Magneto-elastic coupling exploitation for tunable and non-reciprocal rf surface acoustic wave devices
RF signals are everywhere in today’s connected society. Surface Acoustic Wave (SAW) filters are widely used to distinguish between signals at different frequencies. Unfortunately, the performance of SAW filters drops above 5 GHz. Thin films of epitaxial LiNbO3 on sapphire host “guided” AW. These modes comply with the demand for higher frequency and higher efficiency. Unfortunately, despite the success of AW technology and its frequency progresses, two limitations remain inherent to AWs, specifically: (i) The absence of tunability once the geometry and material are defined. If several frequency bands are needed in an application, this requires several AW devices. (ii) The absence of non-reciprocity of acoustical wave propagation. In AW devices, the energy flows as easily in the forward and in the backward directions. The input of any AW device cannot be isolated from the influence of its output, as would be desirable for information processing purposes.
Spin waves (SWs) are the eigenexcitations of the magnetization. They display rich linear and nonlinear physics and they offer the same miniaturization capability as acoustical waves. The dispersion relation of SWs can be engineered by material and geometry, and later adjusted finely by magnetic fields or spin-torque effects; Upon proper design, SWs can be strongly non-reciprocal, i.e. they propagate differently in opposite directions.
The central hypothesis of our research is that coupling AWs with SWs is a route to overcome the intrinsic limitations plaguing acoustic wave technology: by researching at the interface between material science, magnetism, acoustics and microwave engineering, the objective of MAXSAW is to use specific features of spin-waves (SW) –tunability and non-reciprocity– to add new capabilities to state-of-the-art LiNbO3 AW-based filters. We will harness the ability to engineer the dispersion laws of both SW and guided AW to achieve tangential nesting of the propagation characteristics of AW and SW, i.e. match their frequency, wavevector, and group velocities. This last (novel) point, supplemented by the high confinement of the acoustical energy near the interface with the coupled spin-wave medium, ensures that even if the SW-AW coupling (i.e. the magneto-elasticity) is weak, truly magneto-elastic resonance with strong hybridized character can be harnessed and confer non-reciprocity and tunability to the wave propagating medium as well as to dedicated transducers.
The end goal of MAXSAW is to demonstrate new rf components with unprecedented attributes: this includes adjustable delay lines, compact broadband isolators, and frequency-tunable filters all potentially perfectly adapted for 5G standards, that may offer valorization opportunities for us. To achieve its goals, MAXSAW comprises 4 technical work packages: WP1 defines the propagation medium for the hybrid waves by enhancing the magneto-elastic cooperativity. WP2 is devoted to the making of the propagation medium, including the acoustical materials growth and the customization of the magnetic materials. WP3 develops augmented transducers matched to the propagating medium to best benefit from the medium developments. Finally, the WP4 is the demonstration of novel rf devices that harness hybrid AW-SWs in the strong coupling regime.
To demonstrate its objectives, the consortium shall build upon the expertise in state-of-the-art acoustic wave devices (FEMTO-ST, team of Pr. Bartasyte), spin-wave dynamics (C2N, team of Thibaut Devolder, coordinator), their mutual coupling (INSP, team of Laura Thevenard) and optimized non-reciprocal magnetic materials (CEA-SPEC, team of Grégoire de Loubens), complemented by the support of Frec|n|sys as industrial subcontractor.
Coordonné par le C2N