Enhanced in-glass nonlinear optical modulation by coupling thin ENZ film with a femtosecond laser-written skimming waveguide

Glass-based photonic and electronic chips are attractive due to the high transparency, strong mechanical strength and biocompatibility of such substrates, or where low-loss connections to fiber-optic networks are required. However, efficient nonlinear optical modulation is difficult to achieve in ordinary silicate glass substrates, due to their inherent small optical nonlinearity in the broadband transparency window. In this paper, by integrating low-loss skimming waveguides using femtosecond laser pulses, we report a general strategy to significantly enhance the nonlinear response of glass photonic circuits based on the evanescent-coupling of an epsilon-near-zero (ENZ) thin film with the skimming waveguides. The fabricated device exhibits flexible tuning of absorptive optical nonlinearity between saturable absorption and reverse saturable absorption. This work provides what we believe to be a new method for optical switches and modulators that could offer potential solutions for 3D high-density photonic integration with dynamic programmability.