Micromachining using shaped ultrafast pulse trains

Ihtesham H Chowdhury, Purdue University


In wide bandgap dielectric materials, high-intensity amplified ultrafast laser pulses can excite electrons from the valence band into the conduction band by nonlinear photoionization and create free electron plasma. This plasma can absorb more photons by free carrier absorption and also cause impact ionization at high enough energies. The excited electrons can get captured as self-trapped excitons (STEs) which are precursors to defect states that act as absorption sites for subsequent laser pulses. Also, the energetic free carriers can slowly thermalize over a period of picoseconds and couple their energy into the lattice. Finally, the elevation of a significant fraction of the valence electrons to non-bonding states has a destabilizing effect on the lattice that can lead to breakup. This study presents results of pump-probe transmission and reflection experiments in wide bandgap dielectrics that reveal the time-scales within which the free electron plasma exists. The basic motivation is to identify possible ways of improving the energy coupling mechanism and thus optimize the ablation process. To this end, the pump-probe data is correlated with atomic force microscope measurements of the ablated volume. It was observed that the amount of material ablated is related to the temporal separation between the pump and probe pulses. Time-resolved machining using two different pulses at 800 nm and 400 nm was also carried out. It was seen that using two pulses at different wavelengths for machining could lead to about 50% enhancement in the ablation rate. This is attributed to enhanced absorption of the 800 nm pulse due to the creation of defect states and free electron plasma by the 400 nm pre-pulse. Finally, user-defined pulse sequences were created using a Fourier pulse shaping apparatus based on a liquid crystal modulator array. These sequences were used to conduct time-resolved measurements of the ultrafast laser absorption process in fused silica. These experiments reveal a complex dependence of the energy absorption process on the rate at which the laser pulses are repeated. Related ablation studies are also reported.




Weiner, Purdue University.

Subject Area

Mechanical engineering|Optics

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