Ultra high frequency air/aluminum nitride fractal phononic crystals

This paper presents the first design and experimental demonstration of a fractal phononic band gap (PBG) structure in air/aluminum nitride (AlN). The PBG exhibits two frequency band gaps for symmetric Lamb waves centered around 900 MHz and 1.10 GHz, respectively with bandwidths of 11.1% and 9.09% and attenuation as high as 40 dB. The unit cell of the fractal design consists of a center square air scatterer with four scaled-down side squares repeating at its corners. The presence of the side squares effectively shortens the scattering distance between each unit cell and permits the opening of band gaps for higher vibration modes, i.e. higher frequencies, for a given minimum feature size. AlN Lamb wave transducers were integrated with the PBG in order to launch acoustic waves into the structure and characterize its performance. This demonstration not only offers the evidence that it is possible to synthesize GHz PBG structures with relatively large feature sizes, but also shows that these devices can be integrated with more established radio frequency microelectromechanical systems (RF MEMS) and ultimately lead to novel all-acoustic signal processors.