Influence of the polarization state on femtosecond laser-written longitudinal waveguides in silicon

Due to the intrinsic properties of silicon, major hurdles need to be overcome in order to inscribe permanent in-volume modifications with femtosecond laser pulses. This behaviour is explained in terms of the extreme nonlinearities associated with silicon which limit the deposition of sufficient pulse energy at the focal plane [1]. To overcome this challenge, researchers employed the exit-surface-seeded technique to inscribe longitudinally waveguides inside silicon with femtosecond pulse lasers. This technique consists of a pre-modification on the exit surface, which in turn locally enhances the absorption of the subsequent pulses [2,3]. It was shown that the residual tensile stress and crystal defects lead to a positive refractive index change in silicon [4]. While extensive parameter studies have been performed to optimize the waveguide performance, the influence of the polarization state of inscription beam on the guiding properties and morphology of the longitudinal waveguides were not studied yet.