Improved performance of SPR optical fiber sensors

Improved performance of SPR optical fiber sensors with InN as dielectric cover Ó. Esteban1*, F. B. Naranjo1, N. Díaz-Herrera2, S. Valdueza-Felip1, M. C. Navarrete3, A. González Cano2 1

Departamento de Electrónica, Universidad de Alcalá, Escuela Politécnica, 28871 Alcalá de Henares, Madrid (Spain) 2 Sección Departamental de Óptica, Escuela Universitaria de Óptica, Universidad Complutense de Madrid, Arcos de Jalón 118, 28037 Madrid (Spain) 3 Departamento de Óptica, Facultad de Ciencias Físicas, Universidad Complutense de Madrid, Ciudad Universitaria, 28040 Madrid (Spain)

ABSTRACT In this work, the performance of uniform-waist tapered optical fiber sensors based on SPR with Indium Nitride (InN) as dielectric layer has been experimentally studied. The obtained results show that the overall performance in terms of reliability, long term stability and sensitivity is improved. Furthermore, the dependence of the sensitivity with the thickness of the dielectric layer is also studied, showing that increases as the thickness decreases while the well known trend to excite the surface plasmons at longer wavelengths or higher refractive index of the outer medium is kept. Thus, a novel application of the InN to optical fiber sensors is demonstrated. The use of this material would be of great interest to produce new, improved, SPR-based devices for chemical and biological sensing. Keywords: Tapered optical fibers, Surface Plasmon Resonance, Indium Nitride

1. INTRODUCTION Surface Plasmon Resonance (SPR) is a powerful refractometric technique that has been widely used in the field of chemical, environmental and biological sensing, becoming a mature technology in biosensing applications [1]. Several setups have been proposed to excite surface plasmons, being the attenuated total internal reflection technique the most popular in the literature [2]. However, the use of optical fibers can simplify the experimental arrangement providing substantial advantages for all-fibre intrinsic systems, remote measurements, operation in harsh environments, easier handling, multiplexing, etc. In recent years, uniform-waist tapered fibers have been proposed to develop more compact and efficient SPR sensors [3]. One variety of these sensors, namely double-layer uniform-waist tapered fibres (DLUWTs) permits to tune the positions of the surface plasmon resonances for selected refractive index of the outer medium by using of a dielectric layer deposited over the metal layer that supports the plasma waves [4]. Usually, metal-oxides have been used for that second layer, although the use of some other dielectric could be also suitable in order to achieve the desired operation point. In this sense, it has recently been proposed the use of InN as a very convenient material for biosensors [5] due to the concentration of superficial Nitrogen that it provides and that can help to the functionalization of the sensor surface in order to get specific recognition of a particular analyte. This material can act as dielectric layer, given that the spectral range of operation falls out of its semiconductor behaviour range [6]. In this work, we evaluate the overall performance of SPR sensors with an InN layer. The obtained results show a high reliability and long-term stability together with a generally good sensitivity which can be as high as 1.1×104 nm/RIU with the thinnest dielectric layer that has been tested, a really remarkable value never obtained in the past with this kind

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[email protected]; phone: +34 918856566 (5); fax: +34 918856591; http://www2.uah.es/grifo/ 21st International Conference on Optical Fiber Sensors, edited by Wojtek J. Bock, Jacques Albert, Xiaoyi Bao, Proc. of SPIE Vol. 7753, 77530X · © 2011 SPIE · CCC code: 0277-786X/11/$18 · doi: 10.1117/12.885081

Proc. of SPIE Vol. 7753 77530X-1

of sensors based on DLUWTs. This figure is also comparable to the higher performance of SPR sensors reported in the literature and can be the starting point to the generalization of the use of DLUWTs with InN layers in biosensors.

2.

SENSORS FABRICATION AND EXPERIMENTAL SETUP

The method for obtaining uniform-waist tapered fibers has already been depicted elsewhere [4]. We have fabricated and used tapered single-mode fibers with 40μm waist diameter and 12 mm taper length, optimized for transmission at 850 nm. On these tapers we have deposited an Al/InN bilayer at room temperature by RF sputtering. The aluminum layer was grown using a high purity Al target (5N) in Ar plasma and 75 W of DC power. Using these growth conditions, a deposition rate of 2.8 nm/min was obtained. For InN the target a pure In disk (4N5) and pure N2 (6N) were used as the reactive gas. The nitride was deposited with 40 W of radio frequency plasma. The base pressure for the deposition was in the order of 10-5 mTorr, while the whole deposition was carried out at a working pressure of 3.5 mTorr. With this procedure, we have produced several sensors, all of them with an Al layer 8 nm thick and an each of them with a different thickness of InN: 20, 30 and 40 nm. The characterization procedure for the devices has also been reported elsewhere [4]. In short, their spectral transmittance is measured with an OSA for different refractive indices of the outer medium, always in liquid state. In our case, the index of the outer medium can be varied continuously through either a mixture of pure water and ethylene-glycol to cover the range 1.3252-1.4089 or pure water and glycerol to cover the range 1.4082-1.4290. The sensor is submersed in a small volume of the solution whose index is calculated from the values of the indices of water and the added ethyleneglycol or glycerol as a binary mixture. [7] In this way, it is possible to cover a wide range of refractive indices and determine their specific value for a given wavelength if the dispersion relationships are known.

