But when the characteristic heating time is

Fig. 59. Hole diameter and taper as a function of focus position [142].Figure optionsDownload full-size imageDownload as PowerPoint slide
Vukelic et al. [84] have explored the contribution of different mechanism in the formation of features at very high laser intensities. It was observed that for fused silica the pulse energy has GSK461364 impact on feature size and the type of ablation mechanism is governed by the feedrate. At high feedrate the formation of feature is due to explosive plasma expansion whereas at low feedrate the thermal mechanism dominates. Grasset and Bellouard [143] explored the possibility of combining the chemical etching with fs laser for the efficient fabrication of 3D microstructures in fused silica. They demonstrated that fs laser can be utilized for the fabrication high aspect ratio micro-molds as well as shallow pattern with nanoscale resolution in silica.
Zhang et al. [144] demonstrated that holes with a diameter of 20-40 ?m and depth of 300-400 ?m can be obtained in polymers when beam of 100 fs laser pulses at 800 nm wavelength were focused by a plano-convex lens. Arrays of micro-strings having diameter 2 ?m and image length more than 10 mm can be fabricated by fs laser when projection patterning is carried out with a rectangular pinhole. The advantage of using focus dual frequency beam (based on the longitudinal chromatic aberrations) having wavelength of 800 nm and pulse duration of 150 fs was used for the drilling of 3 mm thick PMMA plate by Tan et al. [145]. It was found that the use of dual frequency beam produce high aspect ratio hole with good profile and thus we can overcome the limitation offered by the size and focal depth of laser spot in producing high aspect ratio microhole. The feasibility of using UV and Ti:sapphire fs laser to generate high resolution micropatterns on biodegradable polymers like poly(?-caprolactone) (PCL) and poly(glycolic acid) (PGA) was investigated by Aguilar et al. [146]. It was reported that both UV and fs laser are suitable to generate micron sized channels and holes in these polymers. The ablation depth increases with the increase in number of pulse and the width of channels can be increased with the laser pulse having high pulse energy. The examination of PCL and PGA using scanning electron microscope (SEM), Fourier transform IR spectroscopy in attenuated total reflectance mode (FTIR-ATR) and X-ray photoelectron spectroscopy (XPS) shows that the use UV and fs laser do not degrade the characteristics of polymers.
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