Part of the book: Photonic Crystals
Part of the book: The Delivery of Nanoparticles
Terahertz spectroscopy has great potential for sensing a wide range of elementary excitations. However, terahertz studies are practically limited to macroscopic ensembles of compounds (e.g., thick pellets of crystallized molecules or highly concentrated solutions of nanomaterials) due to the long radiation wavelength (about 300 μm at 1 THz). In this chapter, we show how terahertz nanoantennas can overcome the current limitations of terahertz spectroscopy such as low sensitivity and low spatial resolution. We briefly discuss how to design the resonance characteristics of a dipole nanoantenna through a Fabry-Pérot model, and then we present the experimental characterization of the spectral response of ordered arrays of such devices. Furthermore, we demonstrate how nanoantenna arrays enable the possibility of retrieving the spectroscopic signature of a monolayer of quantum dots and, in principle, of many other organic or inorganic compounds. This technique, based on the idea of increasing the sensitivity through local field enhancement, is named nanoantenna-enhanced terahertz spectroscopy (NETS). A Fano-like interference between the fundamental mode of the nanoantennas and the phonon resonance of the quantum dots is observed, together with an enhancement of the absorption of the dots up to more than a million. Finally, we show how to extract the main spectroscopic information of the quantum dots through a simple coupled harmonic oscillator model. This novel technique can be widely applied in terahertz spectroscopic studies of nanocrystals and molecules, where extremely low concentrations are of concern.
Part of the book: Terahertz Spectroscopy
The fabrication of plasmonic nanostructures with sub-10 nm gaps supporting extremely large electric field enhancement (hot-spot) has attained great interest over the past years, especially in ultra-sensing applications. The “hot-spot” concept has been successfully implemented in surface-enhanced Raman spectroscopy (SERS) through the extensive exploitation of localized surface plasmon resonances. However, the detection of analyte molecules at ultra-low concentrations, i.e., down to the single/few molecule level, still remains an open challenge due to the poor localization of analyte molecules onto the hot-spot region. On the other hand, three-dimensional nanostructures with multiple branches have been recently introduced, demonstrating breakthrough performances in hot-spot-mediated ultra-sensitive detection. Multi-branched nanostructures support high hot-spot densities with large electromagnetic (EM) fields at the interparticle separations and sharp edges, and exhibit excellent uniformity and morphological homogeneity, thus allowing for unprecedented reproducibility in the SERS signals. 3D multi-branched nanostructures with various configurations are engineered for high hot-spot density SERS substrates, showing an enhancement factor of 1011 with a low detection limit of 1 fM. In this view, multi-branched nanostructures assume enormous importance in analyte detection at ultra-low concentrations, where the superior hot-spot density can promote the identification of probe molecules with increased contrast and spatial resolution.
Part of the book: Raman Spectroscopy