Diagnosis of Ultrafast Laser-Induced Optical Breakdown

Abstract

The process of cutting tissue samples into ultrathin slices to study the cross-sectional area is known as microtomy, and is widely used in medical and biological research. Traditional mechanical cutting introduces challenges for tissues like bone, as the hard outer layer and soft marrow core require freezing or fixation that damage cellular structures. These challenges can be overcome by utilizing ultrafast lasers in the femtosecond regime, which leverage precise optical breakdown and negligible thermal transfer to cut micrometer-thin slices of fresh bone with intact marrow cells. The purpose of this thesis is to characterize the underlying physics of this process and suggest methods to improve the overall ablation efficiency. This was achieved by investigating the spatial, spectral, and temporal dynamics of optical breakdown induced by a 775 nm, 1 kHz femtosecond laser across three distinct target media: ambient air, liquid water, and porcine bone. The interactions were characterized using high-speed spatial imaging, VIS–NIR spectrometry, and ultrafast temporal photodetection coupled with an in-house developed statistical deconvolution model. The empirical results demonstrated that ambient air acts as the optimal bulk propagation medium for the laser, exhibiting high pulse-to-pulse stability and rapid nanosecond-scale plasma decay. Conversely, bulk liquid water was dominated by severe nonlinear optical distortions, including filamentation, severe spectral broadening, and macroscopic bubble structures that reflected laser light, rendering it inadequate for precise material ablation. During the ablation of porcine bone, the continuously ejecting plasma plume exhibited distinct calcium transition lines and decayed over hundreds of nanoseconds to microseconds; orders of magnitude slower than the air plasma. This slow dissipation provides direct experimental evidence that restricted geometries cause significantly slower plasma decay than free geometries, and that high-repetition-rate (MHz) laser systems may encounter plasma shielding effects. To maximize cutting efficiency and mitigate damage to the tissue, this thesis proposes utilizing ambient air as the primary propagation medium and employing a gentle ablation strategy with low pulse energy and high repetition rate. Finally, a number of topics for future research are proposed, including an effort to produce a comprehensive review paper of the field; using humidified air streams to reduce tissue dehydration and alleviate debris removal; dynamically tuning pulse energy to account for material heterogeneity and trench geometry; and developing a predictive model for optimal parameter windows for a given laser system.