▎ 摘　　要

Benefiting from its unique electrical and optical properties, graphene as a saturable absorber (SA) has been widely explored. However, it is still challenging to develop SAs with large modulation depths due to the weak absorption of the monolayer graphene. Here a graphene-on-slot waveguide configuration was demonstrated to overcome this limitation. From the experimental study, with only 20 mu m long graphene, the graphene-on-silicon slot waveguide exhibited lower saturation energy of 313 pJ and seven times larger modulation depth, compared with a graphene-on-silicon nitride rib waveguide. Our study paves the way for the development of ultra-compact graphene-on-waveguide SAs for ultrafast lasers and nonlinear optics. (C) 2019 The Japan Society of Applied Physics