Polarization‐Dependence of Nonlinear Optical Effects in Silicon: The Case of Ultrafast Processing

ABSTRACT We investigate the role played by polarization in the propagation of intense ultrashort pulses in the bulk of silicon. Using a pump‐probe experiment and a nonlinear transmission measurement, we first address the differences in the beam evolution between linear and circular polarization in the absence of permanent modifications, comparing the results with numerical simulations based upon a standard model accounting for cubic nonlinearity and plasma defocusing. In the second part of the paper, we investigate the role of polarization when light is capable of inducing permanent modifications in the material. The nonlinear dynamics is now addressed ex post by investigating the nature of the permanent modifications induced in silicon. For this purpose, elongated channels with an increase in the refractive index are inscribed starting from the air‐silicon interface. The resulting laser‐written optical waveguides are eventually characterized using different methods, including near‐field microscopy, optical polarimetry, Raman microscopy, and scanning electron microscopy. Beyond potential implications in the realization of 3D photonic integrated circuits in silicon, our work highlights silicon as a platform for the exploration of photonic spin‐orbit interaction emerging in different nonlinear regimes, despite a linear isotropic response.