Optical sensing of analytes via surface multiplasmonics

Research and industrial fields as varied as biology, agriculture, forensic science, and pharmaceuticals require fast, accurate, and non-destructive identification of substances. There are many technologies for detecting analytes,1 some of which make use of surface-plasmon-polariton (SPP) waves.2 SPP waves propagate guided by the planar interface of a metal and a dielectric material. However, it is only possible to detect a single analyte at a time. Furthermore, we often want to enhance the reliability of the measured concentration of the analyte. Often, SPP-wave-based optical sensors are based on the Turbadar-Kretschmann-Raether (TKR) prism-coupling configuration,3 in which an SPP wave is guided by the planar interface of a thin metal film and an isotropic and homogeneous dielectric material, as shown in Figure 1. On the opposite side of the metal film, there is a prism made of an optically denser material compared to the dielectric material partnering the metal film. Light of a fixed frequency is launched toward one slanted face of the prism. Light refracted into the prism is then incident on the interface of the prism and the metal film at a certain angle with respect to the thickness direction of the film. Oriented toward the other slanted face of the prism, a photodetector measures the intensity of the light reflected from the metal film into the prism and then refracted into air. As the angle of incidence increases toward 90, a sharp drop in reflectance measured by the photodetector indicates an SPP wave has been excited, provided there is no transmittance across the partnering dielectric material. Because the partnering dielectric material is both isotropic and homogeneous, the SPP wave is excited only by ppolarized light.2 With reference to Figure 1, the magnetic field of p-polarized light is oriented perpendicular to this page, and the electric field of s-polarized light is perpendicular to the page. Perturbations in the partnering dielectric material will cause a shift in the angular location of the dip in reflectance.4 This shift is Figure 1. Setup of the Turbadar-Kretschmann-Raether (TKR) configuration, in which a thin metal film deposited on a substrate is affixed to a prism of the same optical density as the substrate. An index-matching liquid between the prism and substrate ensures a minimal refractiveindex mismatch between the prism and the substrate. A fraction of the monochromatic p-polarized light incident on one slanted face at angle is incident on the prism/metal (aluminum, Al) interface at angle . The partnering dielectric material is air. When air on the exposed side of the metal film is replaced by a fluid, a change occurs in the value of , and therefore of , at which a reflectance dip indicative of the launch of an SPP wave occurs. This change can be calibrated for sensing.