  Control of laser writing in transparent dielectrics using temporal contrast
of light pulses is demonstrated. Anisotropic nanopores in silica glass are
produced by high-contrast of 10^7 femtosecond Yb: KGW laser pulses rather than
low-contrast of 10^3 Yb fiber laser pulses. Low-contrast pulses are useful for
creating lamellar birefringent structures, possibly driven by quadrupole
nonlinear current. The difference originates in the fiber laser storing third
of its energy in a post-pulse of 200 ps duration. The absorption of this
low-intensity fraction of the pulse by laser-induced transient defects with
relatively long lifetime and low excitation energy, such as self-trapped holes,
drastically changes the kinetics of energy deposition and type of material
modification.
