Observation of inhomogeneous plasmonic field distribution in a nanocavity

The progress of plasmon-based technologies relies on an understanding of the properties of the enhanced electromagnetic fields generated by the coupling nanostrucutres 1 – 6 . Plasmon-enhanced applications include advanced spectroscopies 7 – 10 , optomechanics 11 , optomagnetics 12 and biosensing 13 – 17 . However, precise determination of plasmon field intensity distribution within a nanogap remains challenging. Here, we demonstrate a molecular ruler made from a set of viologen-based, self-assembly monolayers with which we precisely measures field distribution within a plasmon nanocavity with ~2-Å spatial resolution. We observed an unusually large plasmon field intensity inhomogeneity that we attribute to the formation of a plasmonic comb in the nanocavity. As a consequence, we posit that the generally adopted continuous media approximation for molecular monolayers should be used carefully. The strength of the plasmonic field between a plasmonic particle and a Au surface can be measured at ~2-Å resolution by following the Raman peaks of a suitably labelled self-assembly monolayer.