Ultrafast femtosecond lasers are used increasingly for a wide range of medical purposes. The immediate tissue response to pulses above a certain threshold is optically or laser induced breakdown, which is often visible as gas-filled cavities that persist for some time. In the present study, we attempted to define the cavitation threshold in the human lens in vitro using multiphoton effects based on radiation from a femtosecond 800 nm Ti:Sapphire laser. Cavitations were observed from pulse energy densities exceeding 16 mJ/cm2, but only after several minutes of exposure and not as a result of a single laser pulse. This suggests that cavitations were caused by a process which differs from the single-pulse cavitations observed at higher intensities. To evaluate whether the release of gas was caused by ionization and plasma formation or by thermal effects, we introduced pauses into the pulse train, which did not change the total exposure time needed to form a cavitation. This suggests that local heating did not play a significant role in producing the observed phenomenon, suggesting that photochemical reactions may be involved. These results demonstrate that there are several types of ultrafast laser effects in the lens that have a potential for therapeutic application and treatment of eye disease though further studies are needed to shed light on the nature of the formation of delayed cavitations.
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