Resonant Mirror Enhanced Raman Spectroscopy - The Journal of

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Resonant Mirror Enhanced Raman Spectroscopy De-Bo Hu, Chen Chen, and Zhi-Mei Qi* State Key Laboratory of Transducer Technology, Institute of Electronics, Chinese Academy of Sciences, Beijing 100190, People’s Republic of China S Supporting Information *

ABSTRACT: A resonant mirror as a high-Q dielectric resonator can accumulate a strong evanescent field at its surface, and this field has been proposed for surface/interface Raman enhancement applications for a while. However, the theoretically predicted large Raman enhancement effect of a resonant mirror had never been experimentally demonstrated until our work reported here, primarily due to the difficulties confronting the experimentalists in determining the resonant conditions for this optical device and optimizing the collection efficiency of Raman radiation from molecules at its surface. In this study, taking advantage of the rationally designed and well-fabricated high-quality planar dielectric optical waveguides, and overcoming the two difficulties aforementioned through the use of m-line spectroscopy and waveguide-coupled directional Raman emission techniques, we present the first experimental demonstration of resonant mirror enhanced Raman spectroscopy (RMERS). Considerable signal enhancement that enabled the polarization-division multiplexing (PDM) Raman detection of copper phthalocyanine (CuPc) ultrathin films and cytochrome c (Cyt c) monolayer deposited at the waveguide surface has been achieved. Considering its high Raman enhancement capability, outstanding PDM Raman detection ability, and good affordability, RMERS is believed to be a promising tool for the in situ Raman analysis of analytes on the dielectric flat surfaces and interfaces under ambient conditions.

1. INTRODUCTION Surfaces and interfaces are the playground of a wide range of physical, chemical, and biological processes, including adsorption, catalysis, membrane processes, and so on, the analysis and understanding of which are of great theoretical and practical significance. Along with the upgrade of Raman spectrometers and especially with the evolvement of the surface-enhanced Raman scattering (SERS) technique,1−3 Raman spectroscopy has grown as an important surface analytical tool in the past three decades. However, the SERS technique is inherently incompatible with flat surfaces because the electromagnetic enhancement mechanism of SERS requires roughened or nanostructured metallic surfaces to support the localized surface plasmon resonance (LSPR). Therefore, Raman analyses of any processes taking place on either dielectric or metallic flat surfaces are forbidden to reap the benefits from SERS. To acquire high enough Raman signals from analytes on the flat surfaces, many Raman enhancement methods other than SERS have been invented, including tip-enhanced Raman spectroscopy (TERS),4−6 surface plasmon resonance (SPR) Raman spectroscopy,7−12 and long-range surface plasmon resonance (LRSPR) Raman spectroscopy13 for the metallic flat surfaces, as well as total internal reflection (TIR) Raman spectroscopy,14,15 waveguide Raman spectroscopy (WRS),16−19 and plasmon waveguide resonance (PWR) Raman spectroscopy20 for the dielectric flat surfaces. TERS can provide both a large Raman enhancement factor and high spatial resolution,4 which make single-molecule Raman detection5 and even single-molecule Raman imaging6 on the atomically flat metallic surfaces © XXXX American Chemical Society

possible; this can, however, only be achieved by using highly sophisticated equipment such as scanning tunneling or atomic force microscopes. Moreover, ultrahigh vacuum and cryogenic temperature conditions are usually required. Thus, TERS is technically too demanding to be used routinely as a surface analytical technique. SPR-, LRSPR-, and PWR-based Raman spectroscopies are much more easily accessible than TERS, but they can only provide limited Raman enhancement factors due to the intrinsic dissipation of optical energy in the metal films involved. Being without any resonance mechanism and utilizing only the electromagnetic boundary conditions to achieve slightly higher excitation field strength, the Raman enhancement factor provided by TIR Raman spectroscopy is even smaller. With regard to the WRS, which is lacking a resonance field enhancement mechanism also, detectable, but still very weak, Raman signals can only be obtained using a meticulously designed optical waveguide; thus, the sensitivity and applicability of this method is rather limited. In summary, all the existing techniques mentioned above are not satisfying to some extent as analytical tools for the flat surfaces and interfaces. To provide a high-performance and readily affordable Raman analytical tool for the flat surfaces and interfaces, in this work, a new non-SERS technique named resonant mirror enhanced Raman spectroscopy (RMERS) is presented. Combining the large Raman enhancement factor provided by the high-Q Received: March 3, 2014 Revised: May 5, 2014

