Material Processing of Dielectrics via Temporally Shaped Femtosecond Laser Pulses as Direct Patterning Method for Nanophotonic Applications

Dielectric materials are of great interest for optical applications since they are transparent in the UV, visible and IR spectral range. That makes them very suitable for optical filters, polarizer, waveguides or even reflectors. When dielectrics are processed with conventional techniques based on electron or ion bombardments, they suffer from severe charging effects. For this reason, we present temporally shaped femtosecond laser pulses as a novel direct patterning method of wide band gap materials with very high precision to create photonic crystal structures in dielectrics. Material processing with temporally shaped femtosecond laser pulses overcomes the charging problems. Fabrication of structures well below the diffraction limit is feasible with temporally shaped asymmetric pulse trains due to nonlinear ionisation effects like multiphoton ionisation and avalanche ionisation. For the implementation as optical filters, a thin-film waveguide with a 2D periodic pattern of photonic crystals with circular base elements is investigated. The wave guiding layer consists of a material with a higher refractive index than the surrounding materials, in our case SiO2. Although the refractive index contrast is low, numerical design results prove that light with normal incidence to the plane of periodicity couples to waveguide modes and Fano resonances are excited. This makes the device extremely interesting as a compact narrow-band optical filter.