Abstract
A detailed study of the morphology of nano-craters drilled in borosilicate glass by single shot femtosecond laser ablation near the ablation threshold has been performed by scanning electron microscopy, atomic force microscopy and scanning electron microscopy imaging after focused ion beam sectioning. The influence of the numerical aperture (NA = 0.4 and 0.8), the pulse energy (16 nJ < E p < 600 nJ) and the position of the specimen surface into the focal region were systematically investigated, leading to nanometric or micrometric scales in every spatial dimension. The nanocrater’s size is not restricted by the diffraction limit but determined by the laser pulse stability and the material properties. If the beam is focused inside the glass, two craters are drilled, shaping very distinct morphologies. Their dimensions have been studied in details and different relationships have been proposed for the evolutions of the depths and of the various diameters of these craters as functions of the pulse energy, the numerical aperture and the position of specimen surface in the beam-material interaction region. It is suggested that the long, thin conical profile with very high aspect ratio of the secondary craters is due to a spontaneous reshaping of the beam which transforms the incoming Gaussian pulse into a Gaussian–Bessel pulse. As proposed in the developed model the geometry of the second craters seems to be connected with the one of the main craters.
Highlights
Femtosecond laser micromachining is a versatile materials processing technology for fabricating a wide range of micro and nano-structures in transparent media [1,2]
For Ep/Ep0 b 3.3, all the experimental determinations are close, 100 b L3 b 300 nm, which shows that the L3 diameter is appreciably independent on the numerical aperture (NA = 0.4, 0.8, 0.85) and to the half conical angle αΒ of the Bessel beam, contrary to Ep0 which varies as NA2
If NA2Ep0 = 11 nJ, as previously reported, Ep0 = 304 nJ and 682 nJ, which is in good agreement with the experimental values, Ep0 ~ 320 nJ and 610 nJ, respectively [35]
Summary
Femtosecond laser micromachining is a versatile materials processing technology for fabricating a wide range of micro and nano-structures in transparent media [1,2]. [3,4,5,6,7,8,9,22,23,24] For dielectric materials such as fused-silica or glass, non-linear absorption becomes significant when ultrashort laser pulses are tightly focused by lens with high numerical aperture onto the surface or inside the bulk of the specimen. If the laser pulse energy is adjusted so that only a small part of the focused beam has irradiance above the ablation threshold of the considered material, sub-diffraction nano-structuration can be machined [3,4,5,6,7,8,9]
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