Resonant MHz ultrasonic waves from a one-dimensional phononic crystals

By combining theoretical, numerical, and experimental approaches, this study presents a comprehensive investigation into the transmission spectrum of a one-dimensional phononic crystal (PC) composed of periodically stacked glass coverslips separated by water layers. The results reveal bandgaps, defect states, and band edge resonances in the range of 0.6–7.5 MHz. Wideband acoustic waves are excited within the defect layer of the PC and in free space via the photoacoustic effect. Ultrasound radiation is strongly enhanced at the resonant frequency of 0.836 MHz, with an acoustic energy density enhancement of (30.8 ± 0.6)×, while radiation at other frequencies is significantly suppressed. These findings strongly suggest that the enhancement originates from an increased acoustic density of states, although material absorption suppresses the enhancement at higher frequencies. This work provides direct insights for developing MHz ultrasonic emitters for biomedical applications and advances fundamental research in megahertz ultrasound.