Monolayer Graphene Can Emit SHG Waves

Abstract

Abstract:The usually-held notion that monolayer graphene, a centrosymmetric system, does not allow even-harmonic generation when illuminated at normal incidence is challenged by the discovery of a peculiar e ect we term the dynamical centrosymmetry breaking mechanism. This e ect results in a global pulse-induced oscillation of the Dirac cones which in turn produces second harmonic waves. We prove that this result can only be found by using the full Dirac equation and show that the widely used semiconductor Bloch equations fail to reproduce this and some other important physics of graphene. These results clear the way for further investigation concerning nonlinear light-matter interactions in a wide range of two-dimensional materials admitting either a gapped or ungapped Dirac-like spectrum.

Inhomogeneous Dirac-Bloch Equations for Graphene Interacting with Structured Light

“…This is an indirect result of the fact, that the intensity of the impinging pulse was not uniform overall the graphene sample, but had the characteristic doughnut shape of LG modes. The emergence of even harmonics in the nonlinear spectrum of a monolayer of graphene has been reported before [18] and it has been explained in terms of a pulse-induced oscillation of the Dirac cone, that ultimately amplifies even harmonics. In the context of this manuscript, we observe the same effect, but triggered by the non-uniform spatial intensity distribution of the pulse, rather than its temporal features.…”

“…This enhancement allows to observe a lot of nonlinear effects such as, for example, second- [13] and third-harmonic generation [14], four-wave mixing [15], and optical limiting [16]. While graphene is intrinsically centrosymmetric and therefore lacks second order response, recent works have demonstrated how it is possible to induce secondharmonic generation in graphene by inducing stress or strains [17] or by exploiting the fact that an ultra-intense pulse can open its own gap near Dirac points, thus breaking centrosymmetry [18].…”