Design of Surface Plasmon Resonance Sensor in ...

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ScienceDirect Procedia Engineering 168 (2016) 880 – 883

30th Eurosensors Conference, EUROSENSORS 2016

Design of surface plasmon resonance sensor in plastic optical fibers based on nano-antenna arrays Nunzio Cennamoa*, Ramona Galatusb, Francesco Mattielloa, Reem Sweida, Luigi Zenia a

Department of Industrial and Information Engineering, Second University of Naples, Via Roma, 29, Aversa 81031, Italy b Technical University of Cluj-Napoca, Faculty of Electronics and Telecommunication, Cluj-Napoca, Romania

Abstract

In this work, we report the design process for an optimal response of plasmonic sensors, exploiting gold nanoantenna array geometry realized on a D-shaped plastic optical fiber (POF). We have used different nano-slot arrays geometry, made by realizing rectangular slots on the gold layer. The numerical results obtained with different geometrical parameters, such as Slot’s length, Slot’s width and spacing, are presented. The aim of this work is to consider a different approach to obtain a plasmonic sensor in POFs, in which the geometrical parameters of the slots can modify the sensor’s response to the different analytes placed and bound on the gold layer. The polarization of the Magnetic Field and the different patterns of the nano-slot array, for example different spacing among the slots, influence the plasmonic phenomena: Localized Surface Plasmon Resonance, Surface Plasmon Resonance or both. © 2016 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license © 2016 The Authors. Published by Elsevier Ltd. (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of the 30th Eurosensors Conference. Peer-review under responsibility of the organizing committee of the 30th Eurosensors Conference Keywords: Plastic Optical Fiber (POF); Surface Plasmon Resonance (SPR); Localized Surface Plasmon Resonance (LSPR); Nano-Slot Array; Nano-antenna Array

1. Introduction The study of plasmonic phenomenon is very important because, during the last two decades, it has emerged as a label-free detection method suitable and reliable for clinical analysis and bio-molecular interactions [1]. Authors have presented several bio-chemical plasmonic sensors in D-shaped plastic optical fibers [2-5]. The technique makes it possible to measure interactions in real-time with high sensitivity and without the need of labels, so eliminating the necessity of fluorescent, chemical, or radiolabeled tags. * Corresponding author. Tel.: +39 081 5010367; E-mail address: [email protected]

1877-7058 © 2016 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license

(http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of the 30th Eurosensors Conference

doi:10.1016/j.proeng.2016.11.296

Nunzio Cennamo et al. / Procedia Engineering 168 (2016) 880 – 883

In addition, biosensor technologies are relatively easy to use and offer real-time data collection so that different biochemical interactions can be monitored. Surface plasmon resonance (SPR) phenomenon is the resonant oscillation of conduction electrons at the interface between a negative and a positive permittivity material stimulated by the incident light. The resonance condition is established when the frequency of the incident photons matches the natural frequency of surface oscillating electrons. It is important to mention here that metals play a more important role in plasmonics than in dielectric media [6]. In a metal, the optical as well as the electric properties are very different from the dielectric ones because of the existence of huge free electrons, that have a faster response to varying fields leading to a response different from that in dielectric media. In SPR sensor platforms, it is possible to modify the metal layer thickness, in order to change the sensor’s response according to the refractive index changes of the medium in contact to the metal surface [7]. Another approach to increase the performances of the SPR sensors for biochemical applications is to design a special geometry for the SPR platform [8]. An additional photoresist buffer layer placed under the metal layer, in order to increase the performances of the SPR sensor in POF, has been presented [9]. Alternative SPR sensor devices are based on Nano-antenna arrays [10]. If we consider a structure made by holes in a metal screen, we could study the optical transmission properties, according to the early theory, formulated by Bethe in 1944 [11], for an idealized subwavelength hole, made in a perfectly conducting metal screen, with zero thickness, predicting very weak optical transmission properties. Later, Ebbesen discovered an extraordinary transmission phenomenon through arrays of subwavelength holes made in an opaque metal screen, with orders of magnitude more light than Bethe’s prediction [12]. This phenomenon was called Extraordinary Optical Transmission (EOT), and is mainly due to the match between the wavelength of surface plasmon polaritons (SPPs) and the period of the aperture array, resulting in the larger transmittance through the smaller aperture. Another contribution is due to the Localized Surface Plasmon Resonance (LSPR) of the subwavelength holes. The EOT phenomena exploting holes of different shapes into a metal film were studied in [13-16]. The goal of this work is not to make further investigations about the EOT phenomena, but to consider another way to design a Plasmonic sensor in which a Nano-antennas Array Structure, placed on a D-shaped POF, can improve the sensitivity of the sensor with respect to the changes of the analyte refractive index. In particular, in this paper we show the numerical results obtained with a plasmonic sensor in a D-shaped POF, made by rectangular holes into a gold layer. In order to compare the sensitivity with the classic SPR sensor in a D-shaped POF [9], we report the numerical results obtained with a continuous gold film, too. 2. Optimized geometry and numerical results The numerical simulations were made for different models:  Classic SPR sensor in D-shaped POF  SPR/LSPR sensor based on Nano-Antenna Slotted Arrays in D-shaped POF, in order to observe the behaviour of this kind of Sensor with different geometrical parameters, and different refractive index of the analytes. The simulations were performed by building a unitary cell model, applying the appropriate periodic boundary conditions, in order to simulate a planar infinite structure, using a parametric sweep of the analyte’s refractive index with the following values: 1, 1.332, 1.342, 1.352. After that we evaluate the power that is reflected from the Metal-Analyte interface, which is located on the D-shaped POF (core POF with refractive index 1.49), through the Reflectance parameter. This quantity is related to the power that has gone back into the POF waveguide and propagates until the other end of it. For these simulations we used the Lorentz-Drude model [17] to evaluate the metal dielectric permittivity of the gold, for different wavelengths. In all these models, we used the same thickness of the gold layer of 40nm. In the first step, we show the numerical results obtained with a classic SPR-POF sensor, made by a continuous gold layer [9], as shown in Figure 1a, in order to evaluate the sensor's sensitivity (S). In Figure 1b we show the Reflectance vs wavelength for several refractive indices (the resonance wavelengths are at around 600nm). If we define the Sensitivity as S=δλres/δnanal, we can evaluate the sensitivity of this sensor, that is about 1200nm/RIU. In the second step, starting from the previous numerical results, we simulate a modified version of the sensor, in which some apertures are present into the gold layer, in order to observe the sensor’s response to the analyte refractive index changes, with the Magnetic Field vector parallel to the Slot’s long side (Pol.1). In Figure 2 is shown a top view of the NanoSlot Antenna Array in which are indicated the following parameters: Slot Length (L), Slot Width (W), Vertical Delta (VD) and Horizontal Delta (HD).

