Sensors and Actuators B 211 (2015) 456–461 Contents lists available at ScienceDirect Sensors and Actuators B: Chemical jo u r nal homep age: www.elsevier.com/locate/snb Optical waveguide biosensor based on modal interference between surface plasmon modes Manoj Kumar a,∗ , Arun Kumar a , Saurabh Mani Tripathi b a Department of Physics, Indian Institute of Technology Delhi, New Delhi 110016, India b Department of Physics, Indian Institute of Technology Kanpur, Uttar Pradesh 208016, India a r t i c l e i n f o Article history: Received 10 November 2014 Received in revised form 21 January 2015 Accepted 22 January 2015 Available online 31 January 2015 Keywords: Surface plasmons Waveguide sensors Interaction length Coupling length a b s t r a c t A detailed theoretical study on an optical waveguide biosensor utilizing the modal interference between surface plasmon modes is presented. It is examined that the interaction length i.e. longitudinal dimen- sion of the metallic layer plays a very crucial role in determining the transmission characteristics of such sensors. And as expected, to get a dip in the transmission spectrum around a desired wavelength, the interaction length should be nearly an odd integer multiple of the coupling length corresponding to the surface plasmon modes. It is found that the minimum possible length (equal to one coupling length) of the sensor may not always give the highest figure of merit. Knowing this, we then show that the sensi- tivity of such a sensor can be further enhanced by optimizing the thicknesses of metal and intermediate layers. © 2015 Elsevier B.V. All rights reserved. 1. Introduction Surface plasmon resonance (SPR) based sensors are continu- ously attracting great interest because of the numerous advantages they offer, such as small size, high sensitivity, multichannel sensing capabilities etc. [1–5]. Amongst these, optical waveguide based SPR sensors have extra advantage of their possible integration with other optical components on a single chip [6,7]. These sensors primarily rely on coupling of power between the waveguide and surface plasmon modes. Generally, the sensing region of optical waveguide SPR sensors supports two surface plasmon modes. One of them, having its real part of effective index comparable to the waveguide mode, is the so called long range surface plasmon (LRSP) mode while another one having very high real part of effective index as well as high loss in comparison of LRSP mode is known as short-range surface plasmon (SRSP) mode [8]. In such sensors, mainly the LRSP mode contributes to the output power and a dip in the transmission spectrum is observed at a wavelength for which the real part of effective indices of the wave-guide mode and LRSP mode are closest. The position of the wavelength dip in such sensors is not affected by the length of the metal layer [4,9]. To maximize the sensitivity of such sensors, research studies have been mainly focused on the choice of metal, optimization of metal thickness and ∗ Corresponding author. Tel.: +91 1126596579. E-mail addresses: epmanoj888@gmail.com (M. Kumar), akumar@physics.iitd.ac.in (A. Kumar), smt@iitk.ac.in (S.M. Tripathi). introduction of a high refractive index intermediate layer etc. For example, Fontana [10] has reported the optimum thicknesses of gold, copper, silver, and aluminium metal films to maximize sen- sitivity of such sensors. Combination of different metals has also been reported to enhance the sensitivity [11,12]. Nenninger et al. [13] have shown that by introducing an intermediate buffer layer of Teflon, the sensitivity can be increased by about 7 times than that of conventional SPR sensors. In Ref. [14] Lahav et al. have demon- strated that the sensitivity can even be improved up to 1 order of magnitude by introducing a thin, high refractive index layer of sil- icon in between cover dielectric layer and metal layer. All of these studies focus mainly on optimizing the transverse dimensions with the basic idea being to enhance the evanescent field near the top layer-analyte interface to enhance the sensitivity. Spectral tuning of such sensors can be achieved by selecting the intermediate dielec- tric layer appropriately. Weiss et al. [15] have reported that the resonance wavelength of SPR sensor can be changed from 545 nm to 700 nm by introducing an intermediate layer (TiO 2 ) of thick- ness 80 nm. Ctyroky et al. [16] have reported that a thin layer of Ta 2 O 5 (10–40 nm) can tune the resonance wavelength in the range 600–900 nm. In some SPR sensor designs, more than one LRSP modes may be involved. For example, in the SPR sensor proposed by Nesterov et al. and ˇ Ctyrok ´ y et al. [17,18], two LRSP modes and one SRSP mode are excited. In such designs, the transmission spectrum is mainly the result of interference between two LRSP modes and will be highly dependent on interaction length of the sensor. Nesterov and co-workers [17] have made a detailed study of such sensors http://dx.doi.org/10.1016/j.snb.2015.01.088 0925-4005/© 2015 Elsevier B.V. All rights reserved.