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The realization of efficient high-harmonic generation (HHG) in solid-state systems is anticipated to pave the way for compact ultraviolet and ultrafast light sources, and to provide fundamental insight into quantum many-body electron motion [1-3]. Here we argue that the large light intensity required for HHG to occur can be reached by exploiting localized plasmons in doped graphene nanostructures. In particular, we demonstrate that the synergistic combination of strong plasmonic near-field enhancement and a large intrinsic nonlinearity originating from the anharmonic charge-carrier dispersion of graphene result in efficient broadband high-harmonic generation within a single material [4]. We extract this conclusion from rigorous time-domain simulations using complementary nonperturbative approaches based on atomistic one-electron density matrix and massless Dirac-fermion Bloch-equation pictures, where the latter treatment is supplemented by a classical electromagnetic description of the plasmonic near-field enhancement produced by the illuminated nanostructure.
High harmonics are predicted to be emitted with unprecedentedly large intensity by tuning the incident light to the localized plasmon resonances of ribbons and finite islands, which in turn can be actively modulated via electrical gating. In contrast to HHG in atomic systems, we observe no cutoff in harmonic order, while a comparison of graphene plasmon-assisted HHG to recent measurements in solid-state systems suggests that the HHG yields from bulk semiconductors can be produced by graphene plasmons using 3-4 orders of magnitude lower pulse fluence. Our results support the strong potential of nanostructured graphene as a robust, electrically-tunable platform for HHG.
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