Phononic band gap in honeycomb crystal: Towards simultaneous photonic and phononic band gaps

Periodically structured materials exhibiting simultaneous photonic and phononic, also termed phoxonic band gaps offer unprecedented ways to tailor photon-phonon interactions. The two types of structures rest upon the same underlying physical principle: taking advantage of the scattering phenomena occurring in a periodical structure to give rise to frequency bands in which wave propagation is completely forbidden. The recent demonstrations of planar phononic crystals operating in the GHz range open up interesting prospects for creating integrated devices capable to transport and control both light and sound, while enhancing their interaction. In this work, we report on the fabrication and acoustic characterization of a two-dimensional artificial crystal theoretically capable of exhibiting such a phoxonic band gap. The host material is chosen to be lithium niobate, because of its remarkable piezoelectric and optical properties. A honeycomb lattice based artificial crystal is fabricated using focused ion beam (FIB) milling. The phononic properties of the sample are characterized through electrical measurements of surface acoustic waves propagating through the sample. Optical characterizations achieved by laser heterodyne interferometry clearly demonstrate the existence of a phononic band gap.