Band Gap Trapping of Acoustic Phonons Boosts the Acousto-optical Interaction

Brillouin nonlinearity involving the interaction of photons and acoustic phonons possesses the combined advantages of a large bandwidth of optical waves and a fine spectral resolution of mechanical vibrations simultaneously. Currently, the trapping of the acoustic phonon in the state-of-the-art opto-acoustic hybrid waveguides relies on the acoustic impedance mismatch between the core and the cladding. The energy leakage of the acoustic vibration is inevitable, which brings about the broadening of the Brillouin line width and limits its spectral resolution. Here, we demonstrate nearly complete vibration isolation between the core and the cladding based on the phononic band gap mechanism. The Brillouin line width is 1.1 MHz, which is reduced by 1 order of magnitude and close to the theoretical limitation. The lifetime of the phonon is 145 ns, over 10 times the value in the previous reports. The strong opto-acoustic interaction based on the phononic band gap mechanism provides a host of new coherent signal-processing technologies.