A novel ultrasound generator consisting of a single mode optical fiber with a layer of gold nanoparticles on its tip has
been designed. The generator utilizes the optical and photo-acoustic properties of gold nanoparticles. When heated by
laser pulses, a thin absorption layer made up of these nanoparticles at the cleaved surface of a single mode fiber
generates a mechanical shock wave caused by thermal expansion. Mie's theory was applied in a MATLAB simulation to
determine the relationship between the absorption efficiency and the optical resonance wavelengths of a layer of gold
nanospheres. Results showed that the absorption efficiency and related resonance wavelengths of gold nanospheres
varied based on the size of the gold nanosphere particles. In order to obtain the bandwidths associated with ultrasound,
another MATLAB simulation was run to study the relationship between the power of the laser being used, the size of the
gold nanosphere, and the energy decay time. The results of this and the previous simulation showed that the energy
decay time is picoseconds in length.

Theoretical analysis of a novel ultrasound generator on an optical fiber tip

To perform an ultrasonic marble quarries reconstruction it is necessary to use an echographic method. At the employed frequencies, the commercially available transducers have an active emission surface diameter which is too small in comparison to the ultrasonic wavelength. As a consequence, a very wide radiation lobe propagation occurs leading to energy spreading and to surface propagation which masks the signal coming from the inner portion of the quarry. These effects make the echographic method inapplicable. The goal of our research is to realize a new sensor capable of increasing the ultrasonic beam directivity. A parabolic shaped holder was designed with the piezoelectric element positioned in the focus. By exploiting the parabolic reflector properties, the spherical wave is collimated in a quasi-plane wave on the parabolic focal surface.

M. Donati

Papers

Ultrasonic sensor for reconstruction of marble quarries prospecting