Support loss-free micro/nano-mechanical resonators using phononic crystal slab waveguides

Abstract

Phononic crystals (PnCs) are inhomogeneous materials with periodic variations in their elastic (or acoustic) properties. PnCs, if properly designed, can show frequency ranges in which the propagation of elastic waves is completely prohibited. Within these frequency ranges, called complete phononic band gaps (CPnBGs), elastic energy can be confined and manipulated by the PnC structure. Micro-machined PnC structures with two-dimensional (2D) periodicities and finite thicknesses have been developed to possess large CPnBGs. Such structures have shown to be very effective in confining mechanical vibrations at very high frequencies. It is argued that by replacing the supporting structure of the suspended conventional high-Q micro/nano-mechanical (MM) resonators with PnC structures, the support loss in the resonators can be suppressed. However, a such resonators may give rise to spurious modes in the resonance profile of the resonance. Therefore, the development of more efficient PnC resonators with complete elimination of the supporting structures in all in-plane directions and with large spurious-fee spectral ranges is pending. In this paper we discuss different architectures and properties of support-loss-free PnC micro-mechanical resonators and compare their performance with the conventional MM bar resonators with supporting anchors. We have recently shown that in a thin-film piezoelectric on substrate (TPoS) MM resonator, the quality factor can be greatly improved by replacing the supporting structure with PnC structures. Qs of more than 6,000 are obtained at very high frequencies (∼130 MHz) for the case of PnC resonators compared to Qs the order of about 1,000 for the structures with support. It is though observed that the PnC structure in such resonators may lead to undesirable spurious modes around the main resonant mode. In order to mitigate the spurious modes, in this paper PnC waveguides are engineered and designed to form more effective PnC resonators. Waveguide-based PnC resonators with Qs of more than 7,000 and with a large free spectral range around the resonant mode are hence developed.