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 subpicosecond timescales. This handicap has prompted the search for alternative plasmonic media, with graphene emerging as one of the most capable candidates for infrared wavelengths. We visualized and elucidated the properties of non-equilibrium photo-induced plasmons in a high-mobility graphene monolayer. We activated 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 [Ni et al. Nature Photonics 10, 244 (2016)]. We performed similar experiments for both III-V and II-VI semiconductors. I will discuss merits of graphene and of conventional semiconductors for ultra-fast plasmonic applications.
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