PIERS ONLINE, VOL. 4, NO. 7, 2008 746 High Sensitivity of Phase-based Surface Plasmon Resonance in Nano-cylinder Array Bing-Hung Chen, Yih-Chau Wang, and Jia-Hung Lin Institute of Electronic Engineering, National Dong Hwa University Hualien 97401, Taiwan, R. O. C. Abstract Surface plasmon resonance sensor is used to measure the variation of dielectric constant sample, but its sensitivity is limited to surrounding temperature fluctuation. We present a high sensitivity phase-based surface plasmon resonance sensor made by gold nano-cylinder array. This structure is not only to reduce thermal conduction by the non-continuous gold array, but also to enhance surface plasma by these nano-size surfaces. This device consists of 160 nm diameter, 400 nm height gold cylinders on 47 nm gold film coated on Corning 1737 glass, and match with BK7 prism by Nul LS-5252 optical matching liquid. Surface plasma wave was excited by a 632.8 nm HeNe laser irradiating on this prism at a particular angle. The phase variation between the signal light and reference light are collected by a pair of photo-detectors and recorded by a lock-in amplifier (Stanford Research 830). By considering heterodyne interference optical path, the sensitivity of this tool can reach 10 7 RIU. 1. INTRODUCTION The purpose of sensing technology is qualitative and quantitative analysis the physic phenomenon around environment. Due to nano-scale device manufacturing technologies are increasingly fine and delicate, the accompanied sensing approach also become more important for detecting the tiny variations of physic parameters in the industrial applications and biochemical area. In recent years, surface plasmon resonance (SPR) sensing technology has been proposed and applied in many fields because of its various advantages such as real time, label free, high sensitivity, high throughput screening and qualitative and quantitative analysis interaction between bio-molecules. SPR sensing approach using a thin metal film to couple incident light and plasmon wave is very sensitive to slight changes of surface film thickness and surrounding medium refractive index. However, this kind of SPR based on intensity interrogation has relative low sensitivity compared with phase-based SPR. It has been found that the phase can change much more abruptly than the intensity as the change of thickness of metal thin film or refractive index of media on the sensor surface, and therefore becomes an attractive sensing technology [1]. The theoretical prediction has shown that the phase measurement gives the sensitivity of about 10 2 to 10 3 times higher than the conventional one [2, 3]. Several experimental configurations have been proposed to measure the phase variation of the SPR excitation, for example phase shifting interferometry (PSI) [3], and heterodyne interferometry (HI) [4] etc. PSI has the drawback that is easily disturbed by environmental noise, like temperature and mechanical vibration, and then sensitivity is limited. HI is an optical modulation technique which can suppress the noise caused by environmental disturbance, thus it can offers much higher sensitivity than PSI. In the HI configuration, the reflective light contains a pair of correlated and orthogonal linear polarized p and s light wave with different phase caused by SPR are collected by photodetector and then amplified by lock-in amplifier. On the other hand, the field enhancement or plasmon wave confinement is another critical factor for high sensitivity SPR device. To excite localized surface plasmons (LSP) has been demonstrated and presented high localized electric and magnetic field nearby sensing surface [5]. Compared with conventional SPR sensor, LSP has localized and highly enhanced plasmon can interact with biomolecule binding events close to sensor surface and cause a larger shift of resonant LSP modes [6]. Nano-wire SPR is a kind of LSP, and has been numerically calculated the sensitivity to be more than order than the conventional one [7]. In this paper, we proposed a nano-cylinder array based sensor accompanying heterodyne interferometry configuration to develop a high sensitivity LSP sensor. In which, on the one hand pair of cylinders can be regard as in parallel wires with symmetry electrical resonance mode, on the other hand pair of cylinders with bottom connection with asymmetry magnetic resonance mode. Both of these two modes can enhance light interaction with plasmonic nanowire system [11]. This
