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Mechanism of Laser Induced Filamentation in dielectrics

Laser filamentation in transparent material has a wide range of applications, from three dimensional manufacturing to biological technologies. Various experimental results showed that femtosecond laser pulse filamentation in fused silica strongly depends on laser focusing conditions. However, the physical mechanism governing each regime has not been fully understood. For the first time, single and multiple re-focusing of the laser pulse in interaction of femtosecond laser pulse with fused silica, and consequent single and multiple damage zones (filaments) have been observed in our extensive three-dimensional, high resolution FDTD (finite-difference time-domain) simulations. We show that Kerr nonlinearity plays a crucial role loose laser focusing regime, while it is not an important factor in tight laser focusing regime, where geometrical focusing becomes important. Our simulation results agree well with existing experimental findings. In addition, the improved analytical model prediction gives a reasonable estimate of the shift from Kerr nonlinearity regime to linear geometrical focusing regime.