Acoustic Mode Hybridization in a Single Dimer of Gold Nanoparticles

Abstract

The acoustic vibrations of single monomers and dimers of gold nanoparticles were investigated by measuring for the first time their ultralow-frequency micro-Raman scattering. This experiment provides access not only to the frequency of the detected vibrational modes but also to their damping rate, which is obscured by inhomogeneous effects in measurements on ensembles of nano-objects. This allows a detailed analysis of the mechanical coupling occurring between two close nanoparticles (mediated by the polymer surrounding them) in the dimer case. Such coupling induces the hybridization of the vibrational modes of each nanoparticle, leading to the appearance in the Raman spectra of two ultralow-frequency modes corresponding to the out-of-phase longitudinal and transverse (with respect to the dimer axis) quasi-translations of the nanoparticles. Additionally, it is also shown to shift the frequency of the quadrupolar modes of the nanoparticles. Experimental results are interpreted using finite-element simulations, which enable the unambiguous identification of the detected modes and despite the simplifications made lead to a reasonable reproduction of their measured frequencies and quality factors. The demonstrated feasibility of low-frequency Raman scattering experiments on single nano-objects opens up new possibilities to improve the understanding of nanoscale vibrations with this technique being complementary with single nano-object time-resolved spectroscopy as it gives access to different vibrational modes.

Inelastic Light Scattering by Multiple Vibrational Modes in Individual Gold Nanodimers

“…The Lamb modes of free spheres (computed for stress-free boundary conditions at the nanosphere surface) were considered in our simulations 59 . Omitting environment effects (and, in the dimer case, acoustic interaction ones) appears reasonable here, as acoustic numerical simulations (determined using the same procedure as in our previous work 25 ) show that they lead to modest frequency shifts for the =0 and =2-8 vibrational modes considered here, in strong contrast with the =1 mode case considered in our previous works 24,25 . Note that each Raman peak is the sum of 2 + 1 contributions because of the -degeneracy of the Lamb modes (the frequency depends only on and ).…”

“…Association of the vibrational frequencies measured for nanodimers to vibrational modes is a priori more complex than in the isolated nanoparticle case, due to the acoustic interactions between close nanoparticles induced by the presence of a PVP layer around them. In particular, our previous work on single nanodimers have demonstrated both experimentally and theoretically that the frequencies of the quasitranslational =1 modes can be considerably shifted by this phenomenon 25 . However, reproduction of these calculations for = 0 and =2-8 modes shows that their frequencies are much less impacted by acoustic interactions, undergoing diameter gold nanospheres.…”

“…In a recent work, we investigated in detail the hybridization of these = 1 acoustic modes in dimers by measuring the ultra-low frequency (<15 GHz) inelastic scattering of single nanosystems (isolated nanoparticles and nanodimers) 25 and analyzing the effect of interparticle interactions on their associated frequencies, damping rates and displacement fields using numerical acoustic simulations. Here, we extend these measurements to higher frequencies (up to 45 GHz).…”

“…The used nanoparticle synthesis and sample preparation methods have already been described in detail in previous reports 24,25 . The samples consist in transmission electron microscopy (TEM) substrates with a 40 nm silica membrane, on which a 5 µL drop of a diluted solution of about 100 nm diameter gold nanoparticles coated with polyvinylpyrrolidone (PVP) is deposited and left to dry for several hours.…”

Strong vibrational coupling in room temperature plasmonic resonators

“…Constructing high-frequency/high-quality factor plasmonic nanoresonators will be attractive for cavity optomechanics and electromechanics applications 4,20 , where strongly coupled systems with low losses are needed to observe effects such as Rabi splitting and electromagnetically induced transparency 21,22 . However, the large damping rates that have been reported for plasmonic resonators to date makes strong coupling an unattainable regime 23–26 . Here we improved the vibrational quality factor of Au nanoplates by an order of magnitude by blocking the out-propagating acoustic waves.…”