In the doctoral thesis, a method for amplification of high-intensity pressure waves
generated deep below the irradiated surface with multi-pulsed Nd:YAG laser coupled with
a black-TiOx optoacoustic lens in the water is presented and characterized. The
experimental system consists of a quality-switched Nd:YAG laser source that enables the
generation multiple nanosecond pulses up to an energy of 900 mJ with delays between 50
µs and 400 µs. Using a spherically shaped optoacoustic lens, which is made of titanium
with a surface layer of black TiOx, we created high intensity focusing pressure waves. Size
of generated pressure waves in the focus is about 120 x 300 μm2 (length x diameter), and
the amplitude ~100 MPa. Using schlieren photography and a high-speed camera, we have
shown that it is possible to achieve localized cavitation in the focus of an optoacoustic lens.
The size of the cavitation cloud and its fluctuation in time increases monotonically with the
laser excitation energy. In the case of using four consecutive laser pulses, when the delay
between the laser flashes is equal to the duration of the vapor bubbles above the
optoacoustic lens, the size of the cavitation increases by 10 times, while the amplitude of
the pressure wave increases by more than 75%. This amplification is decisively influenced
by three mutually competing phenomena: obscuration of the laser pulse due to the presence
of a shroud cloud of vapor bubbles, an increase in the temperature of the OL surface, and
the resonance effect that occurs when the next laser pulse is triggered upon the collapse of
the cloud of vapor bubbles. The achieved results are a good basis for the future
development of new minimally invasive medical interventions, where it is necessary to
influence tissue deep below the surface of the skin (several mm), which cannot be directly
illuminated with light.
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