Polarization mode hybridization and conversion in phononic wire waveguides

Abstract

Phononic wire waveguides of subwavelength cross-section support two orthogonal polarization modes: the out-of-plane motion dominated Rayleigh-like and the in-plane motion dominated Love-like modes, analogous to transverse-electric and transverse-magnetic modes in photonic waveguides. Due to the anisotropic elasticity of the substrate material, the polarization states of phonons propagating along certain crystallographic orientations can strongly hybridize. Here we experimentally investigate the orientation-dependent mode hybridization in phononic wire waveguides patterned from GaN-on-sapphire thin films. Such mode hybridization allows efficient actuation of piezoelectrically inactive Love-like modes using common interdigital electrodes designed for Rayleigh-like modes, and further enables on-chip polarization conversion between guided transverse modes. Both are important for on-chip implementation of complex phononic circuits.Over the past few decades, a variety of surface acoustic wave (SAW) devices have been developed for radio frequency electronics and communication systems, including delay lines, filters, resonators and precision oscillators 1,2 . SAW devices are also moving into the labon-a-chip arena and find use in sensing applications 1-4 . Lately, there have been emerging interests in the realization of light-sound and spin-sound interactions using SAW 5-10 . However, in almost all of those applications, the SAW propagates on the chip surface in a two-dimensional (2D) or quasi-2D slab of many wavelength wide without tight lateral confinement. It is anticipated that the strength of these interactions can be greatly enhanced by using phononic strip waveguides of subwavelength cross-section or phononic wires, to confine phonon fields 11,12 . Such high-confinement phononic circuits could perform functions that complement electronic and photonic circuits, at operation frequencies matching that of microwave and wavelengths matching that of optical signals [13][14][15][16][17] .Similar to optical waveguides which support orthogonal polarization transverse-electrical (TE) and transverse-magnetic (TM) modes, phononic waveguides support two types of phonon modes: the vertically polarized (with dominant out-of-plane motion) Rayleighlike mode and the horizontally polarized (with dominant in-plane motion) Love-like mode. While the Rayleighlike mode is piezoelectrically active and has been extensively studied in literature and more commonly utilized in today's RF signal processing devices 2,18 , the Love-like mode is particularly interesting for building high quality factor (Q) acoustic resonators and for sensing applications due to their relative immunity to air and fluid damping 4,19-21 . However, it is difficult to actuate the Love-like mode, which is normally piezoelectrically inactive, by an interdigital transducer (IDT).Here we study the phononic modes propagating along a high-confinement waveguide structure-GaNon-sapphire phononic wire waveguide. Although GaN presents isotropic elastic properties in the c-plane...

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