PIERS ONLINE, VOL. 4, NO. 7, 2008 747 gold nano-cylinder array is not only to reduce thermal conduction fluctuation, but also to increase the resonance effect by array pattern interference. We expect the sensitivity can reached to 10 7 refractive index unit (RIU). 2. EXPERIMENTAL SETUP In this section, we will present the sensor s fabrication procedure and the optical experimental setup. BK7 prism was chosen as coupler of incident light with LSP mode. 2.1. Gold Nano-cylinder Array Fabrication Figure 1 shows the gold nano-cylinder array fabrication process. With the assistance of lithography, molding and UV forming, photo-patterned features of the gold nano-cylinder array were constructed on a glass and mounted on the BK7 prism via optical match liquid. PR coating Demolding Lithography Photoresist UV exposure Mask Photoresist PR development UV cruing Sputtering (Au) Demolding Molding UV forming casting Figure 1: Gold nano-cylinder array fabrication process. The pattern features was outlined with design software and printed out to be a photo mask. Photo-resist (PR) layer then was spin-coated on the silicon substrate surface. After UV exposure and PR development, the desired pattern was transferred to the PR layer. The follow up step was to cove transparency film () to PR layer. Then removed PR and sputtered gold onto. Finally, de-mode and bond pattern gold film on the Corning 1737 glass as shown in Fig. 2(a). The gold nano-cylinder has 160 nm diameter, 400 nm height and with 160 nm space on a 47 nm thickness gold film. Kreschmann prism coupler was chosen to excite surface plasmon wave [10]. In which, gold cylinder array is the active metal material and glass slide mounted on the prism by 0.1cm Index match oil BK7 prism Corning 1737 glass Coring glass slide 2.5cm Gold nano-cylinder array Dielectric (alalyte) gold nano-cylinder 2.5cm (a) (b) (c) Figure 2: (a) The gold nano-cylinder array on a glass substrate. (b) The prism coupler used in our experiment. (c) The SEM of thenano-cyliner array structure.
PIERS ONLINE, VOL. 4, NO. 7, 2008 748 using optical matching liquid as shown in Fig. 2(b). The morphological profile of the array can be seen in SEM picture in Fig. 2(c). 2.2. Experimental Setup The proposed optical path structure of heterodyne interferometry with phase-based LSP sensor is shown in Fig. 3. The proposed optical path structure of heterodyne interferometry with phase-based LSP sensor is shown in Fig. 3. The light source is a frequency stabilized He-Ne laser operating at the wavelength of 632.8 nm having output power 15 mw. The chopper, mirror (2 and 3) and beam splitter (1 and 4) is used for obtaining a linear polarized sensing beam with difference frequency shift. The sensing beam is then split into a reference and a signal beam by beam splitter (6). The reference beam go through a polarizer (polarizer 7, oriented at 45 relative to the horizontal direction) and is receive by photo-detector (8). None the less, the signal beam was used for surface plasmon excitation in the Kreschmann configuration at an incidence angle having maximum SPW coupling efficiency. The reflected beam then go through a polarizer (polarizer 10, oriented at 45 relative to the horizontal direction) and is receive by photo-detector (11). NB 6 4 5 1 He-Ne laser 9 Lock-in amplifier 10 11 7 8 3 2 Figure 3: The optical path structure of heterodyne interferometry with phase based LSP sensor. In which, (1, 4 and 6) are beam splitters, (2 and 3) are mirrors, (5) is chopper, (7 and 10) are polarizers, (8 and 11) are photo-detectors, (9) is Kreschmann coupler. Finally, compare signal and reference beam to obtain phase difference by lock-in amplifier. The advantage of this sensor system is that any unwanted phase drift by mechanical and temperature can be eliminated through nano-cylinder array and heterodyne interfermetry. Figure 4: Experimentally relative phase changes versus different alcohol concentrations. The dash and solid line represent experimental results of a thin-film SPR sensor and a nano-cylinder LSP sensor, respectively.
