Negative Refractive Index Metasurfaces for Enhanced Biosensing

Zoran Jakšić, Slobodan Vuković, Jovan Matovic, Dragan Tanasković
2010 Materials  
In this paper we review some metasurfaces with negative values of effective refractive index, as scaffolds for a new generation of surface plasmon polariton-based biological or chemical sensors. The electromagnetic properties of a metasurface may be tuned by its full immersion into analyte, or by the adsorption of a thin layer on it, both of which change its properties as a plasmonic guide. We consider various simple forms of plasmonic crystals suitable for this purpose. We start with the basic
more » ... case of a freestanding, electromagnetically symmetrical plasmonic slab and analyze different ultrathin, multilayer structures, to finally consider some two-dimensional -wallpaper‖ geometries like split ring resonator arrays and fishnet structures. A part of the text is dedicated to the possibility of multifunctionalization where a metasurface structure is simultaneously utilized both for sensing and for selectivity enhancement. Finally we give an overview of surface-bound intrinsic electromagnetic noise phenomena that limits the ultimate performance of a metasurfaces sensor. OPEN ACCESS Materials 2011, 4 2 Keywords: plasmonics; optical metamaterials; artificial nanomembranes; long range surface plasmons polaritons; chemical sensors; biosensors Introduction The demand for various types of chemical, biochemical or biological (CBB) sensors is constantly increasing [1] . A CBB sensor may be generally described as a device which generates a readable output (most often electrical or optical) proportional to the amount of a targeted analyte in a given environment. Various uses in biomedicine, process industry, forensics, homeland defense and other fields pose demands which include high sensitivity and selectivity, fast response, low noise, possibility of massively parallel multisensor operation and low cost [2]. Among the convenient devices an important role belongs to surface plasmon polariton (SPP) sensors [3] . An SPP sensor is actually a simple waveguide for surface plasmon polaritons, planar waves supported at sensor-environment interface and evanescent in both directions perpendicular to it. The sensor surface is a metal slab with a negative value of relative dielectric permittivity, while the dielectric permittivity of the ambient is positive. When analyte is adsorbed at the sensor surface (either directly or through a target-specific ligand layer) or is brought into contact with it in some other way, for instance by full immersion, it modifies the refractive index -seen‖ by the SPP wave and in this manner modulates the propagating conditions. The SPP wave must use some kind of a coupler with the propagating plane waves used for excitation and readout, because its wavevector is typically much larger than that of the propagating waves. This is done using some external means, for instance a refractive prism in Kretschmann [4] or Otto configuration [5], optical grating, surface corrugations or some other coupling device [6] . A SPP chemical or biological sensor is label-free, contrary to many of other competing types of CBB sensors and, owing to its electromagnetic nature, sufficiently fast to ensure measurements in real time. A recently proposed generalization of the SPP sensor involves the use of electromagnetic metamaterials (MM) instead of simple metal surfaces. A prototype example of a metamaterial is a composite structure with the real part of its effective refractive index being less than zero (-negative refractive index-NIM‖ materials, also sometimes denoted as -left-handed‖ metamaterials or -double-negative‖ structures) [7] [8] [9] . Although the first metamaterials described were NIM structures, the term is nowadays used in a broader sense to denote various synthetic composite structures with properties not readily found in nature. These may also include (but are not limited to) the so-called single negative materials, the artificial structures in which only relative dielectric permittivity or only relative magnetic permeability reaches values lower than zero (ENG-Epsilon Negative or MNG-Mu Negative), as well as various other structures with -unusual‖ properties, an example being 2D arrays of subwavelength apertures with extraordinary optical transmission [10]. Many novel electromagnetic effects appear in the MM, some of which may appear counter-intuitive (opposite directions of phase velocity on one side, and group velocity and Poynting vector on the other side, reversal of the Snell's law, etc.) [11] . An obvious thought is to combine such properties, many of which could be used to enhance freedom in sensor design, with the already existing benefits of the
doi:10.3390/ma4010001 pmid:28879974 pmcid:PMC5448475 fatcat:7c2s2nxnh5g2rpc55hnfvayo5i