A working theory and experiments on photomechanical disruption of melanosomes to explain the threshold for minimal visible retinal lesions for sub-ns laser pulses.

The threshold radiant exposure (Hth [J/cm2]) at the retina which produces a minimal visible lesion (MVL) has been investigated as a function of the laser pulse duration (tn). By considering the optical absorption coefficient of the melanosomal interior, J-A.me1anosome' one can calculate the threshold deposited energy, Qth=k.me1anosomeHth [J/cm3], for the MVL. The tp-dependence of Qth is adequately explained for tp 16 ps by the thermal relaxation of heated melanosomes in the retinal pigmented epitheium (RPE). However, at very short pulses (100 ps), there is an apparent .-10-fold drop in the Qth which is possibly due to the onset of a photomechanical mechanism of damage. Thermoelastic expansion of the laser-heated melanin granules (-10 nm in size) within the 1 .5-j.tm melanosome is induced by laser pulses less than 50 Ps in duration. This expansion occurs faster than the induced pressure can dissipate from the granules at the speed of sound. The stress relaxation time of a 10-nm melanin granule is about 7 ps. As the accumulated pressure attempts to propagate out of the granule as a pressure wave, the pressure wave suffers reflectance at the granule surface boundary due to the difference in acoustic impedances of the granule and surrounding intramelanosomal matrix. About 12 % of the acoustic energy is estimated to be reflected back into the granule as a negative (tensile) pressure wave. This negative stress is hypothesized to elicit cavitation within the melanin granule. This mechanism of intragranule cavitation is a working hypothesis for the mechanism of the MVL in the sub-50-ps regime. An experimental test of feasibility was conducted using a Q-switched laser and a liver/saline interface. A negative refletance of about-22 % was demonstrated at the liver/saline interface, indicating the ease with which negative stress can be generated at interfaces with mismatched acoustic impedances.