PIERS ONLINE, VOL. 4, NO. 7, 2008 749 3. RESULTS AND DISCUSSION To demonstrate the high sensitivity of this nano-cylinder array LSP sensor we prepared two distinguish structure devices, one is the conventional thin-film SPR with film thickness 47 nm and the other is the novel one sensor, both having the same optical circumstances. We chose the alcohol-water mixtures with different alcohol concentrations as our testing samples. These seven samples have alcohol concentration range of 0% to 6% at the same room temperature. According to the literature which describes the mixtures solution concentration following up the rule of weight percentage [8], the refractive index would theoretically variate in the range of 1.3330 to 1.3367. Results of relative phase changes with respect to the alcohol concentration variation are shown in Fig. 4. The sensitivity, σ n, is a criterion to evaluate a SPR measurement system to resolve the smallest change of refractive index in an ambient medium. According to the definition [9], sensitivity should be σ n = ( n/ δ) σ δ, where n is the change of refractive index and δ is the corresponding phase variation, and σ δ is the instrument smallest resolution. Analyzing from measured dada, n, is 3.7 10 3 RIU, σ δ is 0.01 of our lock-in amplifier and δ are 29.71, 103.96 for thin-film SPR sensor and nano-cylinder LSP sensor, respectively. The overall system sensitivity of thin-film SPR sensor can reach to 1.25 10 6 RIU, but the nano-cylinder LSP sensor can reach to 3.55 10 7 RIU. 4. CONCLUSION In this paper, we have present a high sensitivity phase based surface plasmon resonance sensor structure and the accompanying optical path configuration. This structure is not only effectively to reduce thermal fluctuation by the gold array, but also to enhance localized surface plasma by symmetry electrical field resonance mode. Phase measurement between p and s polarized light can eliminate any unwanted phase shift caused by mechanical and temperature by using heterodyne interferometry. From experimental results we have demonstrated that nano-cylinder array LSP sensor has higher sensitivity than thin-film-based SPR sensor, and the sensitivity can reach to 10 7 RIU. We expected that use of nano-cyliner array can further enhance the sensitivity of sensor by adjusting the diameter and space between cylinders to observe the localized magnetic resonance. ACKNOWLEDGMENT This work was supported by the National Science Council of Taiwan under Grant NSC96-2120-M- 002-017. REFERENCES 1. Yu, X., D. Wang, X. Wei, X. Ding, W. Liao, and X. Zhao, A surface plasmon resonance imaging interferometry for protein micro-array detection, Sensors and Actuators B, Vol. 108, 765 771, 2005. 2. Markowicz, P. P., W. C. Law, A. Baev, P. N. Prasad, S. Patskovsky, and A. Kabashin, Phase-sensitive time-modulated surface plasmon resonance polarimetry for wide dynamic range biosensing, Opt. Express, Vol. 15, 1745 1754, 2007. 3. Wu, S. Y., H. P. Ho, W. C. Law, and C. L. Lin, Highly sensitive differential phase-sensitive surface plasmon resonance biosensor based on the mach-zehnder configuration, Opt. Lett., Vol. 29, 2378 2380, 2004. 4. Chiang, H.-P., H.-T. Yeh, C.-M. Chen, J.-C. Wu, S.-Y. Su, R. Chang, Y.-J. Wu, D. P. Tsai, S. U. Jen, and P. T. Leung, Surface plasmon resonance monitoring of temperature via phase measurement, Optics Communications, Vol. 241, No. 4 6, 409 418, November 16, 2004. 5. Prasad, P. N., Nanophotonics, Wiley-Interscience, 2004. 6. Byun, K. M., S. J. Yoon, D. Kim, and S. J. Kim, Experimental study of sensitivity enhancement in surface plasmon resonance biosensors by use of periodic metallic nanowires, Opt. Lett., Vol. 32, 1902 1904, 2007. 7. Kim, K., S. J. Yoon, and D. Kim, Nanowire-based enhancement of localized surface plasmon resonance for highly sensitive detection: A theoretical study, Opt. Express, Vol. 14, 12419 12431, 2006. 8. Lide, D. R., Handbook of Chemistry and Physics, 82nd ed., CRC Press, Boca Raton, FL, 2001. 9. Nelson, S. G., K. S. Johnston, and S. S. Yee, High sensitivity surface plasmon resonance sensor based on phase detection, Sensors and Actuators B, 35 36, 187 191, 1996.
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