Experimental Characterization of the Ultrafast, Tunable and Broadband Optical Kerr Nonlinearity in Graphene

Graphene's giant nonlinear optical response along with its integrability has made it a vaunted material for on-chip photonics. Despite a multitude of studies confirming its strong nonlinearity, there is a lack of reports examining the fundamental processes that govern the response. Addressing this gap in knowledge we analyse the role of experimental parameters by systematically measuring the near-infrared spectral dependence, the sub-picosecond temporal evolution and pulse-width dependence of the effective Kerr coefficient (n₂,_eff) of graphene in hundreds of femtosecond regime. The spectral dependence measured using the Z-scan technique is corroborated by a density matrix quantum theory formulation to extract a n₂,_eff ∝ λ² dependence. The temporal evolution obtained using the time-resolved Z-scan measurement shows the nonlinearity peaking at zero delay time and relaxing on a time-scale of carrier relaxation. The dependence of the n₂,_eff on pulse duration is obtained by expanding the input pulse using a prism-pair set-up. Our results provide an avenue for controllable tunability of the nonlinear response in graphene, which is limited in silicon photonics.