Plasmon–phonon coupling in graphene–hyperbolic bilayer heterostructures
Yin Ge, Yuan Jun, Jiang Wei, Zhu Jianfei, Ma Yungui†, 
Relative permittivity spectra (a) and isofrequency band dispersion (b) of hBN. The two shaded areas in panel (a) indicate the type I (green) and type II (orange) hyperbolic regions of hBN. Curves in panel (b) plot the corresponding isofrequency curves in these two regions at 800 cm−1 (green) and 1500 cm−1 (orange), respectively.
