Hyperbolic Metamaterial-Based Ultrasensitive Plasmonic Biosensors for Early-Stage Cancer Detection
Metamaterials are artificially designed materials that enable extreme light matter interactions that is typically not possible in naturally occuring materials. An important class of Metamaterials is hyperbolic metamaterials (HMMs), which has a hyperbolic
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Hyperbolic Metamaterial-Based Ultrasensitive Plasmonic Biosensors for Early-Stage Cancer Detection G. Strangi, K.V. Sreekanth, and M. Elkabbash
7.1
Introduction
7.1.1 Biosensors: Background, Significance, and Challenges Optical biosensors are analytical devices that detect bioanalytes immobilized on the device surface via monitoring a given characteristic of light, e.g., intensity, wavelength, and phase. Commonly, optical biosensors detect changes in the refractive index of their environment; that is, they are refractometers (Homola et al. 1999). In order to properly attribute changes in refractive index to bioanalytes, the sensing domain should be restricted to the immobilized bioanalytes only. To do so, refractometers based on guided waves have been used as biosensors, where the superstrate (upper surface) of the waveguide is functionalized to immobilize a certain bioanalyte. Guided waves are waves that are forced to propagate in a given plane of propagation. These waves have an evanescently decaying field in the superstrate and thus allow for selective detection to immobilized biomolecules. Coupling to a waveguided mode usually require total internal reflection (TIR) inside an optical waveguide due to momentum (or phase) mismatch between the propagation constants inside and outside the guide. The momentum mismatch is
G. Strangi (*) Department of Physics, Case Western Reserve University, 10600 Euclid Avenue, Cleveland, OH 44106, USA Department of Physics and CNR-NANOTEC UOS di Cosenza, University of Calabria, Rende 87036, Italy e-mail: [email protected] K.V. Sreekanth • M. Elkabbash Department of Physics, Case Western Reserve University, 10600 Euclid Avenue, Cleveland, OH 44106, USA © Springer Nature Singapore Pte Ltd. 2017 P. Chandra et al. (eds.), Next Generation Point-of-care Biomedical Sensors Technologies for Cancer Diagnosis, DOI 10.1007/978-981-10-4726-8_7
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directly related to the refractive index of both the waveguide and the superstrate. The field evanescently decays due to the momentum mismatch. The evanescent field is the sensing element, i.e., transducer. Particularly, changes in the refractive index of the superstrate can be sensed by the evanescent field which in turn changes in the propagation condition of the field inside the guide. This is because the propagation conditions inside the waveguide are dependent on the refractive index of the waveguide and the superstrate. By using the evanescent field for detection, it is possible to constrain the sensing domain to the immobilized analytes (Gauglitz and Proll 2008). An important class of guided waves are surface plasmon polaritons (SPPs). In this case, the wave is guided at the interface between a metal and a dielectric under strict conditions. In general, a wave can be confined (and guided) at an interface between two media if the field is evanescently decaying in both media. For an SPP, the field is evanescently decaying inside the metal because of its high imaginary refractive index component which rej
