“…Within the Reststrahlen band, anisotropic materials can exhibit interesting optical properties because of their crystal asymmetry. [
24,25,31,72 ] The unequal responses between the in‐plane isotropic and out‐of‐plane dielectric tensor components allows for their product to be less than zero (Re(ε xx,yy ) * Re(ε zz ) < 0), and under these circumstances the isofrequency surface becomes an open hyperboloid, allowing the material to support high‐momenta (
) hyperbolic phonon polaritons (HPhPs). [
72–75 ] Conversely, when Re(ε xx,yy ) * Re(ε zz ) > 0, the in‐plane momentum of light within the material is bound by a closed ellipsoid isofrequency surface.…”