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dx.doi.org/10.1021/jp502171k | J. Phys. Chem. C XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry C

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Figure 1. Design, fabrication, and AFM characterization of the waveguides used in the RMERS. (a) Guided modes in waveguides of different PMMA thicknesses. By properly choosing the PMMA thickness, both excitation laser light (λL = 532 nm) and Raman scattered light (λR = 633 nm, corresponding to the Raman shift about 3000 cm−1) can be supported as guided modes. (b) Fabrication processes and optical constants of the resultant waveguides. (c) AFM images of the surfaces of MgF2 buffer layer and PMMA resonant layer. Reasonably low Rq values indicate that high optical quality waveguides can be obtained.

after overcoming the two difficulties aforementioned through the use of methods that will be elaborated on in the following. The design, fabrication, and atomic force microscopic (AFM) characterization of the planar dielectric optical waveguide used in our experimental demonstration of RMERS are illustrated in Figure 1. As shown in Figure 1b, the waveguide is constructed on a glass slide substrate (n1 = 1.52680 at 532 nm and 1.52076 at 633 nm), comprising a magnesium fluoride (MgF2, n2 = 1.39080 at 532 nm and 1.38877 at 633 nm) buffer layer vacuum sublimated onto the glass slide, a poly(methyl methacrylate) (PMMA, n3 = 1.49569 at 532 nm and 1.48936 at 633 nm) resonant layer dip-coated onto the MgF2 buffer layer (other resonant layer materials are possible; see Figure 1S in the Supporting Information), and the air or water superstrate. As long as the MgF2 buffer layer is thick enough (1300−1600 nm for all waveguides in this work), the glass slide can be ignored in determining the guided modes for waveguides of different PMMA thicknesses. As shown in Figure 1a, by properly choosing the PMMA thickness, both excitation laser light (λL = 532 nm) and Raman scattered light (λR = 633 nm, corresponding to the Raman shift about 3000 cm−1) can be supported as fundamental or higher-order transverse electric (TE) or transverse magnetic (TM) guided modes in the waveguide with either an air or a water superstrate. Specifically, the waveguides used in this work are all single mode waveguides, with the PMMA thickness being

resonant dielectric structure with the simple construction and use of the TIR Raman spectroscopy, RMERS is believed to be a promising tool for the in situ Raman analysis of analytes on the dielectric flat surfaces and interfaces under ambient conditions.

2. PREPARATION OF THE RESONANT MIRRORS A resonant mirror, also known as a leaky mode optical waveguide, which is a dielectric resonator composed of a prism and a planar optical waveguide, is far less popular than the SPR structure. Only now and then, it was used as a chemical or biochemical sensing element by monitoring the polarization or intensity changes of the reflected light,21,22 while the transmitted evanescent field at the waveguide surface caught little attention. It was only recently that the great field enhancement ability of a resonant mirror was recognized by Garciá de Abajo et al.23 and soon proposed theoretically by Sipe et al.24 for Raman enhancement applications. However, the theoretically predicted large Raman enhancement effect of a resonant mirror had never been experimentally demonstrated, primarily due to the difficulties confronting the experimentalists in determining the exact resonant conditions for this optical device and optimizing the collection efficiency of Raman radiation from molecules at its surface. Here, taking advantage of the rationally designed and well-fabricated high-quality optical waveguides, we present the first experimental demonstration of RMERS B



