A nano-plasmonic chip for simultaneous sensing with dual-resonance surface-enhanced Raman scattering and localized surface plasmon resonance

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1 Laser Photonics Rev. 8, No. 4, (2014) / DOI /lpor ORIGINAL Abstract A dual-resonance surface-enhanced Raman scattering (SERS) chip which also serves as a localized surface plasmon resonance (LSPR) refractive index sensor is proposed. The dual-resonance SERS chip can simultaneously enhance excitation and Stokes lines for Raman signals detection in a broad wavelength region with virtually no limitation. Thus, it is especially useful for Raman detection at long wave numbers and hyper Raman. The great performance of this chip relies on the highly independent tunability of the two localized plasmonic resonances from the optical to the nearinfrared region and the strict hot spot match in space for both resonant wavelengths. Furthermore, Raman signals of polymethyl-methacrylate (PMMA) from 500 cm 1 to 3300 cm 1 are measured in the experiments and an obvious superiority can be seen compared to a single-resonance SERS chip. In an addition, by using the subradiant magnetic dipole resonance, the LSPR refractive sensor gives a high sensitivity of 577 nm/riu and high figure of merit (FoM) of The experimental results are consistent with the simulated results. This dual-functional sensing chip opens a route for dual-modality detection of the concentration of some specific molecules. A nano-plasmonic chip for simultaneous sensing with dual-resonance surface-enhanced Raman scattering and localized surface plasmon resonance Jiao Lin 1, Yuan Zhang 2, Jun Qian 1, and Sailing He 1,2,3, 1. Introduction In past decades, with the demand for high sensitivity and characterization of specific molecules, biosensors based on localized surface plasmon resonance (LSPR) have attracted much appeal for use in biological detection, medical diagnoses, food safety and environmental monitoring [1 3]. As we know, the spectral positions of metallic nanostructures which support LSPR are highly dependent on the size, shape, material and dielectric environment around the nanostructures, which makes these metallic nanostructures inherently sensitive to a small change in the refractive index of the dielectric environment [4 10]. However, LSPR sensors are incapable of identifying unknown analytes. In this case, Raman spectroscopy is more suitable and information rich because it gives the characteristic and unique spectra (as sort of finger print ) of different molecules [11]. In spite of so many advantages, Raman scattering suffers from an extremely small scattering cross section, which makes the direct measurement of Raman signals very difficult. To solve this problem, most of the previous works aim at using single-resonance localized plasmonic structures to enhance the scattered Raman signal. By utilizing the intense electromagnetic field provided locally by plasmonic structures, Raman signals can be greatly enhanced and can even detect individual molecule [12 14]. However, the main issue for such a single-resonance Raman enhanced structure lies in the factor that it works well only for Stokes line that is quite close to the excitation wavelength. Sometimes, Raman signals at longer wave numbers (that are quite far away from the excitation wavelength; for example, C-H vibration Stokes line is near 3000 cm 1 ) or hyper Raman signals are also required to be detected. In this case, dualresonance surface enhanced Raman chip can instinctively 1 State Key Laboratory of Modern Optical Instrumentations, Centre for Optical and Electromagnetic Research, Zhejiang University, Zijingang campus, Hangzhou , China 2 ZJU-SCNU Joint Research Center of Photonics, Centre for Optical and Electromagnetic Research, South China Academy of Advanced Optoelectronics, South China Normal University (SCNU), Guangzhou, China 3 Department of Electromagnetic Engineering, School of Electrical Engineering, Royal Institute of Technology (KTH), S Stockholm, Sweden Corresponding author: sailing@kth.se

2 ORIGINAL Laser Photonics Rev. 8, No. 4 (2014) 611 give better performance for its strong ability in enhancing Raman signals both at the excitation wavelength and the Stokes lines. In this article, we propose a dual-resonance surfaceenhanced Raman scattering (SERS) chip by utilizing two localized plasmonic resonances. Superior to previous dualresonance SERS chips (where their two resonances were due to the wavelength-splitting caused by the coupling between a non-local SPPR and a LSPR mode) [15, 16], our subwavelength-structured plasmonic chip can provide dual-resonance enhancement for Raman signals in a really broad region with virtually no limitation. The great performance of our chip relies on two key points. First, the two resonances of our chips can be independently and easily tuned from visible to near infrared region by using different structure parameters. Besides an easy tunability of resonant wavelength, another advantage of our dual-resonance chip is that the highest field intensity ( hot spot ) of the two localized plasmonic resonances overlap in space, which