Control of optically generated ultrasound fields using binary amplitude holograms

To obtain high resolution ultrasound images, transducers able to operate at high frequencies are required. Optically generated ultrasound utilising the optoacoustic effect is a promising alternative to piezoelectric transducers to achieve this. To use optically generated ultrasound for imaging, a method to spatially steer and focus the acoustic pulses is desirable. In this paper, the use of binary amplitude holograms to focus broadband ultrasound pulses generated by a pulsed laser was investigated. This was done experimentally with patterned absorbers and in simulation. It was found that applying two or more laser pulses applied to a hologram at its design frequency was sufficient to form a focus. The position of this focus could be moved in 3-D by changing the hologram. Additional focal points to those designed for were also found in both the simulation and experimental data. These were generated by constructive interference of harmonics of the pulsing frequency. Simulations found that increasing hologram resolution and applying greater numbers of laser pulses to the hologram decreased the volume of the primary focal point, and that the volume of this focus decreased more rapidly with increasing laser pulse numbers at higher hologram resolutions.