Ultrafast Charge Separation Induced by a Uniform Field in Graphene Nanoribbons

Abstract

When heteronuclear molecules are illuminated by light of spatially uniform intensity, electronic excitations may, nevertheless, be restricted to parts of the system, depending on the absorption properties of its constituents. Here, we show that this effect is observed also in homogenous carbon based systems, such as graphene nanoribbons (GNRs): a spatially uniform laser pulse can create strongly localized carrier excitations, including excitons, on the sub-nanometer scale within a few femtoseconds. The origin of this effect is the unusual topological-based electronic structure of the GNRs. This opens new avenues for nanoelectronics and brings petahertz switching within reach. Using nonequilibrium Green functions simulations we demonstrate this effect by exciting small GNR heterostructures of suitable geometry with a laser pulse with carefully chosen photon energy, polarization, and carrier-envelope phase.

Figures (4)

• Figure 1: (a) Density of states (bottom) and spatial localization (top) of the indicated states of System 1. (b) Time-dependent carrier density in the bulk (blue) and edge region (red) and their sum (black) in response to a short circularly polarized laser pulse (top) and $y$-component of the dipole moment (bottom) for two values of the CEP. (c) Space resolved differential conductance at $4\,$Å$\,$ above the GNR joost_19_nanolett (top) and occupied LDOS, for three times (bottom). The top-bottom asymmetry is controlled by the carrier-envelope phase: $\phi_{\rm CE}=0$ corresponds to the $E_y$ shown in (b), whereas for $\phi_{\rm CE}=\pi$, $E_y$ has its minimum at $t=0$.
• Figure 2: Ground state properties of System 2 (a): a short cove-edged ZGNR (blue) terminated by two different graphene quantum dots (red and yellow). (b) LDOS of the three system parts of (a) (the same colors). The respective excitonic states are marked by triangles. (c) Momentum distribution of the LUMO states of the two quantum dots. The first Brillouin zone is indicated by the white hexagon. The white arrows represent the interband optical matrix element $\bm{M}^{v,c}_{\bm{k}}$ in dipole approximation for tight-binding pristine graphene bonitz_pssb23.
• Figure 3: Dependence of spatial charge separation in System 2 [measured by the horizontal component of the dipole moment], on (a) photon energy, for circular polarization, and (d) laser polarization, for $\hbar \omega_{\textnormal{L}}=1.52\,$eV, at $t=28\,\mathrm{fs}$, i.e., long after the laser pulse. (b, c, e, f): Spatial distribution of the excited carriers at $t=28\,\mathrm{fs}$ for four combinations of polarization and $\omega_{\rm L}$ indicated in the figures [white triangles in (a) and (d)].
• Figure 4: Excitation of System 2 (free-standing, $U/J=3.54$) with a four-cycle laser pulse of frequency $\hbar\omega_{\textnormal{L}}=1.87\,$eV with linear polarization [arrow in (b)]. (a) Laser pulse and time evolution of the number of excited carriers (full lines) and excitons (dashed lines) in the three system parts. (b) Spatial distribution of the excited electrons at $t=28\,$fs. (c) Average e-h pair correlation function, $\bar{g}_{eh}$, in the red QD for three times. (d) Dipole moment for and two linear polarizations vs. Hubbard $U$. (e) $U$-dependence of the exciton energy in the three system parts, cf. Fig. \ref{['fig:2']}b. In panels (d) and (e) the laser frequency changes from $\hbar\omega_{\textnormal{L}}=1.4\,$eV for $U/J=1.5$, to $\hbar\omega_{\textnormal{L}}=1.9\,$eV for $U/J=3.54$, for details see text.