Piezoelectric devices that manipulate high-frequency sound waves on a chip also generate a co-propagating electromagnetic field—and this project aims to actively control that field for new sensing and signal-processing capabilities. Most existing piezoelectric devices rely on weak confinement of plane acoustic waves, limiting their performance. The researchers propose that by engineering strong transverse confinement in phononic integrated circuits, they can exploit the electromagnetic field that travels alongside the acoustic wave—a field oscillating at gigahertz frequencies but confined to wavelengths roughly 100,000 times smaller than typical electromagnetic waves. This opens qualitatively new possibilities. If successful, the work could improve spin detection sensitivity in electron spin resonance experiments down to the thermal noise limit, enabling more sensitive magnetic sensors. It could also produce acoustic phase shifters and non-reciprocal amplifiers that give active control over acoustic wave propagation, pushing device integration to its limit for radio frequency signal processing. The project also tackles the fundamental engineering challenge of getting gigahertz acoustic waves into and out of micrometre-scale devices with near-unity efficiency.
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The analogy between manipulating light and gigahertz (GHz) frequency acoustic waves in chip-scale platforms has been extensively explored, with ideas from silicon photonics, mainly strong geometric confinement and routing, being applied to acoustic waves to develop phononic integrated circuits (PnICs). It is important to note that while the light-sound analogy is generally applied to the strain field, acoustic waves in piezoelectric materials have a co-propagating electromagnetic (EM) field as well. This EM field, which oscillates at GHz frequencies, but is confined to acoustic wavelengths (~10^5 smaller), underpins the dominance of piezoelectric resonators and filters in radio frequency (RF) devices. Despite the advances made in PnICs in the past decade, the majority of piezoelectric devices, both bulk and surface wave based, still rely on weak transverse confinement and manipulation of quasi plane acoustic waves. This project seeks to answer the question: if one could actively control and manipulate these co-propagating EM fields in waveguide geometries with strong transverse confinement, what qualitatively new sensing and information processing paradigms can one enable? We show that by exploiting strong field enhancement in a PnIC platform, one can design resonant magnetic near field generators for electron spin resonance experiments that can improve the spin detection sensitivity by ~10^7, down to the thermal noise limit. In addition, by engineering acousto-electric interactions in waveguide geometries, acoustic phase shifters and mode-selective, non-reciprocal amplifiers can be realized that exert active control on acoustic wave propagation, and push active passive device integration to its limit, enabling a new class of devices for RF signal processing. To ensure these devices perform as expected, we also address the important question: can we get GHz frequency acoustic waves into and out of micrometre-scale devices with near-unity efficiency?
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