Metal nanoparticles as nano-sized opto-acoustic transducers

There has been much interest in the optical and mechanical properties of metal nanostructures and the possibility to use them as very high frequency opto-acoustic transducers. This interest stems from the ability of such devices to work as sources of ultrasound when a short optical pulse is used to thermally excite them producing GHz acoustic waves. This vibrational response can be detected in the time domain (with other short optical pulse) by measuring the changes in the scattered light using a technique called time-resolved pump-probe spectroscopy. In particular, these devices may allow the generation of ultrasound with very high spatial frequencies (short wavelengths and oblique directions) that will enable very high resolution imaging. In this thesis, the use of spherical metal nanoparticles as nano-sized opto-acoustic transducers will be explored by time-resolved pump-probe spectroscopy measurements using solid and core-shell nanoparticles made by a single metal layer and dielectric-metal layers, respectively. Firstly, the study of the optical and mechanical responses of these devices will be studied by analytical and finite element models. This allows one to obtain the absorption/scattering coefficients and vibrational modes of metal nanostructures by solving Mie and Lamb theory, respectively. Then, time-resolved experimental data will be compared with the modelling achieving two main results. Firstly, the possibility of ``turning off'' the detection mechanism of these devices by tuning the probe wavelength without affecting the generation mechanism. Secondly, the development of a size characterisation technique which can obtain the size of individual particles, their size distribution and also, may be able to obtain information about the surrounding medium. Finally, the main novelty of this work will be described showing a new super-resolution imaging/localisation technique. Here, the optical diffraction limit is overcome by resolving several particles inside the optical point spread function by centroiding and differentiating their vibrational frequencies