified PI film was immersed in an Ag colloid for 12 h for the Ag nanoparticle assembly. An Ag colloid with a nanoparticle size of ∼50 nm (Figure S3) was synthesized by Lee’s method.26 The PI waveguide immobilized with Ag nanoparticles was characterized by the scanning electron microscopy (SEM, JEOL JSM-6700F). 2.3. Simulation of Electric Field Dispersion. The finitedifferent time-domain (FDTD) simulations were carried out by the FDTD solution software (Lumerical Solutions, Inc.). The calculations assumed that all individual layers had a constant n. The indices of refraction used for all calculations at 532 nm TE plane wave light were as follows, prism (n = 1.516 at 532 nm), Ag film (n = 0.14287, imaginary part of refractive index (k) = 3.0518 at 532 nm, thickness = 30 nm), PI film (n = 1.760, which was obtained by fitting the ellipsometric data as stated above, thickness = 300−700 nm for every 50 nm), and air (n = 1.000). The incident angles were scanned from 40 to 70° with a step of 0.1°. The electric field distributions in the waveguide layer were obtained at the resonance angles for the matching TE mode in different thicknesses of PI layers. To simulate the electric field distribution of the LMR-MPR coupling configuration, an Ag nanoparticle (spherical, diameter = 50 nm) is immobilized 2 nm (based on the thickness measurement result of Figure S2) on the PI waveguide (thickness = 300 nm) under the LMR excitation (incident angle = 49.6°) with the 532 nm light source. One PSS/PDDA bilayer between the Ag nanoparticle and PI film can be ignored because it is too thin to form a layer. For comparison, the electric field distribution of a 50 nm Ag nanosphere under direct excitation of 532 nm light source is provided. 2.4. Measurement of Angle-Dependent Reflection Spectra. The angle-dependent reflection spectra were measured by our self-built microspectrometer,27 which was fixed on the basis of a goniometer (Figure 1). A 532 nm laser (Changchun New Industries Optoelectronics Tech. Co. Ltd.) was used as the light source that passes through Lens 1

3. RESULTS AND DISCUSSION 3.1. Optimization of PI Waveguide Thickness. Figure 1A,B shows a photo and an AFM image of a 300 nm PI film prepared by spin-coating on a BK7 glass slide. The PI film 24944


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Figure 2. (A) Electric field distributions of waveguide modes in PI layers with different thicknesses. A 532 nm incident wavelength was set. (B) Plot of the electric field intensities at the PI/air interface under different TE modes along with the PI film thickness.

displays a uniform morphology and the roughness is 0.945 nm. And the PI OWG looks nearly transparent under the thickness of 300 nm. Figure 1C is the UV−vis−NIR spectrum of this PI OWG and it shows that this PI film has a high transmittance over 90% in the range of 450−1100 nm, indicating it is feasible for an OWG material. For a certain thickness of PI OWG, several modes can be found. To obtain the LMR of the PI waveguide, the thickness of the OWG was optimized to guide the strongest electric field at the PI/air interface. Figure 2A shows the FDTD simulation results of electric field distributions within the PI waveguides under the resonance conditions. On the basis of the electric field amplification at the PI/air interface, the electric field intensities of different modes are plotted in Figure 2B. A 20−30 fold enhancement of incident EM field was obtained at the resonance modes. To obtain the highest intensity of the evanescent field (∼30 times enhancement) among the resonance modes of different PI films, a PI film with the thickness of 300 nm was selected with TE1 mode for further experiment. Figure 3 shows a comparison of simulated and experimental resonance curves of a PI waveguide with the thickness of 300 nm under the light wavelength of 532 nm. In the scanning range from 40° to 70°, there is only one mode existing (TE1). The TE0 mode does not appear due to the mismatching of parameters in this scanning range. The resonance band is very narrow and the resonance angle appears at 49.6° in an ideal condition, whereas it becomes broader (peak width at the half height = 3.1°) and the lowest reflectivity is 50.4° in our experimental condition. 3.2. LMR-MPR Coupling Configuration. The Ag nanoparticles were assembled on the PI OWG as plasmonic antennas to increase the scattering cross section and further for SERS. In this work, the choice of Ag nanoparticles is because

Figure 3. Resonance curves of the PI waveguide (thickness = 300 nm) at 532 nm.