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Figure 2. Field enhancement mechanism and determination methods of resonant angles (θ) for the resonant mirror designed for solid−air interface applications (RM#1). (a) Mode profiles of TE and TM (only the Ex component of the TM mode is shown here; x axis is perpendicular to the waveguide surface) resonant guided modes. (b) Electric field enhancement of the buffer modes, TE and TM guided modes at the waveguide surface. The field enhancement factors on the left of the break are multiplied by 5 to make the TE and TM buffer modes visible. (c) Experimental setups for the determination of resonant angles of RM#1, m-line method (top) and spectral method (bottom). (d) Experimental phenomena when RM#1 is on resonance. Top: TM and TE polarized m-lines corresponding to the buffer modes and guided modes. Bottom: color patterns corresponding to the TM and TE resonant guided modes.

resonant mirror shown in Figure 2a are about 40 and 10 for TE and TM polarized incident optical electric fields, respectively; the FEFs of the resonant mirror shown in Figure 4Sa in the Supporting Information are about 55 and 28 for TE and TM polarized incident optical electric field, respectively) due to the finite thickness of the MgF2 buffer layer; but still, they are much larger than those that can be obtained using TIR, SPR, LRSPR, and PWR methods (the FEFs are all less than 10, as demonstrated in Figure 3S in Supporting Information).

equal to 250 nm for those designed for solid−air interface applications and 150 nm for those designed for solid−water interface applications, respectively. Since both the vacuum sublimation of MgF2 and the dip-coating of PMMA are welldeveloped film fabrication techniques, optical quality of the resultant waveguide can be considerably high, which is demonstrated by the low root-mean-squared roughness (Rq) of the MgF2 surface and especially the subnanometer Rq of the PMMA surface, as shown in Figure 1c. The fabricated high optical quality waveguide together with a high refractive index prism (Schott N-SF6, n0 = 1.81565 at 532 nm and 1.79882 at 633 nm, coupled with the waveguide via diiodomethane) can make a high-Q one-dimensional resonator (i.e., the resonant mirror (see Figure 2c); the glass slide has been omitted in all diagrams involved in this work for simplicity), which, once on resonance, can enhance the evanescent field at the waveguide surface enormously. The field enhancement factors (FEFs) of such a resonant mirror are only limited by the thickness of the MgF2 buffer layer. The thicker the buffer layer, the larger the field enhancement factors.23 With regard to the resonant mirrors used in this work, the field enhancement factors are moderate (the FEFs of the

3. DETERMINATION OF THE RESONANT CONDITIONS Besides the higher field enhancement factors, the resonant mirror also puts forward a serious challenge for us: how to determine its exact resonant conditions. Since the energy loss is very low in a high-Q resonant mirror, it is very difficult to determine the resonant angles by simply tracing the reflection minimum like in the cases of SPR, LRSPR, and PWR. As an alternative, the polarization interference method could be used;21 however, it is an over delicate technique and thus quite experience-demanding and easily subject to the effects of material absorption and birefringence, which may shift the polarization interferogram prominently and thus disturb the C



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Figure 3. Determination of the radiation power distribution for fluorescent molecules at the surfaces of resonant mirrors. (a) For RM#1 and (b) for RM#2, numerical simulations of the dispersion of the effective refractive index (Neff) of the guided modes and the recoupling angles of the directionally emitted fluorescence within the wavelength range from 532 to 633 nm. (c) For RM#1 and (d) for RM#2, pictures of the rainbow-like color patterns formed by projecting the directionally emitted fluorescence onto a white screen. The small green spots in the pictures indicated by thick arrows correspond to the reflection spots of the laser beam. The color patterns on the top are excited by a TM polarized evanescent field on resonance, and the ones on the bottom are excited by a TE polarized evanescent field on resonance. Because of the steep dispersion curve of the prism in the wavelength range concerned, the recoupled red light corresponding to the TM guided mode for RM#1 and the TE/TM guided mode for RM#2 would emerge at a larger angle than the green light.