is of great importance for simutaneous enhancement at both the excitation wavelength and Stokes wavelength, especially for single molecule detection. These advantages should give a wide potential application of our SERS chip in Raman detection at ultra-long wave numbers and hyper Raman. The superiority of our structures is experimentally demonstrated by comparing with the cases when only excitation or Stokes enhancement exists. Furthermore, our nano-plasmonic chip also serves as a localized surface plasmon resonance (LSPR) refractive index sensor by utilizing a subradiant magnetic dipole resonance. The LSPR efficiency of a plasmonic nanoparticle is typically evaluated by its FoM, defined as the ratio of the sensitivity (i.e. plasmon wavelength shift per refractive index unit change in the surrounding medium) to the width of the spectral peak. Due to the high sensitivity of 577 nm/riu and narrow bandwidth of this magnetic dipole resonance, the LSPR refractive index sensor in our paper achieves an ultra-high figure of merit (FoM) of 14.2, the typical FoMs of metallic nanoparticles are around [17]. Therefore, a sensitive and selective sensing chip combining LSPR with dual-resonance SERS is quite powerful because it opens a route for dual-modal molecule detection. 2. Design of dual-resonance plasmonic structures for SERS and LSPR sensing Our dual SERS-LSPR sensing chip in this article consists of an array of bowties on a continuous gold film, which is optically opaque. The schematic of the device is shown in Fig. 1a together with a scanning electron micrograph (SEM) image of the experimentally fabricated sample in Fig. 1b. The dimensions of the device are subtracted from the fabricated sample, as determined by SEM. The overall size of thearrayis60µm 60 µm. The array period is 560 nm. The bowtie antennas have a thickness (T) of 55 nm with a gap size (g) of 10 nm. Each constituent triangle has tips with Figure 1 (a) Schematic of the dual-resonance gold device. (b) Scanning electron micrographs (SEM) of the fabricated bowtie arrays on opaque gold film. radii of 15 nm, an arm length (L) of 120 nm, the distance from the midpoint of the edge to the apex, and a vertex (θ)of 50. The inset in Fig. 1a shows the position of the origin and directions of the three axes. The origin is set at the interface of the gold film and air and in the middle of the connection line between the two triangle tips. All the simulations in this article are calculated using the finite-difference-timedomain (FDTD) method. The complex dielectric constant of gold used in the simulation is taken from the experimental data of Johnson and Christy [18]. The whole structure is illuminated from the top by a plane wave with the electric field polarized along the bowtie axis, as can be seen in Fig. 1a. To characterize the optical properties of this device, we calculate the reflection spectrum and compare it with the experimental one. In the experiment, white light reflection spectroscopy is measured with BX51 Olympus microscopy. Light from a tungsten halogen lamp is polarized and focused by a 50 objective (NA = 0.5) to illuminate the sample at normal incidence. The reflected light is collected into a spectrometer. The reflection spectrum is normalized

3 612 J. Lin et al.: Dual-resonance SERS and LSPR sensing Figure 2 Reflection spectrum for bowtie arrays on a gold film from simulation (a) and experimental measurement (b). (c) Intensity enhancement spectrum obtained at the middle point of the gap with Ez profiles of the two resonances on the top surface of the bowtie structure. (d) Electric field profiles at the top surface of the bowtie structure. Structure parameters: Arm length L = 120 nm, vertex of apex θ = 50, gap size g = 10 nm, radius of the corner r = 15 nm and thickness T = 55 nm. Array period is 560 nm. to the spectrum from a standard reflection mirror. Here, only the diffraction of the zero order (in the direction of the mirror reflection) is considered in the simulation to match the measured spectrum of reflection collected backward in the experiment (see similar treatment in e.g. [19, 20]). In addition, both modes studied in the paper are in long wavelengths (longer than the period) where there are no higher orders of diffraction. In Fig. 2a, the black solid curve is the simulated reflection spectrum. Two distinct dips can be observed, with one dip at 605 nm and the other one at 756 nm. One minor dip occurs at 560 nm, which is the same as the array period. If the array period changes, the dip at 560 nm shifts with the period of the arrays while the positions of the other two dips at 605 nm and 756 nm change only slightly because of weak coupling. Therefore, the dip at 560 nm is considered to be a non-local surface plasmon polariton resonance (SPPR) of the present chip and arises from the surface wave being excited at the interface between the air and the periodically-corrugated gold film [19, 20]. Since the loss is large for gold below 600 nm, the resonance at 560 nm is faint and difficult to be observed. In Fig. 2b, the positions of the two resonant dips in the experimentally measured reflection spectrum