  A computer simulation of dynamics of an optical discharge produced in the
volume of a transparent dielectric (fused silica) by a focused femtosecond
laser pulse was carried out taking into account the possibility of developing
small-scale ionization-field instability. The presence of small foreign
inclusions in the fused silica was taken into account with the model of a
nanodispersed heterogeneous medium by using Maxwell Garnett formulas. The
results of the calculations made it possible to reveal the previously unknown
physical mechanism that determines the periodicity of the ordered plasma-field
structure that is formed in each single breakdown pulse and is the root cause
of the ordered volume nanograting formation in dielectric material exposed to a
series of repeated pulse. Two main points are decisive in this mechanism: (i)
the formation of a thin overcritical plasma layer at the breakdown wave front
counter-propagated to the incident laser pulse and (ii) the excitation of the
"internal surface plasmon" at this front, resulting in a rapid amplification of
the corresponding spatial harmonic of random seed perturbations in the plasma
and formation of a contrast structure with a period equal to the wavelength of
the surface plasmon (0.7 of the wavelength in dielectric).
