Ultrasonic tunable focusing by a stretchable phase-reversal Fresnel zone plate

This paper reports a stretchable silicone phase-reversal (PR) Fresnel zone plate (FZP) that can focus ultrasonic energy at different focal lengths with a high transmission coefficient in water. Unlike a traditional FZP that creates focused by constructive interference of waves diffracted through open annular zones in an opaque screen, the silicone PR-FZP takes advantage of all the zones of the FZP contribute to the focal area by adding phase compensation regions instead of opaque regions. More interestingly, the silicone PR-FZP can be stretched, and the focal length increases gradually with the unchanged full width at half maximum as the PR-FZP is stretched. The aforementioned performance aspects are verified in both experiments and simulations. The proposed stretchable PR-FZP with a tunable focal length has potential applications in the broad field of ultrasonics, such as ultrasonic imaging and ultrasound neuromodulation.