Mapping high frequency ultrasonic fields with femtosecond laser generated cavitation

As the resolution of transducers improve, it is increasingly difficult to find single point scatterers small enough for ultrasonic field mapping. Femtosecond lasers can vaporize water in a highly localized area on the nanosecond time scale, generating shock wave sources as small as a few microns for near-optical breakdown threshold laser pulses. Resulting shock waves and cavitation oscillations can be nearly spherical for strong laser focusing conditions. For 50 MHz transducers the size and, shape of these acoustic events represent nearly a, point source with very broad bandwidth. We characterized the acoustic spectra from laser generated acoustic sources and tested their utility for field mapping. A low f-number, 50 MHz, single element transducer was used as the ultrasonic receiver. A focused femtosecond laser produced an optical beam propagating perpendicular to the primary axis of the ultrasound transducer. The optical focus could be positioned anywhere in the three-dimensional space of a water tank housing the ultrasonic transducer. Using a trigger derived from the laser source, ultrasound signals associated with optical breakdown were recorded.