On-chip ultrafast mode-locking lasers are basic building blocks for the realization of a chip-based optical frequency comb. In this paper, an ultrafast saturable absorber made up of a graphene pad on top of a silicon waveguide is applied to implement an ultrafast pulse laser. Benefiting from the small mode area of the graphene/silicon hybrid waveguide, the saturable pulse energy is reduced by two orders of magnitude compared with the fiber. A mode-locked pulse with a duration of 542 fs and a repetition rate of 54.37 MHz is realized. Pump–probe measurement shows that the carrier relaxation process of free carrier recombination with atomic-thin graphene/silicon junctions is three orders of magnitude faster than silicon, which plays a fundamental role in pulse narrowing. The chip-scale silicon ultrafast laser lays a foundation for a new class of nonlinear devices, in which a combination with multiple functional silicon photonic circuits enables efficient nonlinear interaction at the micrometer scale and less than 1 W of power consumption.

Graphene resting on a silicon-on-insulator platform offers great potential for optoelectronic devices. In the paper, we demonstrate all-optical modulation on the graphene–silicon hybrid waveguides (GSHWs) with tens of micrometers in length. Owing to strong interaction between graphene and silicon strip waveguides with compact light confinement, the modulation depth reaches 22.7% with a saturation threshold down to 1.38 pJ per pulse and a 30-μm-long graphene pad. A response time of 1.65 ps is verified by a pump–probe measurement with an energy consumption of 2.1 pJ. The complementary metal-oxide semiconductor compatible GSHWs with the strip configuration exhibit great potential for ultrafast and broadband all-optical modulation, indicating that employing two-dimensional materials has become a complementary technology to promote the silicon photonic platform.