Dispersion curve-based sensitivity engineering for enhanced surface plasmon resonance detection Sherif H. El-Gohary, Seyoung Eom, Soo Yeol Lee, Kyung Min Byun n Department of Biomedical Engineering, Kyung Hee University, Yongin, 446-701, South Korea article info Article history: Received 23 December 2015 Received in revised form 23 February 2016 Accepted 2 March 2016 Available online 19 March 2016 Keywords: Surface plasmons Biosensor Dispersion Sensitivity abstract Manipulation of dispersion curve for enhancing surface plasmon resonance (SPR) detection is proposed. Based on strong correlation between slope of dispersion curve and SPR angle shift, it is confirmed that dispersion curve characteristics can be employed as an analysis tool to evaluate SPR sensor performance and to predict anomalous plasmonic behaviors. Complicated resonance shift in SPR angle, especially in the presence of metallic nanograting, such as negative shift, can be controlled reliably by engineering the dispersion curve. As it has a dependence on geometrical parameters of metallic films and gratings, dispersion relation engineering is also useful in optimizing the sensor sensitivity. For a wavelength of λ ¼630 nm, introduction of a gold nanograting shows a significant improvement in sensitivity by more than 5 times, compared to a traditional thin-film-based SPR structure. In addition, we find that use of a longer wavelength in near-infrared region can be advantageous for avoiding a negative SPR shift and obtaining a narrow and deep SPR curve. Our approach is expected to extend the applicability of dis- persion-based sensitivity engineering technique to a variety of SPR platforms for highly enhanced SPR detection. & 2016 Elsevier B.V. All rights reserved. 1. Introduction Resonant coupling between conduction electrons and polarized incident light, called surface plasmon resonance (SPR) contributes to a collective charge oscillation at a dielectric-metal interface. Since SPR condition is sensitive to a variation in refractive index of a sensing medium in proximity to a metallic substrate, one can measure an adsorption of target analytes by tracking a change in resonance angle. However, despite several unique advantages of an SPR biosensor, such as rapid, quantitative, and label-free de- tection, it often suffers from an insufficient sensitivity, especially for biomolecules at very low molecular weights [1]. Recently, dispersion relation has been investigated to interpret the manner of light coupling to surface plasmons in a variety of SPR configurations such as a curved metal-dielectric interface [2], metal-dielectric slot waveguide [3], sinusoidal metallic gratings [4], and subwavelength Gaussian grooves [5,6]. Enhanced coupling in surface plasmons by means of metallic nanoparticle arrays was explored by measuring a dispersion relation experimentally based on angle and wavelength scanning methods [7,8]. More interest- ingly, manipulation of dispersion curve by an added SiO 2 overlayer on top of a silver film was utilized to expand the color dispersion of SPR-based holography to incident angle because the angular separation for color reconstruction is relatively small for the case without SiO 2 layer [9]. Inspired by those previous works, we found a possible link between dispersion relation and SPR sensor sen- sitivity, since dispersion relation in dielectric-metal interface re- lates the angular frequency of the surface plasmon field to its wave vector magnitude depending on the optical constants of dielectric and metallic materials and its curve is slightly displaced due to a refractive index change in the dielectric medium. This gives us a hint that sensitivity characteristics of SPR biosensor can be con- trolled by engineering the dispersion curve. A number of studies for amplifying the SPR signal have been demonstrated [10,11]. For example, alternative SPR configuration with a periodic gold nanograting on a gold film was suggested to improve the angular sensitivity while maintaining its intrinsic la- bel-free strategy [12,13]. In principle, this approach could serve as an effective way to increase a surface reaction area and to generate a highly confined localized surface plasmon mode, thereby boosting a field-matter interaction [4,14]. While a resonant ex- citation of localized plasmons on a nanostructured metallic film enables to improve the sensitivity by more than an order of magnitude, highly distorted SPR curve associated with multiple localized plasmon mode excitations results in an abrupt change in magnitude or sign of the sensitivity [15,16]. Similar to an anom- alous blue-shift occasionally found in a localized SPR biosensor, a negative shift in an SPR system indicates that the resonance angle Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/optcom Optics Communications http://dx.doi.org/10.1016/j.optcom.2016.03.011 0030-4018/& 2016 Elsevier B.V. All rights reserved. n Corresponding author. E-mail address: kmbyun@khu.ac.kr (K.M. Byun). Optics Communications 370 (2016) 299–305