Figure 3.

Propagation constants ${\beta }_{+}$ (solid red line) and ${\beta }_{-}$ (dashed blue line) for signal SPP modes for the double-layer graphene sheets versus the interlayer distance d numerically calculated from Equation (10). Parameters correspond to the strong coupling regime for wavelength $8.04\mathsf{\mu }\mathrm{m}$ in Table 1.