Strong Vibrational Coupling in Room Temperature Plasmonic Resonators

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 $\mathbf{f}\times \mathbf{Q}=1.0\times 10^{13}$ Hz for the fundamental mechanical modes in room temperature plasmonic nanoresonators reported to date, which exceeds the value of $0.1\times 10^{13}$ 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.

29 pages, 4 figures

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