(2) to switch the polarization state of the incident laser beam between TE and TM using a polarizer inserted between the laser and the prism; (3) to project the TE or TM polarized reflected light as well as the m-lines (formed by recoupling of the scattered laser light within the waveguide25) onto a white screen; and (4) to adjust the incident angle θ of the laser beam until the light spot of the reflected laser beam is coincident with the TE or TM polarized m-line, as shown in Figure 2d (top, photographed with a Nikon-D5200 digital camera); then, the incident angle θ can be determined as the resonant angle for the TE or TM polarized resonant mode. The resonant angles determined in this way agree very well with the theoretically predicted ones (calculated using standard transfer-matrix method for multilayer structures26), as shown in Figure 2b.

determination of the resonant angles. Fortunately, it was discovered in the experiments that the m-line spectroscopy can come to our rescue and help to determine the resonant angles precisely. The experimental setup for determining the resonant angles of a resonant mirror designed for solid−air interface applications (designated as RM#1, thickness of MgF2 d2 = 1300 nm, thickness of PMMA d3 = 250 nm) using m-line spectroscopy is illustrated in Figure 2c (top) (see Figure 4S in the Supporting Information for the case of a resonant mirror designed for solid−water interface applications, RM#2, d2 = 1560 nm, d3 = 150 nm). The whole procedure of the m-line method is (1) to illuminate the resonant mirror with a linearly polarized 532 nm laser (polarization direction oriented at 45° with respect to that of both TE and TM, laser power: 20 mW); D



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tions). Simply by analogy, we can infer that most of the Raman scattered light (0−3000 cm−1) of the probed molecules at the surface of a resonant mirror is also distributed within the small angle ranges corresponding to the resonant angles of the guided modes. In addition, it is worthy of note that the waveguidecoupled directional Raman emission would be more energetically efficient than its surface plasmon-coupled counterpart due to the absence of Raman radiation energy dissipation associated with metal films and the presence of additional TE polarized directional Raman emission (note that only the TM polarized Raman radiation can be effectively coupled out via the surface plasmon-coupled directional emission). Thus, optimal Raman collection efficiency can be achieved by collecting the directionally emitted Raman scattered light with an objective of large enough numerical aperture (NA).

We can further verify the validity of these determined resonant angles using spectral method, as shown in Figure 2c (bottom). By adjusting the incident angle of a collimated beam from a broad-band light source (BBS, tungsten halogen lamp) to the determined resonant angles, color patterns (caused by the higher loss of the light of wavelength on resonance), as shown in Figure 2d (bottom, also photographed with a NikonD5200), can be observed on the screen; thus, the validity of the determined resonant angles is doubly confirmed.

