Single Nanoparticle as a GHz opto-acoustic transducer for local elastic probing at a submicron scale – NanoVibe

The present project aims at developing innovative experimental techniques to perform 3D elastic probing of heterogeneous media at a submicron scale by exploiting the high frequency acoustic field radiated by a nanoparticle (NP). This project will therefore allow getting insight into local elastic characteristics of solid-state devices or biological processes. During the last twenty years, opto-acoustic methods have revealed as a powerful way to investigate elastic vibrational modes of metal or semiconductor NPs, either in the spectral domain (Raman or Brillouin spectroscopy) or in the time domain (pump-probe transient transmission experiments). Measurements have been performed on ensemble of NPs and more recently on a single NP. We have reported transient reflection measurements which have revealed a new detection mechanism of elastic vibrations of a single submicron gold particle. Transient transmission measurements bring less information on elastic properties of the surrounding medium than transient reflection. The latter allows the detection of Brillouin scattering, the characteristic of which being intrinsic to the surrounding medium, i.e. independent of the NP morphology.

This project will be dedicated in a first step to the engineering of nanometric opto-acoustic transducers (NOATs). This part will lean on the preliminary results we have obtained with a single 430 nm diameter gold particle.
In the first task, we will optimize the transduction efficiency by adjusting both the NP size/shape and its constitutive material so that Brillouin frequency (a few tens of GHz) matches the fundamental breathing mode of the NP. We will optimize at the same time the detection efficiency by finely tuning the probe wavelength. A theoretical modelling of the 3D elasto-optic interaction will be developed to include the 3D character of the optical wave.
The second task will be dedicated to the generation of higher frequency acoustic waves (above 100 GHz), in order to become sensitive to nanometric elastic heterogeneities. Besides investigations on core-shell NPs, preliminary studies will be carried out to highlight the potential of magnetic NOATs to control the directivity of the acoustic field radiated.
Finally, the last task will consist in demonstrating the suitability of NOATs to perform a 3D local elastic probing at a submicron scale, either through the Brillouin scattering detection (acoustic wavelengths in the 100 nm – 1 µm range) or the interferometric detection (acoustic wavelengths smaller than 100 nm) of the radiated acoustic field. The latter configuration will be also well suited if the piezo-optic coefficients of the medium of interest are too small.

The strength and the ambitious character of the present project lie thus in the different axes we propose to develop innovative application of the acoustic field radiated by a single NP. We will need at the same time high quality samples, tunable probe wavelength to maximize the magnitude of the Brillouin scattering, a mastery of NPs made of unusual constitutive material (amorphous silica, magnetic ferrite) and with original geometry (nanorods, nano-ellipsoids) and finally a thorough theoretical modelling of the detection process to take into account the 3D character of the optical wave.

This project should lead to an ensemble of NOATs, with different morphologies, each of them affording its own advantage. We should have demonstrated their strong potential to perform 3D local elastic probing at a submicron scale, having in mind to insert such a transducer in a biological.
Furthermore, a positive response to this proposal will allow the principal investigator of the present project to carry on the development of this new scientific axis he has initiated two years ago in the Laser Opto-Acoustic Group of the I2M and to reinforce the collaboration with the CIMMES team of the CRPP.