The progress of plasmon-based technologies relies on an understanding of the properties of the enhanced electromagnetic fields generated by the coupling nanostrucutres. Plasmon-enhanced applications include advanced spectroscopies, optomechanics, optomagnetics and biosensing. In all plasmon-based techniques and applications, field intensity distribution is a decisive factor. The currently available plasmon imaging and molecular recognition techniques take advantage of the inhomogeneous field in the transverse plane (xy). However, little is known about the field distribution in the longitudinal direction (z), i.e., the actual magnitude of the field gradient along the longitudinal direction has never been measured experimentally. Especially, precise determination of plasmon field intensity distribution within a nanogap remains challenging.

 

To overcome these obstacles and to measure the inhomogeneity along the longitudinal direction (z) in a nanocavity, a group of international researchers worked on this mission and came out with their findings which is reported in the latest issue of Nature Nanotechnology journal. This international research team includes Dr. M. Kathiresan, a scientist from the CSIR-Central Electrochemical Research Institute (CSIR-CECRI), Karaikudi, TN.

 

Dr. M. Kathiresan

 

A self-assembled monolayer (SAM) based molecular ruler was made from a set of viologens with which the plasmon field distribution in the longitudinal (z) direction of a nanocavity was measured precisely with ~2-Å spatial resolution. Molecular ruler composes of a set of viologen-based monolayers assembled on an atomically flat Au(111) single-crystal surface. A nanocavity of well-defined size was then constructed around each self-assembled monolayer (SAM) by placing either core/shell Au/glass nanoparticles or bare Au nanoparticles on top. The size of the nanocavity was measured based on the height of the SAM. By acquiring surface-enhanced Raman scattering (SERS) spectra from the set of viologens, relative field strengths at different longitudinal positions in the nanocavity were measured. Spatial resolution for measurement of longitudinal field distribution is limited by the difference in the position of the probe moiety, along with each type of viologen molecule, and it is ~2 Å. We have shown that the electric field strength is inhomogeneous with an unexpectedly large gradient, as a result of the self-focusing effect of individual molecules in the molecular monolayer to form a plasmonic comb in the nanocavity.

 

This research not only enriches the fundamental understanding of the plasmonic field distribution in nanocavities, it may also be applied in variety of applications, including plasmon-enhanced reactions, photo-selective bond dissociation, ultra-high-resolution Raman chemical imaging and optical force-controlled assembly of nanostructures for nanodevices

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