Fig. 3. Plasmon dispersion.

a AGP dispersion obtained from the interference fringes in near-field at the sample with t = 21 nm (circles), and the analytically calculated dispersion for E_F = –0.46 eV: the exact solutions for AGP (red solid), GSP (red dashed; for thick alumina layer), and the imaginary part of the reflection coefficient (color map). b Top row: distribution of the near-field contrast $∣\eta \left(x,y\right)∣$ over the same graphene edge obtained at different excitation frequencies. Bottom row: corresponding $∣\eta ∣$ profiles across the graphene edge, measured along the dashed line (average value for ten-pixel-wide lines). c Spectral dependency of the calculated D_e (solid) and the measured $∣\eta ∣$ above the sample shown in (b) (circles; averaged over the area far from the edge).