Graphene-covered nanoscale waveguides: from fabrication to nonlinear spectral broadening demonstration

Abstract

In recent years the integration of graphene on nanoscale waveguides has attracted much attention as it allows using wellestablished CMOS technology for constructing next-generation photonic integrated circuits. However, important challenges need to be overcome regarding the fabrication and patterning of graphene-covered waveguide devices. In addition, a more in-depth investigation of the fundamental optical properties of graphene-covered waveguides, and in particular their nonlinear optical characteristics, is required. The latter are promising for, amongst others, generating spectrally broadband light useful for a wide range of application domains including telecommunications and sensing. In this paper we present a novel approach for patterning graphene on top of waveguides, and provide new insights in the nonlinear optical properties of graphene-covered waveguides. The patterning approach that we developed is chemicalsfree and based on laser ablation and plasma etching, removing the graphene top layer without damaging the underlying material. Regarding graphene's nonlinear optical properties, we focus on the nonlinear-refraction process of self-phase modulation causing spectral broadening of laser pulses in graphene-covered waveguides. We show that the underlying physics is not based on refraction induced by graphene's conventional third-order susceptibility, but instead on a much more complex phenomenon that we call saturable photoexcited carrier refraction.