Formation of laser-induced periodic surface structures on fused silica upon multiple parallel polarized double-femtosecond-laser-pulse irradiation sequences

► LIPSS formation studied for double-fs-pulses (160 fs, 800 nm) of different fluences. ► Close to the damage threshold predominantly HSFL are observed. ► The HSFL period remains almost constant ∼375 nm (delay independent). ► At high fluences and for short delays a transient metallic state is created (LSFL regime). ► A transition of the LSFL period from 750 to 530 nm is observed in the sub-ps delay range.