71 research outputs found
Femtosecond Self-Reconfiguration of Laser-Induced Plasma Patterns in Dielectrics
Laser-induced modification of transparent solids by intense femtosecond laser
pulses allows fast integration of nanophotonic and nanofluidic devices with
controlled optical properties. So far, the local and dynamic nature of the
interactions between plasma and light needed to correctly explain nanograting
fabrication on dielectric surfaces has been missing in the theoretical models.
With our numerical approach, we show that a self-consistent dynamic treatment
of the plasma formation and its interaction with light triggers an ultrafast
reconfiguration of the periodic plasma patterns on a field-cycle time scale.
Within this framework, a simple stability analysis of the local interactions
explains how the laser-induced plasma patterns change their orientation with
respect to the incident light polarization, when a certain energy density
threshold is reached. Moreover, the reconfigured sub-wavelength plasma
structures grow into the bulk of the sample and agree with the experimental
findings of self-organized volume nanogratings. Mode coupling of the incident
and transversally scattered light with the periodic plasma structures is
sufficient to initiate the growth and the self-organization of the
characteristic pattern with a periodicity of a half-wavelength in the medium.Comment: 8 pages, 7 figure
Dynamical rate equation model for femtosecond laser-induced breakdown in dielectrics
Experimental and theoretical studies of laser-induced breakdown in dielectrics provide conflicting conclusions about the possibility to trigger ionization avalanche on the subpicosecond time scale and the relative importance of carrier-impact ionization over field ionization. On the one hand, current models based on a single ionization-rate equation do not account for the gradual heating of the charge carriers, which, for short laser pulses, might not be sufficient to start an avalanche. On the other hand, kinetic models based on microscopic collision probabilities have led to variable outcomes that do not necessarily match experimental observations as a whole. In this paper, we present a rate-equation model that accounts for the avalanche process phenomenologically by using an auxiliary differential equation to track the gradual heating of the charge carriers and define the collisional impact rate dynamically. The computational simplicity of this dynamical rate-equation model offers the flexibility to extract effective values from experimental data. This is demonstrated by matching the experimental scaling trends for the laser-induced damage threshold of several dielectric materials for pulse durations ranging from a few fs to a few ps. Through numerical analysis, we show that the proposed model gives results comparable to those obtained with multiple rate equations and identify potential advantages for the development of large-scale, three-dimensional electromagnetic methods for the modeling of laser-induced breakdown in transparent media
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