# 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.

## Key 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.

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Source: https://tomesphere.com/paper/1903.01670