  The spatiotemporal evolution of the field and plasma in the optical breakdown
induced in the volume of transparent dielectric (fused silica) by the focused
fs laser pulse is studied under condition of the so-called
plasma-resonance-induced ionization instability that results in the deep
small-scale periodic modulation of the breakdown plasma parameters in the
direction of the laser polarization. In the framework of the model used, the
optical electric field was calculated with allowance for the effects
influencing both its long-scale structure (the beam focusing accounted for in
the given-ray-tube approximation, phase and group delays, and back reflection)
and the small-scale one (quasi-static enhancement in the plasma resonance
regions). The plasma density evolution is described by the rate equation taking
into account the photoionization, avalanche ionization, and ambipolar
diffusion. Based on the fulfilled numerical calculations, we have described the
main types of the breakdown wave originating in the focal region and have found
the laser pulse intensity range where the instability evolving from very small
seed perturbations leads to the formation of the contrast subwavelength
periodic structure containing the number of the narrow zones with overcritical
plasma density and enhanced energy deposition. The latter allows us to consider
this structure as underlying the nanograting formation observed experimentally
in the fused silica irradiated by series of repeated fs pulses.
