Ultrafast time-resolved imaging of stress transient and cavitation from short-pulsed laser irradiated melanin particles

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Abstract
Melanins are naturally occurring pigment particles which strongly absorb visible and near infrared radiation in many pigmented tissues including the eye and the skin. When irradiated by short laser pulses, stress transients and cavitation can be generated from these micrometer-sized particles. The photoacoustic effect is a potential mechanism for short pulse laser injury to the retina. We report here direct observations of the stress wave and cavitation from irradiated melanin particles using the technique of fast time-resolved imaging. Melanin granules were isolated from fresh bovine retina suspended in gels, and irradiated with 100 psec laser pulses at 1.064 micrometers . Stress transients were imaged under a microscope using a delayed visible strobe pulse. With incident laser fluences of about 1 J/cm2 (near the threshold fluence for retinal injury), a spherical pressure wavefront and a cavitation bubble were observed around each irradiated melanin particle. By measuring the radius of the wavefront as a function of strobe delay time the propagation velocity of the stress wave was determined. We observed in initial supersonic shock front which rapidly decays to an acoustic wave within a few nanoseconds. Because the melanin granules are densely packed inside the 10 micrometers diameter RPE cells, the shock range of a few microns is potentially sufficient for creating injury in the retina.

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