Quantum nonlocal effects in individual and interacting graphene nanoribbons

We show that highly doped graphene ribbons can support surface plasmons at near-infrared frequencies when their width is in the nanometer range, leading to important nonlocal and finite quantum-size corrections, such as sizable blueshifts. The magnitude of these effects is assessed by comparing classical and quantum-mechanical models to describe graphene plasmons. More precisely, we examine individual and interacting 6–8 nm wide zigzag and armchair ribbons doped to 0.4–1.5 eV Fermi energies. We find a strong influence of nonlocal effects on the orientation of graphene edges, with plasmons in zigzag ribbons undergoing strong quenching when their energy is below the Fermi level. Nonlocality is also affecting the hybridization between ribbon plasmons in dimers and arrays for separations below a few nanometers. Remarkably, the removal of a single row of atomic bonds in a ribbon produces a strong plasmon frequency shift, whereas the removal of bonds along an array of rows separated by several nanometers in an extended sheet causes a dramatic increase in the absorption. Besides the fundamental interest of these results, our work supports the use of narrow ribbons to achieve electro-optical modulation in the near infrared. A team in Spain shows theoretically that doped nanoscale-wide graphene nanoribbons can sustain surface plasmons at near-infrared frequencies. These plasmons -- light-induced collective excitations of conduction electrons at metal surfaces -- give rise to significant nonlocal quantum effects, including sizeable blue shifts. The researchers, who are from the Institute of Photonic Sciences (ICFO) and Institució Catalana de Recerca i Estudis Avançats (ICREA), analysed these quantum nonlocal effects by comparing the results of classical and quantum-mechanical models for the optical responses of graphene. They discovered that the nonlocal effects are strongly dependent on graphene edge orientations and are also affected by interactions between ribbons a few nanometres apart. The findings highlight the advantages of using ribbons over other methods of exploiting near-infrared plasmons. They are of fundamental interest and should also assist in designing graphene-plasmon electro-optical modulators and switchers.