Unveiling a Counterintuitive Intermode Interplay in a Prototype Plasmonic Nanosystem.

We demonstrate a counterintuitive intermode interplay in the plasmonic system of gold nanorods, i.e., energy transfer (EnT) from the lower-energy longitudinal (L) mode to the higher-energy transverse (T) mode. The opening of this EnT(L→T) channel is enabled by an energy upconversion process with the L mode, in which the solvent environment plays a critical role. Switching from a strong thermal-conductivity solvent (i.e., water) to a much weaker one (i.e., chloroform) brings on prolongation of plasmonic hot-electron lifetime and enhancement of phonon emission, thereby increasing the probability of L-mode energy upconversion assisted by self-absorption of phonon emission. The pertinent justification and further manipulation of EnT(L→T) are provided by control experiments mainly from ultrafast spectroscopy. Besides, a subtle intermode dynamic screening effect in this unary plasmonic system is also addressed. This work refreshes our knowledge about the elusive intermode interplay in plasmonic systems and offers implementable strategies to harness hot electrons toward plasmon-mediated applications.