Data acquisition is performed by a MylabOne clinical ultrasound scanner (ESAOTE Europe, Maastricht) at 50 MHz sampling frequency, which transfers prebeamformed data to a laptop. The unprocessed data from the US scanner are then bandpass filtered and reconstructed offline using a Fourier domain algorithm.28 The axial resolution of the system is , and the lateral resolution is .27 The elevational width ranges from , close to the probe, to 1 mm at the acoustic focus, at an axial distance of from the acoustic lens. The relevant elevational thickness for the experiments is 3 mm wide, 5 mm away from the acoustic lens. Laser pulses of 1 mJ and 130 ns are generated by a stack of diode lasers (Osram Opto Semiconductors GmbH, Regensburg, Germany; Quantel, Paris, France). There are four bars stacked vertically, each bar with 64 individual emitters. The diodes are powered by a custom laser driver (Brightloop, Paris, France), which can be triggered by MylabOne. This one laser driver, if triggered, fires all bars simultaneously. The raw beam from the stack is, after collimation, beam-shaped using a diffractive optical element (SILIOS Technologies, Peynier-Rousset, France). A prism is used to change the angle of the beam at the front end of the probe, from which a rectangular beam of 2.2 mm by 17.6 mm () exits the device under an angle of 51 deg.