Strong Vibrational Coupling in Room Temperature Plasmonic Resonators

TL;DR

This paper demonstrates strong vibrational coupling at ultra-high frequencies in room temperature plasmonic nanoresonators by significantly increasing vibrational quality factors, enabling potential quantum control of phonon modes in metallic nanoparticles.

Contribution

It reports the first experimental observation of strong vibrational coupling in ultra-high frequency plasmonic nanostructures with enhanced quality factors at room temperature.

Findings

Achieved the highest frequency quality factor product of 1.0×10^{13} Hz in plasmonic nanoresonators.

Observed avoided crossing indicating strong intermodal vibrational coupling with a rate of 7.5 GHz.

Expanded the platform for quantum control of phonons in metallic nanoparticles.

Abstract

Strong vibrational coupling has been realized in a variety of mechanical systems from cavity optomechanics to electromechanics.$^{1, 2, 3, 4, 5}$ It is an essential requirement for enabling quantum control over the vibrational states.$^{6, 7, 8, 9, 10, 11}$ The majority of the mechanical systems that have been studied to date are vibrational resonances of dielectric or semiconductor nanomaterials coupled to optical modes.$^{12, 13, 14, 15}$ While there are fewer studies of coupling between two mechanical modes,$^{3, 9}$ particularly, there have been no experimental observation of strong coupling of the ultra-high frequency acoustic modes of plasmonic nanostructures, due to the rapid energy dissipation in these systems. Here we realized strong vibrational coupling in ultra-high frequency plasmonic nanoresonators by increasing the vibrational quality factors by an order of magnitude. This is achieved through blocking an energy dissipation pathway in the form of out-going acoustic waves. We achieved the highest frequency quality factor products of $\mathbf{f}\times\mathbf{Q}=1.0\times10^{13}$ Hz for the fundamental mechanical modes in room temperature plasmonic nanoresonators reported to date, which exceeds the value of $0.1\times10^{13}$ Hz required for ground state cooling. Avoided crossing were observed between the vibrational modes of two plasmonic nanoresonators with a coupling rate of $\mathbf{g}=7.5\pm 1.2$ GHz, an order of magnitude larger than the dissipation rates. The intermodal strong coupling was consistent with theoretical calculations using a coupled oscillator model. Our results expanded the strong coupling systems for mechanical resonators and enabled a platform for future observation and control of the quantum behavior of phonon modes in metallic nanoparticles.