Diagnosis of Ultrafast Laser-Induced Optical Breakdown

Sammanfattning

The process of cutting tissue samples into ultrathin slices to study the cross-sectionalarea is known as microtomy, and is widely used in medical and biological research.Traditional mechanical cutting introduces challenges for tissues like bone, as the hardouter layer and soft marrow core require freezing or fixation that damage cellularstructures. These challenges can be overcome by utilizing ultrafast lasers in thefemtosecond regime, which leverage precise optical breakdown and negligible thermaltransfer to cut micrometer-thin slices of fresh bone with intact marrow cells. Thepurpose of this thesis is to characterize the underlying physics of this process andsuggest methods to improve the overall ablation efficiency. This was achieved byinvestigating the spatial, spectral, and temporal dynamics of optical breakdowninduced by a 775 nm, 1 kHz femtosecond laser across three distinct target media:ambient air, liquid water, and porcine bone. The interactions were characterizedusing high-speed spatial imaging, VIS–NIR spectrometry, and ultrafast temporalphotodetection coupled with an in-house developed statistical deconvolution model.The empirical results demonstrated that ambient air acts as the optimal bulk propagationmedium for the laser, exhibiting high pulse-to-pulse stability and rapidnanosecond-scale plasma decay. Conversely, bulk liquid water was dominated bysevere nonlinear optical distortions, including filamentation, severe spectral broadening,and macroscopic bubble structures that reflected laser light, rendering itinadequate for precise material ablation. During the ablation of porcine bone, thecontinuously ejecting plasma plume exhibited distinct calcium transition lines anddecayed over hundreds of nanoseconds to microseconds; orders of magnitude slowerthan the air plasma. This slow dissipation provides direct experimental evidence thatrestricted geometries cause significantly slower plasma decay than free geometries,and that high-repetition-rate (MHz) laser systems may encounter plasma shieldingeffects. To maximize cutting efficiency and mitigate damage to the tissue, this thesisproposes utilizing ambient air as the primary propagation medium and employinga 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 producea comprehensive review paper of the field; using humidified air streams to reducetissue dehydration and alleviate debris removal; dynamically tuning pulse energy toaccount for material heterogeneity and trench geometry; and developing a predictivemodel for optimal parameter windows for a given laser system.