Optik 125 (2014) 97–100 Contents lists available at ScienceDirect Optik jou rn al homepage: www.elsevier.de/ijleo Transverse magnetic peak type metal-clad optical waveguide sensor Hani M. Kullab, Sofyan A. Taya ∗ Physics Department, Islamic University of Gaza, Gaza, Palestinian Authority a r t i c l e i n f o Article history: Received 28 January 2013 Accepted 2 June 2013 Keywords: Double negative materials Slab waveguide sensor Reflectance a b s t r a c t Transverse magnetic (TM) waves in a four-layer slab waveguide structure are studied for optical sensing applications. The structure consists of a semi-infinite substrate, a thin metal layer, a medium with negative permittivity and permeability as a guiding layer, and a semi-infinite layer as a cover. The proposed sensor is operated in reflection mode in which the angular position of the reflectance peak is used to detect small changes in the refractive index of the cover medium. The optimal structure parameters that correspond to the sharpest and highest peak are presented. The results reveal that for aluminum metal layer, a thickness of about 9 nm represents the optimum metal thickness. Moreover, the thickness, negative permittivity, and negative permeability of the guiding layer are found to have great impacts on the performance of the proposed optical waveguide sensor. © 2013 Elsevier GmbH. All rights reserved. 1. Introduction The need for rapid and accurate processes for detecting microor- ganisms in water, air, food, and medical samples was the main reason behind the tremendous effort conducted in the field of biosensing. Sensing techniques reported include waveguide mode sensor [1–5], surface plasmon resonance (SPR) sensor [6], fiber Bragg grating sensor [7], long period grating sensor [8], and Fabry–Perot interferometer senor [9]. Biochemical sensors based on optical transducers are widely used because of their ability to detect the minute changes in the refractive index of an ana- lyte and thickness which occurs when ligands bind to receptors immobilized at the transducer surface [10]. Another commonly used sensor is the resonant mirror. The principle of operation of resonant mirrors is based on frustrated total internal reflection which combines the enhanced sensitivity of waveguide mode sen- sor [11–14] with the simple construction and operation of SPR sensor. Intensive work has been conducted to improve the sen- sitivity of optical waveguide sensors utilizing nonlinear media [15,16], reverse symmetry slab waveguide configurations [14,17], and media with negative index of refraction [18,19]. Slab waveguides have been used as optical sensors in two dif- ferent operation modes. The first one is based on the evanescent wave that penetrates the analyte medium from the guiding layer and senses the changes in the refractive index of the analyte [20,21]. The second one is operated in reflection mode in which the angular ∗ Corresponding author at: Physics Department, Islamic University of Gaza, Gaza, P.O. Box 108, Gaza, Palestinian Authority. Tel.: +972 8 2860700x1203; fax: +972 8 2860800. E-mail address: staya@iugaza.edu.ps (S.A. Taya). position of the reflectance dip is used as the probe for sensing any change in the index of the analyte [22,23]. Materials with negative effective permittivity (ε) and per- meability () in a certain frequency band have received much attention in the last two decades [24–28]. These materials are referred to as metamaterials, double negative materials (DNMs), and left-handed materials. DNMs were introduced by the Russian physicist Vesalago in 1968 who predicted a number of unusual features [24]. The phenomenon of negative index of refraction was experimentally verified when periodic arrays of split ring res- onators and wire strips were combined together [29]. DNMs have been studied for a number of interesting applications such as optical waveguide sensing [23,27]. This paper presents transverse magnetic waves in a four-layer slab waveguide structure for optical sensing applications. The guid- ing layer is assumed to be DNM and is separated from the substrate by a thin metal layer. The reflection mode is adopted for detection changes in the index of refraction of an analyte homogenously dis- tributed in the cover layer. The structure parameters are chosen so that a reflectance peak is obtained when the reflectance is scanned with the angle of incidence. The angular position of the reflectance peak is used as the probe for detecting small changes in the ana- lyte refractive index. The properties of the reflectance peak such as sharpness, position, and height are studied with the parameters of the proposed structure for optimization purposes. 2. Structure analysis We consider a slab of DNM of thickness d 3 , permittivity ε 3 , and permeability  3 sandwiched between a semi-infinite cover of per- mittivity ε 4 and permeability  4 and a thin metal layer of thickness 0030-4026/$ – see front matter © 2013 Elsevier GmbH. All rights reserved. http://dx.doi.org/10.1016/j.ijleo.2013.06.029