Subsurface photodisruption in pig skin as monitored by high-frequency ultrasound

For diagnostic or therapeutic technologies using femtosecond laser-induced optical breakdown (LIOB) in turbid biological tissues, pulses of sufficient fluence must be delivered to the site of interest. As light attenuates and diffuses rapidly due to wavelength-dependent absorption and scattering, it is important to develop penetration optimization schemes. In this study, we use a high frequency (50MHz) ultrasonic technique to investigate the precision and penetration depth limitations of infrared femtosecond laser-induced photodisruption in excised pig skin. Optical parameters varied include laser fluence (energy density in J/cm) and focusing numerical aperture. Our ultrasonic method uses sensitive detection of laser-induced bubbles to measure breakdown extent. Using a geometrically focused Nd:Glass laser (1053 nm, 800 fs) source, we show that acoustically detectable bubbles can be produced as deep as 900 um into excised porcine skin. As penetration exceeds several hundred microns, however, multiple bubbles stacked at different depths can be produced with a single laser excitation. Secondary bubble creation is more likely at suprathreshold fluences or with low NA (≤ 0.4) focusing, where optical self-focusing may occur near threshold fluences. However, as the numerical aperture is increased (> 0.4) for deeper focusing, aberrations can severely distort the beam, increasing the perceived LIOB-threshold with maximal penetrations of less than 500um. Using an index matching fluid (i.e. aqueous glycerol solutions) to help reduce scattering, we are able to improve penetration. However, multiple breakdown sites and the corresponding reduction in precision is still likely in skin even with glycerol treatment.