Fig. 1

Isofrequency surface. a Elliptical isofrequency surface. b and c Hyperbolic isofrequency surface with (b) ε _⊥ = ε _x  = ε _y  < 0, ε _∥ = ε _z  > 0, and (c) ε _⊥ = ε _x  = ε _y  > 0, ε _∥ = ε _z  < 0