Low f-number photoacoustic lens for tight ultrasonic focusing and free-field micro-cavitation in water

We demonstrate a photoacoustic lens with a low f-number of 0.61 and a high focal gain of 220 at 15-MHz frequency for laser-generated focused ultrasound (LGFU), which enables free-field micro-cavitation in water. Due to tight ultrasonic focusing (90 μm in lateral and 200 μm in longitudinal spot widths at a distance of 9.2 mm), the lens produces a peak pressure of 20 MPa (positive) using an input laser energy of only 1 mJ/pulse (6-ns temporal width). Remarkably, we confirm single-pulsed micro-cavitation in a free-field condition by using this lens, which has not previously been achieved with LGFU. The free-field cavitation was monitored and characterized in terms of a bubble radius, a lifetime, and a probability. Our result demonstrates that LGFU amplitudes can be sufficiently higher than a threshold for free-field cavitation at a microscale spot, which is a crucial step for cavitation-based therapy with high precision.