▎ 摘　　要

Exploring emerging materials with enhanced optical nonlinearitiesat low power levels with ultrafast response and small footprints isof great interest for information processing, communication, sensing,and quantum systems. Recent progress on nonlinear metamaterials andmetasurfaces suggests promising solutions to overcome the limitationsof nonlinear materials in nature. Here we present a design conceptfor highly enhanced saturable absorption effect based on subwavelength-thick(<1/5 lambda(0)) hybrid graphene-plasmonic metasurfacestructures in infrared wavelengths. Our theoretical and experimentalresults demonstrated that, by exciting nonequilibrium carriers insidenanoscale hotspots, one could not only enhance the saturable absorptionin graphene, but also reduce the saturation fluence by over 3 ordersof magnitude (from similar to 1 mJ/cm(2) to similar to 100 nJ/cm(2)). Our pump-probe measurement results suggested anultrashort saturable absorption recovery time (<60 fs), which isultimately determined by the relaxation dynamics of photoexcited carriersin graphene. We also observed pulse narrowing effects in our devicesbased on the autocorrelation measurement results. Such design conceptscan be tailored via structure engineering to operate in broader wavelengthranges up to mid- and far- infrared spectral regions. These ultrafastlow-saturation fluence saturable absorber designs can enable low-threshold,compact, self-starting mode-locked lasers, laser pulse shaping, andhigh-speed optical information processing.