We have studied on the control of molecular properties in the spaces of various optical cavities, such as plasmonic metal nanostructures and Fabry-Perot mirrors. Single-molecule observations based on surface-enhanced Raman scattering (SERS) spectroscopy are used for the findings on unique resonant enhancements of SERS through exotic electronic excitation processes at electrified interfaces. Plasmonic surface lattices and Fabry–Perot mirrors allow the formation of electronic and vibrational strong coupling states, respectively, showing unique properties of molecule polaritons, even under dark conditions, i.e., without photo-illumination.
Optical manipulation has been used for the trapping of micrometer-scaled objects, but it is still difficult to control the motion of small molecules on the nanometer scale at room temperature. Plasmonic metal nanostructures are expected to be useful for the optical manipulation of nanoscale molecules using a highly localized electric field. We use the plasmonic Ag nanostructure for a demonstration of optical trapping through the observation using surface-enhanced Raman scattering (SERS) imaging. The optical measurements were conducted under electrochemical potential control to stabilize the nanostructure with target molecules, 4,4′-bipyridyl (44 bpy). Upon increasing the concentration of 44 bpy molecules in an electrolyte solution at room temperature, the blinking frequency of the SERS signal was different in both the spectra and imaging. The dwell time of the SERS signals was increased from several tens of milliseconds to a few seconds, which suggested the successful observation of plasmonic trapping of small molecules through the surface diffusion control. The observed results prove the importance for the control of the surface coverage of the molecules and its influence on surface diffusion under plasmonic molecular trapping.
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