Coupled surface acoustic wave cavities

Resumen

Over the last two decades surface acoustic wave (SAW) propagating in periodic structures have attracted a great deal of attention and have been the basis of a vast number of investigations. SAW tags, for example, explore the possibility to use active or passive devices to encode information and use it in many applications, from traffic control, to security or identification of parts on conveyer lines. Much similar designs are used for sensing applications, especially of temperature and mass. In addition, a myriad of scientific advances have been made in the study of phononic crystals (PnCs). PnCs are defined as artificial materials made of periodic arrangement of scatterers embedded in a matrix, and they enable the control of the propagation of elastic waves. The growing interest in these structures arise from the exhibition of very interesting features, such as absolute acoustic band gaps – spectral bands where propagation of waves is forbidden independently of the direction of propagation – and also of dispersion curves with a negative slope – when the wave group velocity is antiparallel to the wave vector. Both effects allowed relevant experimental achievements. The former, for perfect mirrors, vibration isolation and acoustic filters. The latter, on flat superlenses, able to focus elastiwaves with a resolution that beats the diffraction limit. Such superlenses have potential applications in the fields of medical imaging or ultrasonic beam-based therapy. Moreover, the confinement of acoustic energy in carefully designed modes made possible the fabrication of very efficient waveguides. Defect modes lead also to functionalities such as filtering and multiplexing. All these functions can be achieved in a very tight space of the order of some acoustic wavelengths. Phononic crystals are similar to photonic crystals except for the peculiarities of elastic waves as compared to optical ones. Elastic waves can be strongly anisotropic and exhibit different combinations of shear and longitudinal polarizations. Also, surface modes almost always exist at the phononic crystal boundaries. Most studies of PnCs focus on the propagation of bulk acoustic or elastic waves. However, bulk elastic waves are generally generated outside the sample of interest. SAWs, on the other hand, can be conveniently excited at the surface of a piezoelectric solid. Motivated to expand further the realm of possibilities granted by this kind of periodic structures we investigated the coupling between defects, i. e., cavities, introduced in the matrix of scatterers. Based on previous results, where the coupling between several cavities was achieved in a one-dimensional grid, we were able to achieve two key experimental results. First, the dynamical tuning of this cavities in one dimension. And second, the extension of this 1D coupling of cavities for a two-dimensional grid composed of rectangular pillars. That is, we realized the simultaneous coupling of cavities on perpendicular directions. The major tool used to investigate these devices was a finite element method (FEM) simulation model. With the successful development of the model, we were able to predict the behavior of our devices with great accuracy. Moreover, we present a third key result, reached in close collaboration with the group of Prof. Dr. Per Delsing of the Chalmers University of Technology, in Gothemburg, Sweden, during a temporary stay there. With the use of a SAW cavity at very low temperature we studied sources of loss in superconducting quantum circuits. More specifically, losses due to two-level systems (TLS). TLSs are tunneling states, regarded as an uncontrolled intrinsic systems, which have a broad distribution of energy splitting and can be thermally activated at low temperatures, being an important source of loss. Our results shed some light into the linewidth of the TLSs ensemble and suggest a way to mitigate TLS loss in superconducting qubits. This thesis is structured as follows: on the Introduction we provide explanation of the basic concepts used throughout our work, accompanied by a brief historical introduction on each subject; on Methods we expose all the simulations models used for designing our devices and show fabrication and measurement details; on Results we display the most relevant simulation results achieved (with 2D and 3D simulation models) together with the experimental results, showing their agreement. Also, we write a section with the motivation, basic concepts and experimental results of the investigation on TLSs derived from the collaboration with Chalmers. On Conclusion we summarize our findings and give future prospects which can arise from our discoveries.