4. DETERMINATION OF THE RAMAN RADIATION DISTRIBUTION The evanescent field at the waveguide surface of a resonant mirror would be 10 of times stronger than the incident field when the incident angle θ is tuned to the resonant angles; thus, the total Raman radiation power of the probed molecules in the vicinity of the waveguide surface will be greatly enhanced. What is more, the spatial distribution of the Raman radiation power will be greatly modified by the presence of the resonant mirror. To know the spatial distribution of the radiation power is desirable in all kinds of Raman spectroscopic techniques, because it can help to enhance the sensitivity of the whole system by improving the light collection efficiency. However, to derive the radiation power distribution of a radiating dipole near a plane dielectric surface analytically is really a formidable task,27 while, to calculate it numerically using commercial software like FDTD Solutions, is money- and resourceconsuming. Here, we provide an elegant way out of the dilemma by simply taking advantage of the analogues between different physical phenomena. Inspired by the well-known surface plasmon-coupled directional fluorescence and Raman emission,28,29 we speculated that the fluorescence and Raman light emitted by the molecules in the vicinity of the waveguide surface could couple into the waveguide as guided modes (this is the very reason why the waveguides are designed to support both the excitation and Raman wavelengths as guided modes) and recouple out directionally via the prism. This speculation has been positively confirmed by the waveguide-coupled directional fluorescence emission experiments. The experimental setup is the same as the one used in the m-line experiments, except that a long-pass filter (Semrock, LP03-532RU-25) is placed between the prism and the screen to block the reflected laser beam. In the experiment for RM#1 working in an air superstrate, a Rhodamine 6G (R6G) monolayer was first deposited onto the surface of RM#1 by dipping it into the 20 μM R6G water solution for 30 min, whereas, in the experiment for RM#2, the 20 μM R6G water solution was used directly as the superstrate. The experimental results (i.e., the fluorescence m-lines) are presented as photographs (taken with a NikonD5200 with proper exposure times indicated at the bottom of the images) shown in Figure 3c,d,), from which we can conclude that the R6G molecules distribute most of their radiation power within the small angle ranges corresponding to the resonant angles of the TM and TE polarized guided modes, and a bit of their radiation power within the larger angle ranges corresponding to the TM and TE polarized buffer modes, no matter how they are excited (by TM polarized evanescent field on resonance or TE polarized evanescent field on resonance). This conclusion agrees very well with the theoretical predictions, as shown in Figure 3a,b, which give out the recoupling angles of light within the wavelength range from 532 to 633 nm (pay attention to the color orders in the pictures, which are amazingly concerted with the theoretical predic-

5. EXPERIMENTAL DEMONSTRATION OF RMERS 5.1. Experimental Setup. Having known the exact resonant angles of the resonant mirror and the spatial distribution of the Raman scattered light, we are now all set to investigate the true Raman enhancement power of RMERS experimentally with both TE and TM polarized excitation fields (referred to as polarization-division multiplexing (PDM) RMERS). As shown in Figure 4a, the experimental setup for RMERS is the same as the one used in the m-line experiments (power of the 532 nm laser is 20 mW), except that a liquid chamber is mounted onto the surface of the waveguide and the white screen is replaced by a commercial Raman probe (Avantes, AvaRaman-PRB-532), whose light collection angle (NA = 0.27, corresponding to a collection angle about 31° in air) is large enough to encompass the divergence angle (only about 1°, as shown in Figure 3a,b) of the directionally emitted Raman scattered light. The collected Raman scattered light is transported to the Raman spectrometer (Avantes, AvaRaman532TEC) by an optical fiber, and the Raman spectra of the probed molecules on the waveguide surface could be obtained. In all the experiments mentioned below, the integration time of the Raman spectrometer was set to 20 s (IT = 20 s) for a single scan, and every recorded Raman spectrum was an average of four successive scans. 5.2. RMERS of CuPc Ultrathin Films. We took vacuum sublimated copper phthalocyanine (CuPc) ultrathin films of the thickness of about 3.5 nm (corresponding to about three layers of CuPc molecules, as indicated in the insets of Figure 4b,c) as samples to verify the Raman enhancement power of RM#3 (with the same waveguide parameters as RM#1) and RM#4 (with the same waveguide parameters as RM#2), which are designed for working in air and water superstrates, respectively. Both RM#3 and RM#4 can be switched on and off resonance during the experiments by changing their superstrates either from water to air or vice versa without any further adjustment of the optical arrangement. The experimental results of the PDM RMERS for RM#3 and RM#4 are shown in Figure 4b,c, respectively, from which we can conclude that the Raman signals of the CuPc ultrathin films have been enhanced considerably by the resonant mirrors on resonance, whether with a TE or TM polarized excitation field, whether in air or a water superstrate, and the Raman enhancement ability of RM#4 exceeds that of RM#3. However, the absence of the CuPc characteristic Raman peaks in the off-resonance Raman spectra makes it difficult to evaluate the Raman enhancement power of RMERS directly; therefore, we resort to theoretical calculations to quantify the Raman enhancement power of RMERS. E



