(color online) Real-space optical imaging of graphene stripe. (a) AFM topography image of mechanically exfoliated graphene stripe. The darker area corresponds to the SiO₂/Si substrate. The outline of graphene is marked by blue dashed lines. (b) The corresponding nano-infrared optical image of graphene stripe with incident frequency of 930 cm⁻¹. The outline of graphene is marked by red dashed lines. The cavity mode and dipole mode are marked by white and green dashed circles, respectively. The scale bar is 200 nm. (c) AFM line-profile obtained from the cyan dashed line in panel (a). The experimental topography data (black squares) agrees well with Boltzmann fitting method. (d) Plasmonic line profile of cavity mode shown in panel (b). The experimental result (black squares) is well fitted by Gaussian shape. (e) Plasmonic line profile of dipole mode shown in panel (b).