The success of metal-based plasmonics for manipulating light at the nanoscale has been empowered by imaginative designs and advanced nano-fabrication. However, the fundamental optical and electronic properties of elemental metals, the prevailing plasmonic media, are difficult to alter using external stimuli. This limitation is particularly restrictive in applications that require modification of the plasmonic response at sub-picosecond timescales. This handicap has prompted the search for alternative plasmonic media1,2,3, with graphene emerging as one of the most capable candidates for infrared wavelengths. Here we visualize and elucidate the properties of non-equilibrium photo-induced plasmons in a high-mobility graphene monolayer4. We activate plasmons with femtosecond optical pulses in a specimen of graphene that otherwise lacks infrared plasmonic response at equilibrium. In combination with static nano-imaging results on plasmon propagation, our infrared pump–probe nano-spectroscopy investigation reveals new aspects of carrier relaxation in heterostructures based on high-purity graphene.
We thank P. Kim, Z. Sun, A. Sternbach, S. Dai and J.-S. Wu for helpful discussions. Research on static plasmon interferometry of high-mobility graphene is supported by DOE-BES DE-FG02-00ER45799. Work on ultrafast imaging of non-equilibrium plasmons is supported by ONR N00014-15-1-2671. The development of ultrafast pump–probe spectroscopy is supported by DOE-BES DE-SC0012592 and DE-SC0012376. The development of nano-imaging is supported by AFOSR and ARO. D.N.B is supported by the Gordon and Betty Moore Foundation's EPiQS Initiative through Grant GBMF4533. J.H. acknowledges support from ONR N00014-13-1-0662. G.X.N., B.O., and A.H.C.N. acknowledge the National Research Foundation, Prime Minister Office, Singapore, under its Medium Sized Centre Program and CRP award ‘Novel 2D materials with tailored properties: beyond graphene’ (R-144-000-295-281). B.O. acknowledge NRF-Competitive Research Programme (CRP award no. NRF-CRP9-2011-3).