Enhanced spectroscopies and surface plasmon thin-film sensors

Surface plasmon resonance (SPR)-based sensors have seen tremendous developments during the past decade, and some ofthese sensors are becoming a mature technology now, although improvements in their sensitivity and detection limit are ongoing. Together with the recent investigations in localized SPR (LSPR) phenomena, extraordinary optical transmission through nanoapertures in metals, and surface-enhanced spectroscopies, drastic developments are expected to revolutionize the field of optical sensing. Sensitivity enhancement techniques based on SPR phenomena are reviewed, focusing both on physical transduction mechanisms and system performance. It is shown that in the majority of cases, sensitivity enhancement is associated with the enhancement of the electromagnetic (EM) field overlap integral describing the EM interaction energy within the analyte. Examples are given starting from the well-known Kretschmann configuration to the addition of high-index dielectric thin films to metals, the addition of gratings, the use of nanosculptured thin films (nano-STFs), and enhanced transmission of nanoslits. Special attention will be given to nano-STFs, which are assemblies of shaped, parallel, and tilted nanorods, prepared using many variants of the basic oblique angle deposition (OAD) technique. Because of the special shapes and nanoscale dimensions of STFs, they exhibit a great potential in the biosensing field. The fact that the nanofeatures composing these films exhibit LSPR, which falls in the visible-near infrared (Vis-NIR) range, gavethese structures the property of enhancing significantly the EM field in their vicinity. Enhancing the field by some orders of magnitude leads to amplification of many interesting optical phenomena such as surface-enhanced fluorescence (SEF), surface-enhanced Raman scattering (SERS), and surface-enhanced infrared absorption (SEIRA). A comparison is presentedbetween diferent materials, such as silver, gold, copper, and silicon, and between various shapes, such as columns, screws, spheres, and helices, which are deposited on diferent substrates, with diferent porosities and orientations.