CC BY
12LMI-I-3
Effects of Laser Energy Delocalization in the Regimes of Bulk Modification of Transparent Dielectrics
M. Zukerstein1, V.P. Zhukov1'2'3, N.M Bulgakova1
1- HiLASE Centre, Institute of Physics ASCR, Za Radnici 828, 25241 Dolni Brezany, Czech Republic 2- Federal Research Center for Information and Computational Technologies, 6 Lavrentyev Ave., 630090
Novosibirsk, Russia
3- Novosibirsk State Technical University, 20 Karl Marx Ave., 630073, Novosibirsk, Russia
bulgakova@fzu. cz
Femtosecond laser irradiation of dielectric materials enables deposition of laser energy in a highly localized region whose dimensions can be comparable with laser wavelength. This property of fs laser pulses is widely used for direct writing of multidimensional optical structures inside transparent materials for various photonic and optoelectronic applications [1] that requires gentle modifications of material properties without creation of cracks around the laser-affected zone. On the other hand, creation of extreme states of matter in the material bulk requires strongly localized absorption of laser energy at high energy density levels [2] that is hindered by the intensity clamping effect [3]. We have demonstrated numerically [4] that, using doughnut-shaped laser pulses can lead at certain irradiation conditions to overcoming the clumping effect and reaching very high densities of absorbed energy localized at nano-scales. Such localization is envisioned to result in generation of pressures up to hundreds of gigapascals, yielding in formation of unusual material polymorphs.
To verify the numerical data, experiments on laser modification of fused silica have been carried out using 35-fs, 800-nm laser (Astrella from Coherent). The experiments were performed with Gaussian and doughnut-shaped laser pulses in a wide range of beam energy with focusing inside fused silica bulk at NA = 0.25 as in [4]. At laser energies above 5 J modification was not clearly observed at single laser pulses for both Gaussian and doughnut-shaped beams. Moreover, for multi-pulse irradiation, modification by the Gaussian beam was more pronounced than for doughnut-shaped pulses, contrary to the results presented to [4] for low beam energies. Numerical simulations performed for high-energy irradiation conditions revealed strong delocalization of the laser energy, similar to that reported for silicon in [5], with the maximum of the absorbed energy density insufficient for fused silica modification. Furthermore, the maximum of the absorbed energy density is higher for the Gaussian pulses, in agreement with the experimental data. At decreasing laser beam energy, situation is overturned: the maximum of the absorbed energy density becomes considerably higher for the doughnut-shaped pulses with achieving the conditions of material modification at single laser pulses. The physical processes responsible for the delocalization/localization effect and the dependence of the latter on the focusing conditions will be discussed.
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