Colorimetric sensing using Fourier plane imaging of surface plasmons

Abstract

Surface Plasmons (SPs) propagating at a metal-dielectric interface are highly sensitive to minute changes in the near-field refractive index and thickness of surrounding medium. Moreover, coupling of incident light to the SPs at normal incidences and inherent filtering nature of plasmonic nanostructures resulting in color selective reflection and transmission can be used for applications in SP imaging sensing [1]. Recently, fluorescence coupled leakage radiation microscopy has gained importance wherein, the information pertaining to SPs is radiated out through the substrate and imaged [2]. However, this approach requires fluorescent tagging of analytes to be used in a sensing experiment, that may not be desirable in certain cases. A different approach to image sub-wavelength thick analytes without fluorescent tagging is by using colorimetry, wherein different refractive index films/regions appear as distinctly different colors in an ordinary microscope. In this work, we demonstrated a dark field SP imaging technique by fabrication of engineered 1D and 2D plasmonic substrates and microscopy configuration for real and Fourier plane (FP) imaging to capture surface changes at sub-wavelength thickness. The substrates were designed by sandwiching a thin layer of homogeneous metal between the patterned metal and glass substrate to convert the signature of SPs from transmission dips to transmission peaks [3]. The engineered fabricated substrates were placed in between two crossed polarizers (θP = 45° and θA = 135°) to diminish direct 0th order transmission and capture bright SPs emission against a dark background in real and FP images using a bright field optical microscope. In this specific configuration (θP = 45° and θA = 135°) of cross axis polarizer-analyzer, when the polarizer was at 45° with respect to grating vector, both Transverse Electric (TE) and Transverse Magnetic (TM) could excite SPs equally. The strong coupling between these modes induced a relative phase shift between TE and TM components which led to a polarization rotation of transmitted light by 90° [3].