Ag nanoparticles support stronger SERS enhancement ability compared with Au and Cu (their excitation lasers prefer red and near-infrared lasers) when they are SERS substrates. Also, for an Ag nanoparticles self-assembled film, there is a broad SPR band in the visible range28 (different from these Ag nanoparticles in a colloid system, showing the maximum extinction peak at ∼420 nm, Figure S3), which allows for many options of incidence lasers. Figure 4A displays the SEM image of Ag nanoparticles self-assembled on a PI waveguide. It shows that Ag nanoparticles (∼50 nm) are uniformly distributed on the PI film. The electric field distributions of the Ag nanoparticles under the LMR of the PI waveguide was simulated by FDTD method. A ∼2 nm thickness air gap between the PI OWG and Ag NPs was set. Because a bilayer of polyelectrolytes as the linker cannot form a uniform layer over the surface, they would form many clew-like molecular islands and most of the PI-OWG surface would be exposed to air. So the refractive index of PSS/ 24945


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Figure 4. SEM image (A) and the FDTD simulated electric field distribution (B) of an Ag nanoparticle (sphere, diameter = 50 nm) assembled on the PI waveguide (thickness = 300 nm) surface under the LMR excitation (incident angle = 49.6°) with a 532 nm light source. (C) FDTD simulated electric field distribution of an individual Ag nanoparticle in free space (in air) under the direct irradiation with a 532 nm light source. E and E0 are the electric fields of the designed configuration and the incident light.

for the highest light harvesting effect and we used the anglescanning way to easily find the resonance condition. Under this TE1 mode excitation (the incident angle is 50.4°), the strongest SERS signal is achieved and its spectrum is shown in curve I in Figure 5.

PDDA molecules can be ignored in this simulation model. Panels B and C of Figure 4 show the simulated electric field distributions of the Ag nanoparticles under the LMR of the PI waveguide. When the individual Ag nanoparticle is located in the waveguide-concentrated EM field, its LSPR can couple with LMR, providing the enhancement of the local electric field more than 450 times (|E|2/|E0|2, E is the amplitude of light in the waveguide and E0 is the amplitude of the incident light). Also, the strongest electric field previously at the PI/air interface (Figure 2A) is guided to the side surface of the Ag nanoparticle (Figure 4B), not at the gap site, which is favorable for measuring large-sized probe molecules that hardly penetrate into the gap site. For comparison, we simulated an Ag nanoparticle with the diameter of 50 at 532 nm wavelength in a free space (Figure 4C) and it can be observed that the amplified electric field (|E|2/|E0|2) is about 20-fold for this case, which displays the electric field only from the LSPR contribution. By comparison, it indicates that the LMR-MPR coupling configuration can effectively improve the local electric field, which confirms its remarkable contribution on SERS. 3.3. LMR-MPR Coupling Configuration for SERS. To evaluate the SERS property of the LMR-MPR coupling configuration, 4-MBA was chosen as a probe to be assembled on the Ag surface (Scheme 1). The LMR-MPR slides were dipped into the analyte saples for 20 min and then measured by a Raman spectrometer after they were dried. The incident angle was set as 50.4°, which is achieved from the dip angle in the reflectance spectrum of the LMR-MPR configuration (curve not shown) and is the same as the incident angle of the TE1 mode (black curve in Figure 3). The unchanged resonance angle from LMR to LMR-MPR indicates that the main contribution on the dip in the reflectance spectrum of the LMR-MPR configuration is from the OWG effect. Therefore, the more important thing is to couple the OWG mode rather than the LSPR effect, which is different from most SERS enhancement strategies based on the MPR coupling with excitation wavelength. In our present strategy, we need to optimize the resonance condition by tuning the incident angle

Figure 5. SERS spectra of 4-MBA (1.0 × 10−4 mol/L) under the LMR-MPR coupling excitation (I) and under the prism-type SPR excitation (II).

For comparison, we also measured the SERS spectrum of 4MBA (1.0 × 10−4 mol/L) in the evanescent field under the prism-type SPR excitation while the thickness of Ag film is 40 nm, using a p-polarized 532 nm laser with the same laser power, which is also called as the “SPR-SERS spectroscopy” in our previous publication.27 This prism-type SPR supports the SPR angle at 43.2° (Figure S4 in the Supporting Information), and the SERS spectrum is shown as curve II in Figure 5. Compared with the SPR excitation mode, the LMR-MPR coupling excitation supplies 5.1 times improvement on the peak area of 1069 cm−1 band (7.2 times for 1575 cm−1 band), indicating the high SERS enhancement ability of the designed LMR-MPR coupling way. 24946


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The Journal of Physical Chemistry C Different from the SPR excitation mode where only the ppolarized laser was adopted, the s-polarized light can also cause the waveguide resonance. The SERS enhancement abilities under the TE/TM modes via the LMR-MPR coupling excitation were compared. The SERS spectra of 4-MBA excited with the TM1 mode and TE1 mode are shown in Figure 6. The

Figure 6. SERS spectra of 4-MBA (1.0 × 10−4 mol/L) under the LMR-MPR coupling in TM1 mode (black line) and in TE1 mode (red line). The power of the laser was 1.5 mW, and the integration time of the CCD was 15 s. The incidence angles for TM1 and TE1 are 43.5° and 50.4°, respectively.

