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Fabrication inside transparent materials using femtosecond laser has important applications in optical storage, microfluidic devices, and other fields. However, the manufacturing efficiency and quality are limited due to the Rayleigh-length of Gaussian beam and the saturation effect in multi-pulses processing. In this paper, plasma distribution and morphology inside PMMA was modulated by adjusting the focus depth (0~ 200 μm) of femtosecond laser single pulse, which realized the fabrication of modification lines with controllable length and high efficiency. Ultrafast pumpprobe imaging system was used to investigate the evolution of filaments and shockwave inside PMMA during processing. The results showed that the filaments and shockwave inside PMMA transformed from the combination of hemispherical shockwave and filaments into only filaments gradually. Besides, it was observed that the length of filaments extended from 50 μm to 325 μm with the increasing focus depth. The simulation of finite-different time-domain was executed and the results agreed well with the observation results. It indicated the changed optical field distribution inside PMMA caused by different focus depth resulted in the corresponding changes of filaments and the effects of self-focusing and spherical aberration dominated in the mechanism of filaments extension on both sides. The morphology of structure processed inside PMMA was further characterized by optical microscope. It showed that uniform modification lines can be fabricated by single pulse with all focus depth and the length of which increased from 73 μm to 135 μm. These controllable modification lines have great potential in fields like optical waveguide, optical storage and hologram.
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