Resonators and devices based on elastic waves obtained through the hybridization of surface and bulk waves

Published : 8 February 2020

Bulk or surface elastic wave devices are currently an enabling technology for radiofrequency emission/reception circuits used in mobile phones. Since, at constant frequency, the wavelength of elastic wave is close to 100,000 times smaller than electromagnetic wavelengths, the treatment of a signal carried by elastic waves instead of an electrical signal offers a tremendous miniaturization. With the increase in frequency bands operated simultaneously by each single mobile phone, requirements brought onto radiofrequency filters become more and more stringent. This motivates the research on new types of components exploiting new elastic waves. Conventional technologies rely on bulk acoustic waves (BAW) or surface acoustic waves (SAW) propagating respectively along the thickness or the surface of a piezoelectric material. Such kind of materials offer the possibility to couple electric signals into elastic waves, and conversely. In the last few years, a new kind of propagation mode, called “hybrid SAW/BAW” has been proposed, based on the excitation of waves by a periodic array of piezoelectric stubs. First realizations have been proposed, but their properties are not yet fully determined.

This PhD subject focuses therefore on the study of the potentialities offered by these new kinds of modes. On one hand, the properties of such waves are strongly related to the combination of piezolectric material, of the nature of the substrate, on their respective crystal orientations as well as on the geometric dimensions of the piezoelectric stubs. The candidate will therefore investigate the design space in order to reveal what performances can be expected from such structures and optimise their design towards applications such as RF filters or time references, ideally for applications above 3 GHz. This work will leverage the simulation models available at CEA-LETI and those developped by the FrecNSys company.

A second part of the PhD is expected also to explore more fundamental possibilites opened by these modes arising from the coupling between elastic surface waves and a periodic array of electrically active structures. Such periodic structures belong to the broader range of so-called elastic metamaterials, which offer unusual propagation properties such as frequency ranges in which wave propagation is forbidden, artificial slowing of waves, strong confinement or nonreciprocal propagation. Since active structures are involved, additional interesting effects may be explored. The candidate will leverage the expertise on elastic metamaterials brought by the acoustic department of ISEN.

Eventually, an experimental part will be devoted to the proposition of designs to be implemented in the clean rooms of CEA-LETI and participation to the technological developments. The goal is here to assess the exprimental feasibility of such structures.

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