Nano-infrared imaging of the localized plasmon resonance modes in graphene nanoribbons

We performed a nano-infrared imaging study of the localized surface plasmon resonance (SPR) modes of graphene nanoribbons by using the scattering-type scanning near-field optical microscope (s-SNOM). From the real-space imaging data, we found symmetric plasmonic interference fringes when the excitation field is parallel to ribbons and asymmetric ones in the case of perpendicular field excitation. Based on the finite-element simulations, we conclude that the observed asymmetric fringes are formed due to the interference between the localized SPR mode excited by the ribbons and the propagative surface plasmon polariton mode launched by the s-SNOM tip. Furthermore, by varying the ribbon widths, we observed different orders of SPR modes that lead to distinct asymmetric plasmonic fringes.