3. EXPERIMENTAL RESULTS Since the main target for this kind of devices is biosensing applications, the first test was carried out with a transducer designed to resonantly excite a surface Plasmon with pure water as outer medium. For this purpose the transducer was fabricated with an InN layer of 40 nm thick. The measured transmittance in this case is shown in figure 1 where the shortest resonance wavelength belongs to pure water (refractive index of 1.3252) as outer medium while the largest one comes from an outer refractive index of 1.3905. With the values of the measured data, it is possible to evaluate the average sensitivity of the device around 4000 nm/riu, which is a value comparable with the reported previously in literature for DLUWTs working with TiO2 as dielectric. An additional test to check the long-term stability and the reliability of the transducers was performed after one month. The result of this test is also shown in figure 1 where both measurements appear superimposed with only a small drift in some cases. This feature is of great interest since some drift usually appeared in former SPR transducers based on tapered optical fibers with a metal oxide as dielectric layer. With our method the transducers become more reliable; a decisive characteristic to generalize its use as SPR based sensors.

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Figure 1 Transmittance of a device with a bi-layer Al/InN (8 nm/ 40 nm thick respectively) for outer refractive indices in the range 1.3252-1.3905

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To prove the tunability of the devices, an additional design was fabricated with an InN layer 30 nm thick. The characterization procedure gives the transmittance for this transducer, which is shown in figure 2. In this case the shortest resonance wavelength is obtained for an outer refractive index of 1.3950 while the largest one comes from a value of 1.4089. These results are in agreement with the expected ones that predict the resonance at higher values of refractive index in the same spectral range when the thickness of the dielectric layer decreases. As it can be seen from figure 3, the behavior of the response is quite linear, obtaining from the slope of the plotted curve the very high average sensitivity value of 10800 nm/RIU. Again the stability test was done after some time without any special care in the conservation of the fabricated transducers. The transmittance remained unchanged between several transducers fabricated with the same method while there was no drift in the resonance wavelength for the same values of outer refractive index. Since these results suggest that the sensitivity increases as the dielectric layer thickness decreases (although the resonance appears for higher values of outer refractive indices), an additional device was fabricated with the InN layer 20 nm thick. The transmittance obtained for outer refractive indices in the range 1.4082-1.4221 is shown in figure 4. In this case, the increase of the outer refractive index to higher values does not allow the plot of a SPR feature since the cutoff wavelength of the fiber is below the expected resonance one. In any case, the average sensitivity can be estimated around 11000 nm/RIU, again a remarkably high value. 1080 1060

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Figure 3 Displacement of the resonant wavelength with the change of the outer refractive index for a device with a bi-layer Al/InN (8 nm/ 30 nm thick respectively)

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In table 1 we show the sensitivity experimentally obtained for the different sensors. As it can be observed, it increases as the dielectric layer decreases, although, of course, the resonance that can be observed inside the detection spectral range appears for higher values of outer refractive index each case, which must be taken into account when selecting the application of the devices. Table 1. Resume of samples tested with InN layer thickness and average sensitivity

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4. CONCLUSIONS In this work the suitability of the use of Indium Nitride for the development of SPR-based sensors has been demonstrated. This material offers some advantages when the transducers must be functionalized in order to provide recognition of a specific analyte, since it has a high number of free nitrogen links on the surface that can be used to fix


the recognizing elements. However, the minimum resonance wavelength must be located always above 800 nm, since below this wavelength the InN becomes absorbent. Furthermore, the relatively high refractive index of the InN makes the transducers more sensitive than the previously reported ones where the dielectric layer was a metal-oxide, since the thickness of the dielectric layer can be reduced to tune the sensor response inside a specific spectral range for a range of refractive indices. This result opens the way for the use of high refractive index dielectric materials in SPR sensors to get higher sensitivities than the current ones. From the experimental results, it can be seen how the reliability and long-term stability are also improved. These features, together with the high values of sensitivity can be the starting point to replace the current setups of refractive index based sensors by devices based on tapered optical fibers, which are more compact, cheaper and mechanically stable than other.

ACKNOWLEDGEMENTS This work has been partially supported by Spanish Ministry of Science research projects SPRINT (reference CTQ200910550) and FASTCOM (reference TEC2009-14423-C02-02), by Community of Madrid project FACTOTEM II (reference S2009/ESP-1781) and by the European Social Fund and the European Fund for Regional Development.

REFERENCES [1] J. Homola, “Surface Plasmon Resonance Sensors for Detection of Chemical and Biological Species” Chem. Rev., 108, 462-493 (2008). [2] I. Abdulhalim, M. Zourob, A. Lakhtakia, “Surface Plasmon Resonance for Biosensing: A Mini-Review”, Electromagnetics 28, 214 – 242 (2008) [3] D. Monzón-Hernández, J. Villatoro, “High-resolution refractive index sensing by means of a multiple peak surface plasmon resonance optical fiber sensor”, Sens. Act. B., 115, 227-231 (2006). [4] F.J. Bueno, Ó. Esteban, N. Díaz-Herrera, M.C. Navarrete, A. González-Cano, “Sensing properties of asymmetric double-layer covered tapered fibres”, Appl. Opt. 43, 1615-1620 (2004). [5] N. Chaniotakis, N. Sofikiti, “Novel semiconductor materials for the development of chemical sensors and biosensors: A review”, Anal. Chim. Acta 615, 1-9 (2008). [6] N. Díaz-Herrera, A. González-Cano, D Viegas, J.L. Santos, M.C. Navarrete, Ó. Esteban, " Moving the wavelength detection range in Surface Plasmon Resonance sensors based on tapered optical fibers", Proc. SPIE 7653, 76531F (2010). [7] S. Sharma, P. B. Patel, R. S. Patel, J. J. Vora, “Density and comparative refractive index study on mixing properties of binary liquid mixtures of eucalyptol with hydrocarbons at 303.15, 308.15 and 313.15k”, E-Journal of Chemistry 4, 343-349 (2007).
