Direct laser writing below the diffraction limit using spatially and temporally tuned ultrashort pulses

We report on the experimental and theoretical study of direct laser writing on dielectrics with reduced feature size by using temporally and spatially separated pulses. Features on the front surface of fused silica within the overlapping area of two laser pulses are obtained by tuning the delay time between the two pulses. The observed dependence of feature position on delays longer than the free-carrier lifetime indicates an ionization pathway initiated by self-trapped excitons. The sensitivity of feature size with the increase of laser intensity is studied by simulating the free-carrier density distribution for different temporal and spatial separation of two laser pulses. It is found that the sensitivity is influenced by the spatial and temporal separation of the two pulses.