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Nunzio Cennamo et al. / Procedia Engineering 168 (2016) 880 – 883

Figure 1: Pol.1 (a) Side view and top view of the SPR Sensor with continuous gold film, (b) Reflectance vs wavelength for various refractive indices of the analyte

Figure 2: Top view of Nano-slot antenna array

From the results of Figure 3a, it is possible to see that the dip observed for the SPR Sensor with continuous metal layer (around 600nm) is not present. On the other side, a very sharp dip appears around 800nm, due to the effects of the LSPR phenomenon, showing a higher sensitivity of around 1800 nm/RIU. Increasing the period between the slots, we can see other dips into the Reflectance plot (Figure 3b), always related to the LSPR phenomenon, that show a lower sensitivity to the analyte’s changes.

Figure 3: Results for Pol.1 (a) HD=80nm, VD=200nm, L=160nm, W=40nm, (b) HD=360nm, VD=480nm, L=160nm, W=40nm

From Figure 3b it is possible to see that the dip related to the SPR phenomenon is appearing (around 600nm), because a larger period between the slots gives an increase in the electron oscillation length on the metal film. In particular, for the spacing related to Figure 3b, are present both the “resonant” effects of the SPR (around 600nm) and LSPR (the other dips). In order to confirm this assumption we have made some simulations (not shown here) with the same periods of Figure 3b, changing the Slot’s Width and the Length, and we have observed that the resonance related to the SPR (around the 600nm) doesn’t change its position. On the other hand, the other three dips of Figure 3b show a definite shift, because they are related to the LSPR phenomenon. The dip related to the SPR phenomenon is not sensitive to the geometrical parameters of the slots, like width or length. Finally, we are interested in the effects of a change of polarization. The results of Figure 4a, obtained with a polarization in which the H-Field is parallel to the slot’s short side (Pol.2), give a dip, which occurs at higher wavelengths, with a better sensitivity (about 3000nm/RIU) than the previous polarization state (Pol.1). Increasing again the period between the slots, we observe that the dips obtained from the previous polarization, given by to the LSPR phenomenon, are not present (see Figure 4b), probably because with this polarization a reduction of the effects of the LSPR occurs, resulting in the vanishing of the dips into the Reflectance plot.

Figure 4: Results for Pol.2 (a) HD=80nm, VD=200nm, L=160nm, W=40nm, (b) HD=360nm, VD=480nm, L=160nm, W=40nm

Nunzio Cennamo et al. / Procedia Engineering 168 (2016) 880 – 883

In the Table 1 we show the summary of the results, obtained with different geometries and different polarizations. Table 1. SPR and LSPR Sensitivity Sensor Platform Continuous gold layer SPR Nano-Antenna SPR Pol.1 – HD=80nm, VD=200nm Nano-Antenna SPR Pol.1 – HD=360nm, VD=480nm Nano-Antenna SPR Pol.2 – HD=80nm, VD=200nm Nano-Antenna SPR Pol.2 – HD=360nm, VD=480nm

