  Exploring novel materials with enhanced optical nonlinearities at low power
levels with ultrafast response and small footprints is of great interests for
information processing, communication, sensing and quantum systems. Recent
progress on nonlinear metamaterials and metasurfaces suggests promising
solutions to overcome the limitations of nonlinear materials in nature. Here we
present a design concept for highly enhanced saturable absorption effect based
on subwavelength-thick (<1/5$\lambda$0) hybrid graphene-plasmonic metasurface
structures in infrared wavelengths. Our theoretical and experimental results
demonstrated that, by exciting nonequilibrium carriers inside nanoscale
hotspots, one could not only enhance the saturable absorption in graphene, but
also reduce the saturation fluence by over 3 orders of magnitude (from ~ 1
mJ/cm2 to ~100 nJ/cm2). Our pump-probe measurement results suggested an
ultrashort saturable absorption recovery time (<60 fs), which is ultimately
determined by the relaxation dynamics of photoexcited carriers in graphene. We
also observed pulse narrowing effects in our devices based on the
autocorrelation measurement results. Such design concepts can be tailored via
structure engineering to operate in broader wavelength ranges up to mid- and
far- infrared spectral regions. These ultrafast low-saturation fluence
saturable absorber designs can enable low-threshold, compact, self-starting
mode-locked lasers, laser pulse shaping, and high-speed optical information
processing.
