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The theory of ionization-field instability in a continuous homogeneous medium is generalized to the heterogeneous nanoporous one with the purpose to clear and analyze the mechanism of nanograting formation during the optical breakdown process in a transparent dielectric (fused silica) containing multiple small inclusions (nanobubbles) with a lowered ionization threshold. Based on the Maxwell Garnett approximation extended to include the size effect caused by the finite ratio of the bubble sizes to the scale of unstable perturbations, we have obtained the equation system describing the spatiotemporal evolution of the plasma density, average field, and effective dielectric permittivity and have derived the characteristic equation connecting the temporal growth rate of these perturbations with their spatial period. Analysis of the roots of this equation shows that the unstable periodic perturbation structure having the maximal growth rate is close in character to the nanogratings observed experimentally (modulation in the direction of the pump wave polarization with the period approximately equal to the half-wavelength in the host material). [ABSTRACT FROM AUTHOR]

The steady-state structure of a nonequilibrium discharge maintained in the medium by a focused electromagnetic wave beam is determined based on the physical model treating the discharge as a distributed nonlinear system with a given level of the electric field amplitude self-limitation. In this model, finding the self-consistent spatial distributions of the plasma density and the electric field phase in the ionized region is reduced to some inverse electrodynamic problem, whose solution is found in application to the axially symmetric geometry. The electromagnetic field outside the discharge plasma is found in the approximation of geometric optics with the ray reflection and refraction at the boundary being taken into account. The differential equations determining the form of the plasma-vacuum interface are derived and integrated numerically and the distributions of the plasma density and the electric field within and around the discharge are calculated as applied to the conditions of interest for the microwave and optical discharges in gases and solids. [ABSTRACT FROM AUTHOR]