SPR 1200 nm/RIU 1150nm/RIU 1250nm/RIU

LSPR 1800nm/RIU 3000nm/RIU -

3. Conclusions In this work we performed numerical tests about the sensitivity of plasmonic sensors, made by a NanoSlot array on a D-shaped POF. In this type of structure the combined effects of SPR and LSPR phenomena are present. The presence of this latter effect is strongly related to the geometrical parameters of the slots. In particular, changing the period between the slots is possible to observe the occurrence of SPR, LSPR or both phenomena. When we consider a spacing period for which the LSPR phenomenon is present, the polarization of the H-Field and the geometrical parameters of the pattern play a fundamental role. The obtained numerical results seem to indicate the possibility of obtaining an increase in sensitivity, using an SPR Sensor with slots, with respect to the sensor made by a continuous gold film. References [1] H. H. Nguyen, J. Park, S. Kang, M. Kim, Surface Plasmon Resonance: A Versatile Technique for Biosensor Applications, Sensors 2015 15 (2015) 10481-10510 [2] N. Cennamo, A. Donà, P. Pallavicini, G. D’Agostino, G. Dacarro, L. Zeni, and M. Pesavento, Sensitive detection of 2,4,6-trinitrotoluene by tridimensional monitoring of molecularly imprinted polymer with optical fiber and five-branched gold nanostars, Sensors Actuators, B Chem. 208 (2015) 291–298 [3] N. Cennamo, L. De Maria, G. D'Agostino, L. Zeni, M. Pesavento, Monitoring of Low Levels of Furfural in Power Transformer Oil with a Sensor System Based on a POF-MIP Platform, Sensors 2015 15 (2015) 8499-8511 [4] N. Cennamo, G. D'Agostino, M. Pesavento, L. Zeni, High selectivity and sensitivity sensor based on MIP and SPR in tapered plastic optical fibers for the detection of L-nicotine, Sensors and Actuators B Chemical 191 (2014) 529-536 [5] N. Cennamo, M. Pesavento, L. Lunelli, L. Vanzetti, C. Pederzolli, L. Zeni, and L. Pasquardini, An easy way to realize SPR aptasensor: A multimode plastic optical fiber platform for cancer biomarkers detection, Talanta 140 (2015) 88–95 [6] C. P. Huang, Y. Y. Zhu, Plasmonics: Manipulating Light at the Subwavelength Scale, Active and Passive Electronic Components, Vol. 2007 Art. No. 30946 (2007) 1-13 [7] M. Iga, A. Seki, K. Watanabe, Gold Thickness Dependence of SPR-Based Hetero-Core Structured Optical Fiber Sensor, Sens. Actuat. B Chem. 106 (2005) 363-368 [8] J. Homola, S. S. Yee, G. Gauglitz, Surface plasmon resonance sensors: review, Sens. and Actuat. B Chem 54 (1999) 3–15 [9] N. Cennamo, D. Massarotti, L. Conte, L. Zeni, Low Cost Sensors Based on SPR in a Plastic Optical Fiber for Biosensor Implementation, Sensors (Basel) 11 (2011) 752–11760 [10] M. Eitan, Y. Yifat, Z. Iluz, A. Boag, Y. Hanein, J. Scheuer, Nano slot-antenna array refractive index sensors: Approaching the conventional theoretical limit of the Figure of merit, Optical Sensors 2015 (2015) Proc. of SPIE 9506 [11] H. A. Bethe, Theory of diffraction by small holes, Phys. Rev. 66 (1944) 163–182 [12] T. W. Ebbesen, H. J. Lezec, H. F. Ghaemi, T. Thio, P. A. Wolff, Extraordinary optical transmission through sub-wavelength hole arrays, Nature 391 (1998) 667–669 [13] Y. Wang, Y .Qin, Z. Zhang, Extraordinary Optical Transmission Property of X-Shaped Plasmonic Nanohole Arrays, Plasmonics 9 (2014) 203-207 [14] Y. Shen, M. Liu, J. Li, X. Chen, H. X. Xu, Q. Zhu, X. Wang, C. Jin, Extraordinary Transmission of Three-Dimensional Crescent-like Holes Arrays, Plasmonics 7 (2012) 221-227 [15] L. Yuan, F. Chen, Characteristics of surface plasmon resonances in thick metal film perforated with nanohole arrays, Optik Inter. Jour. Light Elect. Opt. 127 (2016) 3504-3508 [16] Y. Hu, G. Liu, Z. Liu, et al., Extraordinary Optical Transmission in Metallic Nanostructures with a Plasmonic Nanohole Array of Two Connected Slot Antennas, Plasmonics 10 (2015) 483-488 [17] A. D. Rakić, A. B. Djurišić, J. M. Elazar, and M. L. Majewski, Optical properties of metallic films for vertical-cavity optoelectronic devices, Appl. Optics 37 (1998) 5271-5283

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