Broadband All-Optical Ultrasound Transducer

Real-time three-dimensional (3D) high-resolution ultrasound imaging requires 2D phased arrays operating at frequencies higher than 50 MHz, which poses a great challenge for piezoelectric technologies. A promising alternative is an optoacoustic array relying on optical generation and detection of ultrasound. We present an all-optical ultrasound transducer, which consists of a 2D gold nanostructure on a glass substrate, topped by a 2.7 μm polydimethylsiloxane (PDMS) layer and then a 30 nm gold layer. A laser pulse with a wavelength matches that of the surface plasmon resonance of the gold nanostructure caused the absorption of a large fraction of the incident optical energy and heat a highly localized volume; and a rapid thermal expansion launches an acoustic wave. The gold nanostructure also reflects over 90% of optical energy at wavelengths far from the resonance, which, together with the continuous gold layer and the PDMS bulk in between, forms a Fabry-Perot etalon structure for ultrasound detection. Large bandwidth (57 MHz) and high resolution are demonstrated by pulse-echo imaging experiments.