Wave Guiding in Air/Silicon Phononic Crystal With Complete High Frequency Band Gap

Abstract

Phononic crystals (PC) are structures with periodic variations in their elastic properties. PC structures with complete phononic band gaps (CPBGs) are of special interest as they can be used to add new functionalities and make improvements to the conventional acoustic/elastic wave based devices. Recently, there has been a growing interest in PC structures with two-dimensional (2D) periodicity and a finite thickness in the third dimension, called PC plates (or slabs). PC slabs are interesting as the elastic waves are manipulated by the PC structure in the plane of periodicity while it is confined within the finite thickness of the slab structure in the third dimension, preventing the loss of energy out of the plane of periodicity. Although there have been a few reports on the implementation of PC plates with partial band gaps at low frequencies (below 5 MHz), the possibility of utilizing PC structures with high frequency CPBGs which can be used in wireless communication and sensing systems is still unverified. In this paper, we fist show (to the best of our knowledge) the first experimental proof for the existence of large high frequency (119MHz–150MHz) CPBGs in Si-based PC plates, and then using this CPBG, we create a waveguide by introducing a line defect in the PC structure. The structure is made by etching a hexagonal lattice of air holes in a free standing silicon plate by the use of a CMOS-compatible fabrication procedure. The transmission of elastic waves through the PC structure is measured by the use of a network analyzer and embedded interdigitated transducers. More than 30 dB attenuation in transmission through eight layers of PC structure is observed in the frequency range of the CPBG (119MHz–150MHz) with very good agreement with theoretical predictions of the CPBG. By adding a line defect to the PC structure, a PC waveguide with low loss propagation characteristics is formed and experimentally demonstrated. This result adds a new powerful functionality to micro/nano-mechanical devices that can lead to new high-performance, high-frequency devices with possible use in various wireless communication and sensing systems.