Photoresponsive material for opto-acoustical applications

Stimuli responsive polymers are showing an increasing popularity in the field of smart devices. The many advantages come from their versatility, ease of use and in particular their tunability. In particular, light-responsive poly- mers offer different opportunities in terms of remote control, allowing the possibility to take advantage of different polarisation and/or frequencies. For instance, scientific literature shows the possibility of controlling optical and mechanical properties of membranes or structured polymeric samples doped with a photo-responsive molecule, upon proper irradiation with light beams. Interesting results come from the possibility to control the refractive index of a membrane or even the shape of structured micro-pillars. The main goal of this thesis is to design, fabricate and validate an acous- tic metamaterial based on photo-responsive polydimethylsiloxane doped with azobenzene molecules. The photo-responsivity is exploited to locally change the Young’s modulus in such a way that the acoustic behaviour of the system is deterministically affected. In doing this, two cases are considered. Accord- ing to a first implementation, a homogeneous illumination is used to modify the acoustic response of pre-patterned samples. Conversely, in a second im- plementation, a projected light pattern is used to induce local modifications on a homogeneous sample. In both cases, an experimental evidence of the light-dependent acoustic behaviour is provided. In the first part, the fundamental aspects and model describing elastic wave propagation in solid media are presented. This part includes also a brief description of ultrasonic waveguides and metamaterials. Next, a numerical model based on finite element methods is built in order to characterise the acoustic behaviour of the samples. The typical methods and models used in literature based on eigenmode analysis are reproduced, while the implemented model is created based on stationary solvers. For these models realistic values for the chosen material properties are taken into account. Next, an experimental part follows in which the elastomeric samples are fabricated and characterised. The processes used for the samples production are described together with the relative geometries and structures. Finally, the experimental setup setup allowing acoustic characterisation while providing arbitrary light patterns for illumination is described. The polymeric samples are illuminated by means of a spatial light modulatorused to project a pattern that locally modifies the properties of the material. The periodic pattern is chosen in order to induce the structurisation of the polymeric matrix or to enhance the response of an already present structure. Then acoustic waves are excited inside the sample by means of piezoelectric probes and the transmission spectrum is analysed with and without the illu- minated pattern. This thesis work aims to explore the pioneering field of tunable acous- tic metamaterials. These materials offer new opportunities for using light- responsive compounds in devices meant for the elastic wave control, such as elastic beam-splitters, switches, or filters, thus bringing novel effects and unprecedented degrees of control on the metematerial response.