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High-harmonic generation (HHG) in condensed-matter systems is both a source of fundamental insight into quantum electron motion and a promising candidate to realize compact ultraviolet and ultrafast light sources [1-3]. Here we argue that the large light intensity required for this phenomenon to occur can be reached by exploiting localized plasmons in conducting nanostructures. In particular, we demonstrate that doped graphene nanostructures combine a strong plasmonic near-field enhancement and a pronounced intrinsic nonlinearity that result in efficient broadband HHG within a single material platform [4]. We extract this conclusion from time-domain simulations using two 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 self-consistent field produced by the illuminated nanostructure. High harmonics are predicted to be emitted with unprecedentedly large intensity by tuning the incident light to the localized plasmons of ribbons and finite islands. In contrast to atomic systems, we observe no cutoff in harmonic order, while a comparison of the predicted HHG from graphene to that observed in solid-state systems suggests that the HHG yields measured in 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.
[1] S. Ghimire et al., “Observation of High-Order Harmonic Generation in a Bulk Crystal,” Nat. Phys. 7, 138 (2011).
[2] O. Schubert et al., “Sub-Cycle Control of Terahertz High-Harmonic Generation by Dynamical Bloch Oscillations,” Nat. Photon. 8, 119 (2014).
[3] T. T. Luu et al., “Extreme Ultraviolet High-Harmonic Spectroscopy of Solids,” Nature 521, 498 (2015).
[4] J. D. Cox, A. Marini, and F. J. García de Abajo, “Plasmon-Assisted High-Harmonic Generation in Graphene,” Nat. Commun. 8, 14380 (2017).
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