Conical Third Harmonic Generation from Volume Nanogratings Induced by Filamentation of Femtosecond Pulses in Transparent Bulk Materials

Irradiation of transparent materials with high repetition femtosecond laser pulses produces laser-induced periodic surface structures (LIPSS) [1] , which find many applications in plasmonics, photonics and microfluidics [2] . The transition from LIPSS to low spatial frequency structures, called laser-induced periodic annular surface structures (LIPASS) [3] and more importantly, the formation of nanogratings in the volume of various materials [4] has been observed. More recently, conical third harmonic (TH) generation originating from in-bulk optical damage was reported and its correlation with extinction of supercontinuum radiation was unveiled [5] .