Nonreciprocity of Gigahertz Surface Acoustic Wave Based on Mode Conversion in an Inclined Phononic Crystal Heterojunction

In this report, a rectifying surface acoustic wave (SAW) device is proposed and simulated based on a simple inclined phononic crystal (PnC) heterojunction, consisting of monolithic pillars on Si substrate. The designed nonreciprocal operation principle is initially based on the frequency alignment of the surface-coupled guiding bands in the first half of the PnC with the local-surface-resonance (LSR) band gap in the second half of the PnC, along two different equivalent incident directions. Benefiting from flexible LSR band-gap engineering, we tune the band-gap central frequency by optimizing the structural dimensions of the pillars in a small chip area, which is not achievable in conventional Bragg band gaps without varying the lattice constant. The other physical principle that dominantly affects the broken reciprocity in our proposed structure is the induced SAW shear-to-sagittal mode conversion at a limited frequency range in the trapezoidal PnC half, which depends on the incident direction with respect to the inclined cut line of the PnC. Moreover, we optimize the spacing gap between the PnCs to modulate the elastic coupling strength between the half PnCs, and prove a significant SAW nonreciprocity of 34 dB at a frequency of 6.9 GHz, in addition to an acceptable rectified transmission of about -10.68 dB, by the proposed PnC-based operation principles. The presented design benefits from a simple Si-based structure and a CMOS-compatible fabrication process, without the need for any external excitation, and it is a promising miniature and efficient SAW rectifying candidate for wireless-communication applications.