coincide very well with the simulated results. However, the band widths of the two resonances are both broadened in the experiments. This is possibly due to the non-uniformity of the fabricated array of bowties. Figure 2c shows the electric field intensity enhancement (normalized by the corresponding electric field intensity when the plane wave illuminates the same space without the whole structure). A point monitor is put on the top surface of the bowtie and in the middle of the gaps. Very high field intensity enhancement is achieved at both resonance wavelengths and thus the device is highly efficient in enhancing the SERS signal. As we know, for bowtie arrays embedded in a uniform dielectric environment, the main resonance arises from the electric dipole binding resonance (EDBR) where each arm behaves like an electric dipole according to the mode hybridization theory [21]. Moreover, this resonance can also be considered to be caused by two horizontal IMI (insulator/metal/insulator) Febry-Perot (F-P) cavities coupling with each other through an air gap [22 24]. With a metallic substrate, the symmetry in the

4 ORIGINAL Laser Photonics Rev. 8, No. 4 (2014) 613 z axis is broken. It is known that E z profiles reflect charge distribution to some degree, so E z profiles are plotted and shown near the two field intensity enhancement peaks. It can be seen that the oscillating resonance at 605 nm is very likely to an EDBR in a bowtie array. However, compared to bowtie arrays embedded in a homogeneous dielectric environment, a metallic substrate induced a blue-shift to this resonance. From the field distribution at 756 nm, we can see that positive charges and negative charges are distributed separately in each arm. We further investigate the electric field profile in the symmetric plane perpendicular to the y-axis and find that this resonance oscillates like a magnetic dipole (a vertical MIM (metal/insulator/metal) Febry-Perot cavity of a vertical waveguide section formed by the two triangular arms and the air gap acting as the insulator layer). With a gold substrate, this mode can be efficiently excited and coupled to the propagating wave and vice versa. The relatively small scattering cross section of the magnetic dipole resonance (MDR) leads to small bandwidth as well as the shallow dip in the reflection spectrum. However, from Fig. 2c, we see that the field intensity enhancement is very high despite the relatively low excitation efficiency. Moreover, the narrow line shape of the magnetic dipole resonance also makes this chip a very useful refractive index sensor, which will be discussed later. To achieve the highest Raman enhancement, we need to design two resonances and the wavelengths of the two resonances need to match the excitation wavelength and Stokes line. Moreover, the highest field intensity of the two resonances needs to overlap in the same small area so that simultaneous enhancement at excitation and Stokes wavelengths can occur. Figure 2d plots the electric field profiles at the top surface of the bowtie structure. A strong field intensity enhancement can be achieved for both resonances in the same small area, namely, the gap region. Next, we investigate the characteristics of the chip by changing various schematic parameters to get a better understanding and easy tune of the chip. In our experiment, the whole device is covered with a thin film of poly-methyl-methacrylate (PMMA) with a thickness around 100 nm to measure its Raman signals. The refractive index of the dielectric environment in all the following simulations is taken to be Figures 3a c show the electric field intensity enhancement with respect to changing arm length, gap size and thickness, respectively. All the other dimensions have the same value with the previous structure discussed above. Since there are weak coupling between the neighboring bowties, the field enhancement of the two plasmonic resonances will be slightly changed when the periods of the bowtie arrays are changed. We choose an array period of 560 nm to avoid proximity effect in e-beam lithography in our experiment. With a period of 560 nm, the non-local SPPR couples with the first LSPR mode and introduces a dip around 700 nm (such a coupling was studied in [13]). In Fig. 3a, the arm length of each constituent triangle is varied with the values of 100 nm, 120 nm and 140 nm. The gap size and thickness of the bowties are set to be 10 nm and 50 nm, respectively. It can be seen that the first peaks are quite sensitive to arm length changing and are greatly red-shifted from 731 nm to 773 nm while Figure 3 Intensity enhancement spectrum with array periods in the x and y axes to be 560 nm. The black, red and green curves correspond to (a) L = 100 nm, 120 nm, 140 nm, T = 50 nm, g = 10 nm, θ = 50 ; (b)g= 18 nm, 14 nm, 10 nm, T = 60 nm, L = 140 nm, θ = 50 ; (c)t= 50 nm, 55 nm, 60 nm, L = 120 nm, g = 10 nm, θ = 50. the second peaks endure a slight blue-shift around 1000 nm. As we have mentioned before, the bowtie array structure can also be regarded as a horizontal IMI F-P cavity, so the redshift is considered to be an increase in the cavity length.

