Relation between birefringence parameters and the hierarchical spatial structure of microtracks induced in the bulk of fused silica by ultrashort laser pulses

In this paper, we study changes in the phase shift and length, as well as the spatial structure of birefringent microtracks induced in the bulk of fused silica in the tight focusing regime (numerical apertures $\mathrm{NA} = 0.45$ and $0.55$) by laser pulses with a wavelength of $1030$ nm, durations of $0.3$ and $0.6$ ps, and variable energy. It is shown that the phase shift and the length of the microtracks increase monotonically with the pulse energy, while their spatial structure is transformed from an array of longitudinal channels with transverse subwavelength periodicity ${{\Lambda }_{E}}$ along the polarization vector into a three-dimensional hierarchical structure with additional periodicity ${{\Lambda }_{k}}$ about the wavelength along the radiation propagation axis. For the first time, it has been found that the period ${{\Lambda }_{E}}$ in three-dimensional hierarchical structures decreases by almost a factor of $2$, which corresponds to an increase in the refractive index difference induced in the microtracks to $\Delta n \sim 4.5 \times {{10}^{{ - 3}}}$.