As extensively discussed in this book, a classic RF antenna provides a means for channeling radio waves to/from a subwavelength receiver/emitter . Similarly, optical antennas bridge the lengthscale difference between the free-space wavelength of light and subwavelength objects, thereby defining the size of the optical antenna to be in the nano to micron regime. But metals, which are the basis for almost all antenna structures, respond differently to electromagnetic waves in the RF and optical frequency ranges. In particular, metal nanostructures support LSPRs for UV, visible and near-IR wavelengths . The LSPRs strongly influence antenna design and offer an unparalleled means to effectively address nanoscopic objects, such as individual molecules, using light [38, 169]. Nanoplasmonic antennas, ranging from single colloidal NPs to elaborate lithographic structures, have therefore become the basis for a variety of surface-enhanced molecular spectroscopies, such as SERS [168, 176, 177], SEIRA [189, 792] and SEF . These methods thus focus on using nanoplasmonic antennas to increase the interaction between external radiation and the molecule, thereby amplifying the strength of the molecular spectroscopic fingerprint. However, the antenna and the molecule is a coupled system, which means that the presence of the molecule will affect the antenna resonance. Nanoplasmonic refractive index sensing is essentially about this effect, that is, to register a change in the dielectric environment of the antenna through an optical measurement of the antenna's LSPR properties.
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