High-order harmonic generation in graphene and carbon nanotubes

Abstract

This thesis presents a comprehensive theoretical study of the process of high-order harmonic generation (HHG) induced by intense fewcycle infrared laser pulses in two different types of low dimensional carbon allotropes: 2D single layer graphene (SLG) and 1D singlewall carbon nanotubes (SWNTs). Our results show the emergence of a non-perturbative spectral plateau at large intensities but, unlike other more common systems, such as atoms, molecules or bulk solids, there is no simple law governing the scaling of the cut-off frequency. Interpreting this particular behavior allows to unveil the fundamental mechanism for HHG in those low dimensional carbon allotropic structures. Using a model for the emission dipole based on the saddlepoint approximation, we show that the first step for HHG in these carbon compounds is radically different from the tunneling ionization/excitation process found in gas systems and finite gap solids, and that is closely related to the singular geometry of their band structure. In this sense, we demonstrate the crucial role that Dirac points in graphene and van Hove singularities in SWNTs play in the creation of electron-hole pairs. We also show that the high-order harmonic response in SLG is highly anisotropic, making it possible to emit elliptically polarized harmonics from linear-polarized drivers, and linearly polarized harmonics from elliptically-polarized pulses.