Figure 7. (A) SERS spectra of 4-MBA at different concentrations obtained under the LMR-MPR coupling excitation. The power of the laser was 1.5 mW, and the integration time of the CCD was 15 s. (B) Working curve of SERS intensities at 1575 cm−1 for a series of concentrations of 4-MBA ranging from 1.0 × 10−4 to 1.0 × 10−7 mol/ L. Each black point represents an average of three measurements.

incidence angles for TM1 and TE1 are 43.5° and 50.4°, respectively, due to the dip angles of their reflectance spectra. Figure 6 demonstrates that the SERS intensity in the TM1 mode is almost the same as that in the TE1 mode under the same conditions. We also compared SERS enhancement abilities under TM1 and TE1 mode excitation by using another Raman probe, 4-mercaptobenzoic acid (4-Mpy) (Figure S6, Supporting Information), and prove again that they possess almost the same enhancement ability. By means of the FDTD simulation, the EM field distributions of an Ag nanoparticle under TM1/TE1 modes were obtained (Figure S5, Supporting Information, the incident angles are set as 49.6° and 41.9° for the TE1 and TM1 modes on the basis of the optimization of the incident angles) and showed that these two situations (TE1 and TM1) display comparable local EM field intensities, which is consistent with our experimental results that no obvious difference in SERS enhancement was observed between the TE1 and TM1 excitation. The sensitivity of the LMR-MPR coupling excitation mode was evaluated. Figure 7A demonstrates the SERS spectra of 4MBA under different concentrations and they show an increasing trend with the probe concentration. The limit of detection of 4-MBA is 1.0 × 10−7 mol/L (signal-to-noise ratio is 3) by using the LMR-MPR coupling excitation, and a working curve about the concentration of 4-MBA was shown in Figure 7B, which demonstrated the fitting relation between the signal and sample concentration. The uniformity of the Ag nanoparticles on the PI film can be seen from the SERS spectra of 4-MBA randomly selected from ten sites. The SERS intensities of characteristic peaks at 1069 and 1575 cm−1 are shown in Figure 8, with the variances of 14.09% and 12.03%, which are lower than variance requirement on spot to spot as 20%.

Figure 8. SERS spectra of 4-MBA in random locations obtained under the LMR-MPR coupling excitation. The probe concentration of 4MBA was 1.0 × 10−6 mol/L. The power of the laser was 1.5 mW, and the integration time of the CCD was 15 s.

4. CONCLUSIONS In this study, a SERS excitation configuration using a PI waveguide immobilized with Ag nanoparticles was proposed to the waveguide LMR and surface plasmon localization. PI was chosen to fabricate the waveguide layer on the basis of its distinct advantages of a small extinction coefficient in the visible and near-infrared light range and almost no natural signal in a broad Raman detection range. Also, it is easy to fabricate the PI films and control their thickness by spin-coating to match the optical waveguide mode. In this LMR-MPR configuration, the 24947


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The Journal of Physical Chemistry C local electric field increases as a fold of more than 450 times around the side surfaces of an Ag nanoparticle, supporting a stronger coupling efficiency and wide applicability for the SERS measurements of large molecules. This configuration has been applied for SERS excitation of analytes (such as 4-MBA and 4Mpy). The detection limit of 4-MBA is 1.0 × 10−7 mol/L by using the LMR-MPR coupling excitation. In a conclusion, this study is a proof of the concept on the SERS excitation light paths and coupling elements for integration. This configuration can be extended to a function system that integrates the SERS analysis with SERS substrates and the excitation/collection light paths.

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4-MBA, 4-mercaptobenzoic acid; 4-Mpy, 4-mercaptopyridine; FDTD, finite-different time-domain; AFM, atomic force microscopy

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b04791. (1) Preparation of PI films with different thickness, (2) measurement of PI refractive index and thickness by ellipsometry (including the optical model for ellipsometric data fitting, parameters of the simulated waveguide, the correlation matrix), (3) Raman spectrum of PI, (4) measure of the thickness of bilayer of PSS/PDDA, (5) UV−vis−NIR spectrum and TEM image of Ag nanoparticles, (6) the resonance curves of the prism-type SPR excitation at 532 nm, and (7) comparison of the excitation modes of TE and TM (including FDTD simulated EM field distributions and SERS spectra) (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*S. Xu. E-mail: [email protected]. Tel: 86-431-85168505. Fax: 86-431-85193421. Present Address §

Science and Technology on Surface Physics and Chemistry Laboratory, P.O. Box 718-35, Mianyang 621907, China. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by National Instrumentation Program (NIP) of the Ministry of Science and Technology of China No. 2011YQ03012408, National Natural Science Foundation of China (NSFC 21373096), and Innovation Program of the State Key Laboratory of Supramolecular Structure and Materials. We appreciated Dr. Qidan Chen (Jilin University) for her help on revising this manuscript.

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ABBREVIATIONS SERS, surface-enhanced Raman scattering; SP, surface plasmon; LMR, leaky mode resonance; OWG, optical waveguide; MPR, metal plasmon resonance; LSPR, localized surface plasmon resonance; SPR, surface plasmon resonance; PI, polyimide; n, refractive index; nam, new amorphous; PSS, poly(sodium-p-styrenesulfonate); PDDA, poly(diallyldimethylammonium chloride); NA, numerical aperture; 24948


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Leaky Mode Resonance of Polyimide Waveguide Couples Metal

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