Fig. 4: Isofrequency curves for the plasmon dispersion.

a Isofrequency curves for u = 0. All other parameters for the Dirac model are identical to parameters used in Fig. 2. Color scale quantifies the quality factor \({q}^{{\prime} }/{q}^{{\prime\prime} }\) which gives the number of cycles the plasmon goes through before decaying or amplifying by e. c Isofrequency curves for u = 1 eVÅ(u/v = 0.41). Negative values of the quality factor denote regions of the plasmon dispersion that experience gain. b, d Top view of the isofrequency curves shown in Fig. 4a, c. Arrow direction shows direction of increasing frequency.