Strong vibrational coupling in room temperature plasmonic resonators

Strong vibrational coupling has been realized in a variety of mechanical systems. However, there have been no experimental observations of strong coupling of the acoustic modes of plasmonic nanostructures, due to 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. We achieved the highest frequency quality factor products of f × Q = 1.0 × 1013 Hz for the fundamental mechanical modes, which exceeds the value of 0.6 × 1013 Hz required for ground state cooling. Avoided crossing was observed between vibrational modes of two plasmonic nanoresonators with a coupling rate of g = 7.5 ± 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 enabled a platform for future observation and control of the quantum behavior of phonon modes in metallic nanoparticles.

I have already stated what I think is novel and important in the manuscript in my previous report. I would like to add that it is valuable for science in general that this study combines in a novel way plasmonics and optomechanics, and introduces a system where time-domain studies of strong coupling are highly feasible. The manuscript is now, after revision, of high quality and presents novel findings of interest to a broad audience.
For all these reasons, I recommend publication in Nature C ommunications. However, I recommend that the authors consider first the following point: Several researchers indeed give g/omega > 0.1 as a limit for ultrastrong coupling, but others stick to g/omega ~ 1. I find the latter the more correct one, since the effects typical for the ultrastrong coupling regime become prominent only when g/omega ~ 1. I find the authors claims about ultrastrong coupling kind of overselling, and would recommend the authors take away those claims (they can just say that the g/omega = 0.14 is quite large). This would by no means decrease the quality of the manuscript, on the contrary. This manuscript has been previously reviewed at another journal that is not operating a transparent peer review scheme. This document only contains reviewer comments and rebuttal letters for versions considered at Nature Communications. Mentions of the other journal have been redacted.

reviewers' comments
Reviewer #1 (Remarks to the Author): The authors have considerably improved the manuscript. The definition of the coupling is now given (and it is correctly defined), so one can meaningfully compare the coupling to the damping rates. The authors present additional experimental data measured at different positions on the nanoplates, which allows them to do a statistical analysis and assign error bars to the observed quantities. These improvements make the reported observation of strong coupling convincing. The authors have also satisfactorily responded to my other concerns, and done further improvements based on the comments of other referees.
I have already stated what I think is novel and important in the manuscript in my previous report. I would like to add that it is valuable for science in general that this study combines in a novel way plasmonics and optomechanics, and introduces a system where time-domain studies of strong coupling are highly feasible. The manuscript is now, after revision, of high quality and presents novel findings of interest to a broad audience.
For all these reasons, I recommend publication in Nature Communications. However, I recommend that the authors consider first the following point: Several researchers indeed give g/omega > 0.1 as a limit for ultrastrong coupling, but others stick to g/omega ~ 1. I find the latter the more correct one, since the effects typical for the ultrastrong coupling regime become prominent only when g/omega ~ 1. I find the authors claims about ultrastrong coupling kind of overselling, and would recommend the authors take away those claims (they can just say that the g/omega = 0.14 is quite large). This would by no means decrease the quality of the manuscript, on the contrary.