5 614 J. Lin et al.: Dual-resonance SERS and LSPR sensing Figure 4 (a) The intensity enhancement spectra and (b) the measured Raman spectra of PMMA for three structures in our experiment. The black, red and green spectra respectively correspond to structure A: L = 100 nm, g = 25 nm, T = 60 nm, θ = 50 ; structure B: L = 120 nm, g = 10 nm, T = 55 nm, θ = 50 ; and structure C: L = 140 nm, g = 10 nm, T = 60 nm, θ = 50. The dashed violet line marks the wavelength of the pump laser and the dashed cyan line marks the position of the Raman signal at 2960 cm 1. Moreover, since a short range SPP is excited here, the field is tightly confined within the metal layer. With an increase in cavity length, more loss is induced, which can be the possible reason for the decrease in the intensity enhancement. For the vertical MIM F-P cavity with a metal layer of finite thickness(which is truly the arm length), the resonant wavelength will be shifted to a longer wavelength as the thickness of the metal layer decreases according to previous studies [22], which is the same trend shown in our simulation results. Since the field is confined within the gap, the change in arm length from 100 nm to 140 nm only slightly influences the vertical MIM F-P resonance wavelength. Additionally, the field is more confined in the gap as the arm length increases, which leads to an increase in the intensity enhancement. Horizontal F-P resonances gain a higher intensity enhancement on the whole compared to the vertical F-P cavity, due to the higher excitation efficiency. In Fig. 3b, when the gap size decreases from 18 nm to 10 nm with a step length of 4 nm, the horizontal and vertical FP resonance are both shifted to longer wavelengths. The redshift of the horizontal IMI F-P resonance is due to the stronger coupling between the two arms [25]. As with the vertical MIM F-P cavity, decreasing the gap size will drag the dispersion curve away from the light line and thus the resonant frequency redshifts as well. Furthermore, the intensity enhancement evidently increases with smaller gap size because of a more tightly squeezed field for both resonances. In Fig. 3c, as the height of the bowtie antenna is changed from 50 nm to 60 nm, the vertical MIM F-P resonance redshifts because of an increase in the cavity length (i.e., the height of the bowties). Meanwhile, for MIM cavity, excitation efficiency increases when the bowtie becomes thicker but there are more losses. Thus, intensity enhancement does not show a clear increase or decrease because of the trade-off between excitation efficiency and loss. However, an anomalous feature occurs for the horizontal IMI cavity. As we know, for IMI cavities, resonant frequency redshifts when decreasing the thickness of the metal layer. However, for IMI on a continuous metallic film, the resonant frequency shifts in the opposite direction. It is possibly because the slow plasmon resonating not only exists in the bowtie plane but also vertically around the IMI cavity surface with the addition of a metallic film. Thus, the increase of thickness can shift the IMI F-P cavity to a longer wavelength. Also, intensity enhancement decreases due to more losses. 