Advanced MEMS Department, Sandia National Laboratories, Albuquerque, New Mexico, USA
Phononic crystals are an emerging field that has applications across a broad range of systems from radio frequency (RF) communications to thermal management and acoustic imaging. Over the past decade phononic crystals have been scaled from large hand assembled balls in acoustically lossy materials such as water and epoxy to micro and nano fabricated 2D structures realized in low loss materials operating at frequencies from 10-10,000 MHz.
Micro/Nano fabrication of phononic crystals has presented many opportunities and challenges. Batch fabrication and the ability to cointegrate piezoelectric transducers with phononic crystals for rapid electrical interrogation have lead to experimentation on a wide variety of geometries and devices. The ability to suspend 2-D phononic crystals above the substrate using micromachining techniques has allowed for low loss phononic waveguides and cavities to be demonstrated. The wide range of materials available and the implications of finite membrane thickness require careful consideration when designing a phononic crystal for a specific application.
Phononic crystals and devices have been fabricated and studied theoretically and experimentally in several material systems including Si-air, SiC-air, Si-W and SiO2-W. Phononic band gaps at GHz frequencies have been demonstrated in each of these material systems with complete phononic bandgap widths exceeding 10% of the center frequency and band gaps for longitudinal waves in excess of 50%. Solid-Solid phononic crystals realized in Si-W and SiO2-W material systems have demonstrated less sensitivity to membrane thickness and lithography when compared to Solid-Air phononic crystals. Certain devices, such as cavities, requiring very low material damping have been best realized in Solid-Air materials systems using high-Q materials such as SiC. Recently, a nanofabricated Si-Vacuum phononic crystal with its tailored phonon dispersion has demonstrated over an order of magnitude reduction in thermal conductivity of a Si slab potentially leading to improved performance in thermoelectric devices.
While much recent progress has been achieved in realizing micro and nano scale phononic crystals several challenges and opportunities are emerging including: the need for much wider bandwidth piezoelectric transducers for interrogating the phononic crystals, the ability to combine phononic and photonic crystals to increase phonon-photon interactions and for processing light signals in the acoustic regime and the introduction of non-linearity with phononic bandgap materials.