JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 28, NO. 1, JANUARY 1, 2010 111 Lossy Mode Resonance Generation With Indium-Tin-Oxide-Coated Optical Fibers for Sensing Applications Ignacio Del Villar, Carlos R. Zamarreño, Miguel Hernaez, Francisco J. Arregui, Member, IEEE, and Ignacio R. Matias, Senior Member, IEEE Abstract—Surface plasmon resonances and lossy mode reso- nances (LMRs) can be generated with indium tin oxide (ITO) coated optical fibers. Both phenomena are analyzed and com- pared. LMRs present important advantages: they do not require a specific polarization of light, it is possible to generate multiple attenuation bands in the transmission spectrum, and the sen- sitivity of the device to external parameters can be tuned. The key parameter is the thickness of the ITO coating. The study is supported with both theoretical and experimental results. The main purposes are sensing and generation of multiple-wavelength filters. Index Terms—Indium tin oxide (ITO), optical fiber filters, op- tical fiber sensor, optical resonance, thin films. I. INTRODUCTION D URING the last decades, much research has been done in the field of semiconductor and metal-clad optical waveg- uides [1]–[6]. In both cases, the clad introduces losses to the propagation of light in the optical waveguide [2]. Depending on the properties of the cladding or thin film, three main cases can be distinguished [3]. The first case occurs when the real part of the thin film permittivity is negative and higher in magnitude than both its own imaginary part and the permittivity of the material surrounding the thin film (i.e., the optical waveguide and the surrounding medium in contact with the thin film). In this case, coupling occurs between light propagating through the waveguide and a surface plasmon, which is called surface plasmon polariton [3], or surface plasmon resonance (SPR) [4]–[6]. The second case occurs when the real part of the thin film permittivity is positive and higher in magnitude than both its own imaginary part and the material surrounding the thin film. Some authors consider these modes as long-range guided modes [3], whereas others call them lossy modes [2], [7]. Since lossy modes are a specific type of guided modes, we will use henceforward the name “lossy modes.” The third case occurs Manuscript received September 02, 2009; revised October 23, 2009. First published November 24, 2009; current version published December 23, 2009. This work was supported in part by the Spanish Ministry of Education and Sci- ence under Grant FEDER TEC2006-12170 and Grant TEC2007-67987-C02-02/ MIC. The authors are with the Electrical and Electronic Engineering De- partment, Public University of Navarra, 31006 Pamplona, Spain (e-mail: ignacio.delvillar@unavarra.es). Digital Object Identifier 10.1109/JLT.2009.2036580 when the real part of the thin film permittivity is close to zero, while the imaginary part is large [3]. This case, known as long-range surface exciton polariton, falls beyond the scope of this study and will no longer be studied. Hence, the study will be focused on the first two cases mentioned. Some theoretical studies have been devoted to light propaga- tion through semiconductor-cladded waveguides [1], [8]. The characteristics of these materials are adequate for generation of lossy modes. Moreover, for specific thickness values, attenua- tion maxima of the light propagating through the optical wave- guide are obtained [1]. This is due to a coupling between a wave- guide mode and a particular lossy mode of the semiconductor thin film, which depends on two conditions: there is a consider- able overlap between the mode fields and the phase-matching condition (i.e., the equality of real parts of propagation con- stants) is sufficiently satisfied [2]. Since the phenomenon occurs when the lossy mode is near cutoff, it is stated in [1] that there are cutoff thickness values that lead to attenuation maxima. In other works, similar conclusions are extracted after a thorough analysis of the modes. As the thickness of the thin film on the waveguide is increased, some modes guided in the optical wave- guide become guided in the thin film, which causes a modal re- distribution or modal conversion [8], [9]. Previous studies have been focused on the variation of thick- ness. However, if the thin film thickness is fixed, a resonance will be visible in the electromagnetic spectrum for those inci- dent wavelength values where there is a mode near cutoff in the overlay. This is of great interest because one of the basic ways of using waveguides as sensors is by analysis of resonance wave- length shift. Hence, the phenomenon studied in this paper is the generation of resonances in the electromagnetic spectrum based on near cutoff lossy modes. The right term should be near cutoff lossy mode resonance (NCLMR). However, for the sake of sim- plicity, the term LMR will be used, which indeed is similar to that mentioned in [7]. During the last few years, hundreds of publications have been devoted to the SPR, whereas the number of publications devoted to lossy modes is quite low [1]–[3], [7]. Moreover, the utiliza- tion of LMR for sensing purposes has not been used before this study. The main reason is that the selection of the thin-film ma- terial is critical. Among the materials whose characteristics meet the criteria for the generation of LMR, indium tin oxide (ITO) is chosen for this study. There are various reasons for selecting ITO. First, it belongs to transparent conductive oxides (TCOs), which have made a breakthrough in many scientific areas during 0733-8724/$26.00 © 2009 IEEE