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where FEF(λL) and FEF(λRaman) are the field enhancement factors of the system at the excitation laser wavelength and the Raman-shifted wavelength, respectively. In the case of SERS, FEF(λL) is nearly the same with FEF(λRaman) due to the relatively large full width at half-maximum (fwhm) of the LSPR. Therefore, it is well-known that the REF of SERS is approximately proportional to the fourth power of the FEF provided by the LSPR.32 Quite similarly, the REF of RMERS also follows this fourth power rule as long as the frequencies of the Raman signals are low enough; however, for the highfrequency Raman signals, this fourth power rule fails due to the much narrower resonant wavelength range of the resonant mirrors compared with that of LSPR. Thus, the REFs of RMERS are prominently different for Raman signals of different frequencies and need to be evaluated independently by taking polarization states of the excitation light and dispersion of the resonant mirrors into consideration (dispersion curves of all the materials used in the resonant mirrors are shown in Figure 2S in the Supporting Information). The theoretically calculated FEFs of RM#1 (RM#3) and RM#2 (RM#4) within the wavelength range from 532 to 633 nm (corresponding to the Raman frequency range of 0−3000 cm−1 ) are illustrated in Figure 5a,c, respectively; the theoretically calculated REFs of RM#1 (RM#3) and RM#2 (RM#4) within the same wavelength range are illustrated in Figure 5b,d, respectively. As shown clearly in Figure 5, the REFs of RM#1 (RM#3) for the TE polarized excitation field are in the range from 1.9 × 105 to 2.5 × 106, and the REFs of RM#1 (RM#3) for the TM polarized excitation field are in the range from 1.5 × 103 to 1.1 × 104; the REFs of RM#2 (RM#4) for the TE polarized excitation field are in the range from 1.0 × 106 to 8.9 × 106, and the REFs of RM#2 (RM#4) for the TM polarized excitation field are in the range from 1.0 × 105 to 5.8 × 105. Specifically, the on-resonance REFs of RM#3 for the CuPc Raman peak at 1534 cm−1 (corresponding to the wavelength 579.3 nm) are about 6.1 × 105 and 3.5 × 103 for the TE and TM polarized excitation fields, respectively; the onresonance REFs of RM#4 for the same CuPc Raman peak at 1534 cm−1 are about 2.7 × 106 and 2.2 × 105 for the TE and TM polarized excitation fields, respectively. As illustrated in section 5 of the Supporting Information, the off-resonance REFs of RM#3 for the CuPc Raman peak at 1534 cm−1 are about 4.6 and 2.0 for the TE and TM polarized excitation fields, respectively; the off-resonance REFs of RM#4 for the CuPc Raman peak at 1534 cm−1 are about 5.7 and 2.8 for the TE and TM polarized excitation fields, respectively. The calculated REFs can explain the experimentally observed enhanced Raman signals on resonance compared to off resonance and the superior Raman enhancement ability of RM#4 over RM#3 very well; however, there is a serious contradiction between the experimental results and the theoretical predictions: the Raman signals excited by TE polarized resonant guided modes are weaker than those excited by TM polarized resonant guided modes. A probable reason for this contradiction is that the CuPc molecules in the ultrathin films orientate themselves anisotropically and this makes them unable to be excited by the TE polarized field as effectively as by the TM polarized field (this explanation also holds for the case of cytochrome c mentioned below). In principle, this polarization-dependent effect can be utilized to determine the average orientation of molecules in the ultrathin films,33,34 providing the Raman tensors of the probed molecules are wellknown. Thus, this seeming contradiction between theory and

Figure 4. (a) Experimental setup for RMERS. (b) For RM#3 and (c) for RM#4, PDM RMERS spectra of CuPc ultrathin films. (d) For RM#5, PDM RMERS spectra of Cyt c monolayer. Note that the spectra of TE and TM have been offset vertically for clarity in (b)− (d).

