This firs part of the project has been dedicated to two main actions.
The first one pertains to the acquisition and installation of a dilution refrigerator. Preliminary measurements of surface acoustic wave (SAW) devices (quartz and lithium niobate one and two-port resonators operating at various frequencies) have been performed and allowed us to get a clearer view of the first instrumental challenges to address. Investigation of the supraconductivity of the materials used for the fabrication of the SAW devices have also been conducted.
In parallel, we have been working on different strategies for surface acoustic wave propagation control, first at room temperature and normal pressure, to start identifying the most relevant phononic devices to be pushed and used in the remaining of the project. We worked in particular on the design, fabrication and characterisation of phononic structures for SAW waveguiding and for sub-wavelength elastic energy confinement. In particular, we demonstrated the feasibility of a SAW wave guiding and splitting in a phononic waveguide constructed by managing line defects in a square-lattice phononic crystal made pillars deposited atop a piezoelectric substrate. We showed that the guiding effect could be achieved through the optimisation of pillar geometry, mode selection, and channel design. We also showed the possibility to confine elastic energy by exploiting interactions between coupled pillar resonators and surface acoustic waves. Sub-wavelength scale confinement, accompanied by a six-fold field enhancement were hence obtained.
Eventually, we also demonstrated the existence of particular elastic nonlinearities triggered by the interaction of surface acoustic waves with micron-scale phononic resonators. These nonlinearities are characterised by a frequency softening of the pillar pair response for short gap distances, while isolated resonators retain a linear behavior. This points at a mechanism lying outside the usual geometric Duffing nonlinearities linked to the resonator’s geometry. A preliminary analytical model was therefore developed and shown to describe the experiment well by introducing a nonlinear coupling term in the Duffing equation. The nonlinear coupling strength is conditioned by the SAW amplitude, but also by the SAW wave vector, that sets the resonator mode symmetries. The oscillation of the resonator itself was shown to lead to a mechanical back-action towards the substrate surface, that in turns allowed for the occurrence of circular-polarisation states,hence constituting promising results towards the implementation of polarisation-based signal processing functionalities. An experimental method aimed at characterising the nature of these nonlinearities is currently being developed. We also observed nonlinear effects in a surface phononic crystal resonator made of periodical high-aspect-ratio nanostrips fabricated atop a lithium niobate substrate and applied these effects to demonstrate the possibility to implement tunable phononic resonators in the GHz freiqncy range.