3. Experimental results In the experimental part, the superiority of the device as a dual-resonance SERS chip is demonstrated by comparing its abilities in SERS with other SERS chips of different schematic parameters. We fabricated three structures, named A, B, and C, and according to their different schematic parameters, we calculate the intensity enhancement curves as in Fig. 4a. The black, red and green curves correspond to the intensity enhancement spectra of structures A, B, and C, respectively. The dashed violet line marks the positions of the excitation wavelength at 785 nm, and the cyan line at 1010 nm marks the special Raman vibration line at the wave number of 2960 cm 1. Compared with structures B and C, structure A has the largest gap size of 25 nm so it provides weak intensity enhancement at both the excitation wavelength and Raman wavelength. From the red and green curves, we can see that both structures B and C have intensity enhancement peaks very close to the pump line and thus both provide strong excitation enhancement. In Fig. 4b, Raman vibrations from 500 cm 1 to 3300 cm 1 are collected experimentally with the Renishaw confocal Raman microscope. To show the Raman peak at 2960 cm 1 more clearly, the integration time after the blue line is four times longer compared with the integration time before the blue line, which leads to an almost four times intensity enhancement of the Raman signals after the blue line. The black Raman spectrum of structure A shows the weakest Raman intensity. The red Raman spectrum from structure B shows a slightly larger Raman intensity than the green one from structure C in some wave numbers but weaker in other wave numbers. This is because the total Raman intensity enhancement is contributed by the excitation enhancement and the Stokes enhancement. Consequently, the total enhancement value varies in different wave number regions. From Fig. 4a, at the cyan line which corresponds to the Raman signal at 2960 cm 1, we can see that structure B provides a stronger Raman intensity enhancement than structure A and C. Thus, the Raman signal at 2960 cm 1 from the red Raman spectrum is enhanced by both a strong enhancement of the excitation light and a moderate Stokes enhancement, while the Raman signal at 2960 cm 1 from the green Raman spectrum is enhanced only by a strong excitation enhancement. With a similar analysis, one can easily explain that the Raman signal at 2960 cm 1 from

6 ORIGINAL Laser Photonics Rev. 8, No. 4 (2014) Summary In conclusion, we have successfully demonstrated both theoretically and experimentally the superority of a dualresonance nano-plasmonic structure as a dual-resonance- SERS chip (for sensing the concentration of some specific moleculars) and experimentally obtained a high FoM of 14.2 as a LSPR sensor (for sensing a small change in the refractive index of the dielectric environment). We have also extensively analyzed the characteristics of the two resonances through the reflection spectrum, electric intensity enhancement spectrum and field profiles. We envisage that this dual SERS-LSPR sensing chip with powerful local refractive index detection and Raman enhancement can find many future applications. Figure 5 Linear plot of the LSPR resonances of the dualresonance device as the refractive index of the embedding medium varies. structure B is much larger than that from structure C. Another important feature in Raman spectra is that the intensity of the black Raman signals at 2960 cm 1 is relatively larger compared with the Raman signals at shorter wave numbers. It can be seen that at 2960 cm 1, the intensity of the black Raman signal is nearly half that of the green one while at other wave numbers the black Raman intensity cannot reach even fifth of the green Raman intensity. This is because the second peak of structure A is around 1010 nm, which exactly corresponds to the enhancement at 2960 cm 1. In addition to its ability in enhancing local electric fields both at the excitation wavelength and the Raman wavelength, this dual SERS-LSPR sensing chip is also a good refractive index sensor. In Fig. 5, we test the sensitivities of the two LSPRs. The refractive index is changed from 1 