Transient time-delay focusing of shock waves in thin liquids

Abstract

In this work we propose and demonstrate a Fresnel-lens-inspired method to focus multiple laser-induced shock waves through time-delay superposition at arbitrary locations. While the principle works for any geometry, we demonstrate that this method already achieves focusing with two pairs of photoacoustic shock wave emitters located on a line centred around the acoustic axis (z) in a quasi-two-dimensional liquid geometry. Each emitter pair is created by focusing one laser pulse simultaneously at two spots with a spatial light modulator at z = 0 μm with y = ±145 μm and y = ±75 μm. The delays between the emitters necessary to vary the location of the focus from z ≈ 0 to ∼206 μm are 35 and 0 ns, respectively. We find that the location of constructive superposition is significantly closer to the origin than what would be expected for linear waves in homogeneous media. This is confirmed with simulations using an Euler solver that shows the importance of finite-amplitude effects. The simulated dynamics are in reasonable agreement with our measurements. Finally, pressure gains at various locations along the acoustic axis are tested with the response of gaseous microbubbles acting as pressure probes. The measurements agree with calculated pressure ratios at different positions.