As a general principle, the Raman enhancement factor REF of an electromagnetic Raman enhancement system at the Raman-shifted wavelength λRaman can be given by30,31 REF(λRaman) = FEF2(λL) ·FEF2(λRaman)

(1) F



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Figure 5. Field enhancement factors of (a) RM#1 (RM#3) and (c) RM#2 (RM#4, RM#5) within the wavelength range from 532 to 633 nm. Raman enhancement factors of (b) RM#1 (RM#3) and (d) RM#2 (RM#4, RM#5) within the same wavelength range (corresponding to the Raman frequency range of 0−3000 cm−1).

collection efficiency, considerable enhancement of the Raman signals from the probed molecules deposited at the waveguide surface has been achieved; this is especially true for the molecules at the solid−water and solid−solution interfaces. Even higher Raman enhancement factors can be expected if a more optically perfect optical waveguide is used. In view of its high Raman enhancement capability, outstanding PDM Raman detection ability, and good affordability, RMERS is believed to be a promising tool for the in situ Raman analysis of analytes on the dielectric flat surfaces and interfaces under ambient conditions.

experiments does not necessarily negate our RMERS method; rather, it presents one of the potential usages of RMERSto determine the average orientation of the probed molecules utilizing PDM Raman detection. 5.3. In Situ RMERS of Cyt c Monolayer. Considering that the globular protein cytochrome c (Cyt c; see the inset of Figure 4d) has a diameter of 3.1 nm,35 which is comparable to the thickness of the CuPc ultrathin films that have been successfully detected, it is reasonable to infer that a Cyt c monolayer can be detected by RMERS, too. To verify this inference, a PDM RMERS experiment for in situ detection of a Cyt c monolayer using RM#5 (with the same parameters as RM#2) was carried out. In the experiment, the 50 μM Cyt c solution (phosphate buffered saline, PBS, pH = 7.4) was first pumped into the liquid chamber and left still for 30 min before the Raman spectra were recorded. As shown in Figure 4d, the low-frequency Raman peak of Cyt c 36,37 at 268 cm −1 (corresponding to the wavelength of 539.7 nm; the theoretical REFs are about 7.1 × 106 and 4.8 × 105 for TE and TM polarized excitation fields, respectively, as shown in Figure 5d) presents itself clearly by virtue of RMERS; thus, the ability of RMERS for in situ Raman detection of analytes adsorbed on the dielectric flat surfaces has been proven.

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ASSOCIATED CONTENT

S Supporting Information *

The contents of the Supporting Information include the design principle and method for waveguides used in RMERS, the optical properties of the materials used in the fabrication of resonant mirrors, the calculated field enhancement factors of other prism-based configurations, a pictorial explanation of the field enhancement mechanism and determination methods of resonant angles for RM#2, the calculated field and Raman enhancement factors of resonant mirrors off resonance, the transmission absorbance spectrum of the 3.5 nm thick CuPc ultrathin film, and some details about film fabrication and characterization. This material is available free of charge via the Internet at http://pubs.acs.org.

6. CONCLUSIONS We took four crucial steps to achieve a successful proof of concept experimental demonstration of RMERS in this work. First, rationally designed waveguides that support both the excitation laser light and Raman scattered light as guided modes were fabricated with high optical quality; second, exact resonant angles of the resonant mirrors were determined using m-line spectroscopy; third, aiming at optimizing the Raman collection efficiency, spatial distribution of the Raman radiation from molecules at the surfaces of resonant mirrors was determined using waveguide-coupled directional fluorescence emission; and finally, CuPc ultrathin films and a Cyt c monolayer were chosen as samples to verify the validity of RMERS. Attributed to the resonantly enhanced excitation field and the improved Raman

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +86 10 5888 7196. Fax: +86 10 5888 7196. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Major National Scientific Instrument and Equipment Development Project of China G



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(2011YQ0301240802), the National Natural Science Foundation of China (No. 61377064), the Beijing Natural Science Foundation (No. 3131001), and the State Key Laboratory of NBC Protection for Civilian (No. SKLNBC2012-01K2).

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