Generation of a focused pressure wave and localized cavitation clouds using a metal-semiconductor Ti/black-TiOx optoacoustic lens

Abstract

The optoacoustic (OA) generation of pressure waves above the cavitation threshold causes the rupture of the liquid and the formation of dispersed cavities. The OA lenses involved in this process are generally associated with an ultra-short laser pulse to generate high-frequency ultrasound. OA elements based on carbon nanomaterials or ultrathin metal layers have shown they can deliver high-pressure ultrasound for therapeutic treatments that require high spatial resolution. Secondary processes, induced by a transient pressure, such as cavitation bubbles and shock waves, can lead to a more intense pressure transient. Hence, understanding their dynamics within an OA process and harnessing the spatial localization have become relevant for biomedical applications. Here, we show a single-laser-pulse-induced acoustic wave and the localization of inertial cavitation in an OA process employing a metal–semiconductor Ti/black-TiOx lens as an active component. Detection methods based on Schlieren photography and a laser beam-transmission probe (BTP) were used to study the dynamics. The phenomena were described at different laser-excitation energies (EL), showing an increase in the bubble population confined to the focal region of the lens. The size of the bubbles increases monotonically with the laser’s fluence and their distribution becomes denser and more elongated as a result of the increasing of the pressure-activated nucleation sites and of the bubble–bubble overlapped area. The stochastic nature of the cavities is spatially mapped, showing a probability distribution close to the focal region, which reaches 50% of its value for the cavitation fluence Fcav = 1.2 J/cm2. Our results demonstrate how the spatial distribution and penetration depth of the bubble cloud can be shaped by tuning the EL. The possibility to tailor the localized cavities and secondary ablative effects paves the way for the development of inexpensive technologies based on the photodisruption of localized subsurface tissues.