We studied the micromachining of high-aspect-ratio holes in poly(methylmethacrylate) using a visible double-pulse femtosecond laser based on free-electron-density adjustments. Hole depth and aspect ratio increased simultaneously upon decreasing the wavelength in the visible-light zone. When the pulse energy reached a high level, the free-electron density was adjusted by using a double-pulse laser, which induced fewer free electrons, a lower reflectivity plasma plume, and more pulse energy deposition in the solid bottom. Thus, the aspect ratio of the hole was improved considerably. At a moderate pulse energy level, a 1.3-1.4 times enhancement of both the ablation depth and the aspect ratio was observed when the double-pulse delay was set between 100 and 300 fs, probably due to an enhanced photon-electron coupling effect through adjusting the free-electron density. At a lower pulse energy level, this effect also induced the generation of a submicrometer string. In addition, the ablation rate was improved significantly by using visible double pulses.
We have conducted an experimental investigation on highly efficient femtosecond laser micromachining of silicon through N-type doping. We found that the material removal amount has a close relationship with the doping concentration rather than with the doping types. The amount of material removal was enhanced gradually as doping densities increased. When the doping density reached higher than 10(18) cm(-3), the ablation threshold was considerably reduced, up to 15%-20%. The results of the experiment indicate that the high density of initial free electrons by doping is the fundamental reason for efficiency improvement, and bandgap shrinkage also plays an important role. The electrons are excited more easily from the valance band to the conduction band and acquire higher initial kinetic energy, which then promotes the material ablation process.
We present a doping method to improve the femtosecond laser ablation rate and promote ablation selectivity. Doping transition metal ions, Co 2+ or Cu 2+ , in silicate glass apparently change absorption spectroscopy and induce resonant absorption at wavelengths of 600 and 800 nm, respectively. Comparing with femtosecond laser processing of the same glass without doping, we find that the threshold fluence decreases and the ablation rate increases in resonant absorption in doped silicate glass. Resonant absorption effectively increases multiphoton ionization for seed-free electron generation, which in turn enhances avalanche ionization.
This study investigates the resonant effects in nonlinear photon absorption in femtosecond laser ablation of Nd-doped silicate glass (Nd:glass). During the femtosecond laser ablation process, the resonant ablation threshold fluence is decreased by up to 40% compared with that of ordinary ablation. However, it is found that the resonant effect is closely related with laser intensity, and lower laser intensities are required to achieve a significant enhancement. When the intensity is lower than 2.28×10(14) W/cm(2) at which multiphoton ionization dominates, resonant effect is enhanced by a factor of 1.4 to 4.4. When the intensity is higher than 2.28×10(14) W/cm(2), at which intensity tunnel ionization dominates, the resonant effect becomes weak and gradually fades away. It is shown that the resonant effect is still important for multiphoton ionization yet insignificant for tunnel ionization.
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