Faculty of Mechanical Engineering – University of Ljubljana
Focused high amplitudes and high frequency ultrasounds pulses are difficult to achieve with a traditional piezo-electric based technologies. Photoacoustic conversion employing pulsed optical excitation of photoacoustic materials results in ultrasound pulses with high amplitudes as well as high frequency. This occurs by light-induced material ablation or thermoelastic effect. If a focusing scheme is used, the localized high-amplitude pressure wave induces the formation of cavitation bubbles and shock waves emission. We present a single-laser-pulse-induced ultrasound wave and the localization of cavitation bubbles in an optoacoustic process employing a spherical Ti/black-TiOx lens, operating in ablative regime. Our observations demonstrate the possibility to tailor localized cavities as well as secondary ablative effects from their collapse by adjusting the laser energy, from a cluster more confined (<1 mm3) to a deeper distribution. We also show the development of a photoacoustic graphene/polymer composite operating in thermo elastic regime and planar geometry. Preliminary results show an achievable frequency >20 MHz and an optoacoustic pressure of 10 MPa at 350 mJ/cm2 laser fluence, that is above the damage threshold of carbon-nanotube composites. The possibility to embedded the film in a focusing scheme bear the scope for compact, high-efficient photoacoustic lens for biomedical application or sonochemistry of nanomaterials.
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