to 1.5 with a step interval of 0.1 and the green and red curves correspond to the MDR and EDBR, respectively. The slope of the curves represents the sensitivity (S). It is clear that the MDR has a larger sensitivity of 590 nm/riu compared to the EDBR s sensitivity of 354 nm/riu. The experimentally measured sensitivities of the MDR and the EDBR are 577 nm/riu and 326 nm/riu, respectively. These measured values coincide very well with the simulated ones. In addition to sensitivity, another factor affecting the performance of a plasmonic sensor is the plasmonic line width, typically represented by the full width at half maximum (FWHM). Therefore, a figure of merit (FOM), defined as FOM = S/FWHM, is used to evaluate the overall performance of the plasmonic sensor. The measured FOMs of our sensing device from the experiment are 3.8 for the EDBR and 14.2 for the MDR. However, typical FoMs of metallic nanoparticles are around [17]. The high FoM of our MDR sensor is due to the high sensitivity and high quality factor of the magnetic dipole resonance. Acknowledgements. This work is partially supported by the National High Technology Research and Development Program (863) of China (No. 2012AA030402), the National Natural Science Foundation of China (Nos and ) and Swedish VR (No ). Received: 25 January 2014, Revised: 21 February 2014, Accepted: 28 February 2014 Published online: 31 March 2014 Key words: dual-resonance, plasmonic, SERS, LSPR, plasmonic sensor. References [1] A. P. F. Turner, Science 290, 1315 (2000). [2] J. N. Anker, W. P. Hall, O. Lyandres, N. C. Shah, J. Zhao, and R. P. Van Duyne, Nat Mater. 7, 442 (2008). [3] A. Z. K. D. Mortazavi, A. Kaynak, and W. Duan, Progress In Electromagnetics Research 126, 203 (2012). [4] S. Maier, Plasmonics Fundamentals and Applications (Springer, New York, 2007). [5] M. Pelton, J. Aizpurua, and G. Bryant, Laser & Photon. Rev. 2, 136 (2008). [6] J. F. O Hara, R. Singh, I. Brener, E. Smirnova, J. Han, A. J. Taylor, and W. Zhang, Opt. Express 16, 1786 (2008). [7] J. A. Schuller, E. S. Barnard, W. Cai, Y. C. Jun, J. S. White, and M. L. Brongersma, Nat. Mater. 9, 193 (2010). [8] C. Sher-Yi, R. Singh, Z. Weili, and A. A. Bettiol, Appl. Phys. Lett. 97, (2010). [9] Y. Sonnefraud, A. Leen Koh, D. W. McComb, and S. A. Maier, Laser & Photon. Rev. 6, 277 (2012). [10] X. Liu, J. Lin, T. F. Jiang, Z. F. Zhu, Q. Q. Zhan, J. Qian, and S. He, Progress In Electromagnetics Research 128, 35 (2012). [11] K. Kneipp, H. Kneipp, and M. Moskovits, Surface-Enhanced Raman Scattering: Physics and Applications (Springer- Verlag Berlin Heidelberg, Berlin, Heidelberg, 2006). [12] K. Kneipp, H. Kneipp, and J. Kneipp, Accounts of Chemical Research 39, 443 (2006). [13] A. Ahmed and R. Gordon, Nano Lett. 12, 2625 (2012). [14] D. Wang, W. Zhu, M. D. Best, J. P. Camden, and K. B. Crozier, Nano Lett. 13, 2194 (2013).

7 616 J. Lin et al.: Dual-resonance SERS and LSPR sensing [15] M. G. Banaee and K. B. Crozier, ACS Nano 5, 307 (2010). [16] Y. Chu, M. G. Banaee, and K. B. Crozier, ACS Nano 4, 2804 (2010). [17] H. Liao, C. L. Nehl, and J. H. Hafner, Nanomedicine 1, 201 (2006). [18] P. B. Johnson and R. W. Christy, Phys. Rev. B 6, 4370 (1972). [19] Y. Chu and K. B. Crozier, Opt. Lett. 34, 244 (2009). [20] A. Ghoshal, I. Divliansky, and P. G. Kik, Appl. Phys. Lett. 94 (2009). [21] E. Prodan, C. Radloff, N. J. Halas, and P. Nordlander, Science 302, 419 (2003). [22] S. I. Bozhevolnyi and T. Søndergaard, Opt. Express 15, (2007). [23] D. K. Gramotnev, A. Pors, M. Willatzen, and S. I. Bozhevolnyi, Phys. Rev. B 85, (2012). [24] T. Søndergaard and S. I. Bozhevolnyi, Opt. Express 15, 4198 (2007). [25] T. Søndergaard and S. Bozhevolnyi, Phys. Rev. B 75, (2007).