Hypersonic phononic crystal for surface acoustic waves

A phononic crystal exhibiting a band gap in the near-gigahertz frequency range for surface acoustic waves was fabricated in a lithium niobate substrate. Reflection and transmission properties of the sample were characterized both electrically and optically, by means of embedded broadband interdigital transducers and optical heterodyne interferometry, respectively. Measurements performed for (XZ) propagating surface waves show the existence of a band gap between 660 and 900 MHz. Optical measurements confirm that the phononic crystal behaves as a perfect mirror for waves propagating at frequencies within the band gap. Outside the band gap, transmission can be observed for frequencies below, but also above the forbidden frequency range, hence showing that losses experienced by high frequency surface acoustic waves, i.e. for modes located beyond the sound line, can be partially overcome.