From Localized Laser Energy Absorption to Absorption Delocalization at Volumetric Glass Modification with Gaussian and Doughnut-Shaped Pulses

Abstract

Volumetric modification of transparent materials by femtosecond laser pulses is successfully used in a wide range of practical applications. The level of modification is determined by the locally absorbed energy density, which depends on numerous factors. In this work, it is shown experimentally and theoretically that, in a certain range of laser pulse energies, the peak of absorption of laser radiation for doughnut-shaped (DS) pulses is several times higher than for Gaussian ones. This fact makes the DS pulses very attractive for material modification and direct laser writing applications. Details of the interactions of laser pulses of Gaussian and doughnut shapes with fused silica obtained by numerical simulations are presented for different pulse energies and compared with the experimentally obtained data. The effect of absorbed energy delocalization with increasing laser pulse energy is demonstrated for both beam shapes, while at relatively low pulse energies, the DS beam geometry provides stronger local absorption compared to the Gaussian geometry. The implications of a DS pulse action for post-irradiation material evolution are discussed based on thermoelastoplastic modeling.

Volumetric glass modification with Gaussian and doughnut-shaped pulses: from localized laser-energy absorption to absorption delocalization

“…The details of the experimental arrangement are described in [15]. Shortly, a Ti:sapphire laser system Astrella (Coherent) was used with a pulse duration of τFWHM ≈ 40 fs and wavelength of 800 nm (see the experimental scheme in [15]).…”

“…The details of the experimental arrangement are described in [15]. Shortly, a Ti:sapphire laser system Astrella (Coherent) was used with a pulse duration of τFWHM ≈ 40 fs and wavelength of 800 nm (see the experimental scheme in [15]). The laser beam was focused inside 1-mm thick plates of fused silica 200 μm below the surface of the sample using an objective with 10× magnification (NA = 0.25).…”

“…Numerical simulations of fused silica irradiation for the conditions used in the experimental studies were based on nonlinear Maxwell's equations supplemented by the equations describing the excitation of electrons, their trapping, possible re-excitation, and oscillations of the conduction electrons in the field of the laser wave [14,15,17]. The key equations are as follows:…”

“…where r and z are the beam radius and the direction of the beam propagation. Further details on modeling equations and used parameters can be found in [15]. Here we add specification for the distribution of the absorbed laser energy density as this value determines the material modification level.…”

“…Recently, we demonstrated numerically and experimentally that, using doughnut-shaped (DS) laser pulses, it is possible at a certain range of pulse energies to achieve much higher absorption within a strongly localized volume inside fused silica glass as compared with Gaussian pulses [14][15]. This effect can be used for optimizing 3D direct writing as well as for achieving extreme thermodynamic states of matter [16].…”