Abstract
Terahertz (THz) spectroscopy is a promising method to measure the spectrum of low-frequency modes of molecules or ensembles, such as crystals and polymers, including proteins. However, the main drawback of THz spectroscopy is its extremely low sensitivity. In the present study, we report on signal enhancement in THz spectroscopy achieved by depositing amino acid molecules or their derivatives on a gold rod structured silicon substrate whose localized surface plasmon resonance is exhibited in the THz frequency region. The distinct peaks derived from the enhancement of the inherent spectrum based on a molecular crystal were clearly observed when a longitudinal plasmon resonance mode of the gold rod structure was excited and the plasmon resonance band overlapped the molecular/intermolecular vibrational mode. We discuss the mechanism by which surface-enhanced THz spectroscopy was induced from the viewpoint of the enhancement of light-matter coupling due to plasmon excitation and the modulation of the plasmon band by dipole coupling between the plasmon dipole and molecular/intermolecular vibrational modes.
Highlights
Nanoparticles of metals, such as gold and silver, exhibit very intense colors derived from localized surface plasmon resonance (LSPR) [1,2]
We successfully demonstrate surface-enhanced THz spectroscopy of an amino acid molecule and its derivatives deposited on gold rod structures fabricated on a silicon substrate
The dipole resonance mode and higher-order plasmon resonance modes were observed. This observation was especially prevalent in the gold rod structure with a lower aspect ratio, which was judged from the asymmetrical spectrum shape
Summary
Nanoparticles of metals, such as gold and silver, exhibit very intense colors derived from localized surface plasmon resonance (LSPR) [1,2]. Plasmon resonances, which are collective oscillations of conduction electrons, promote the enhancement of the electromagnetic field in the vicinity of metallic nanoparticles [3,4]. Various optical effects, such as surface-enhanced Raman scattering and fluorescence enhancement, are promoted by LSPR excitation [5,6,7]. Terahertz (THz) spectroscopy has received considerable attention because the energy of a THz wave is close to that of intermolecular interactions, such as hydrogen bonding, van der Waals force, the lattice vibration of a crystal, and the translational and rotational energy of a molecule [10,11,12]. One of the drawbacks of THz spectroscopy is its extremely low sensitivity due to the low probability of interaction between the THz wave and the molecular/intermolecular vibrational modes
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