Fig. 2.

Dielectric permittivity tensor of h-BN in the upper Reststrahlen band. Real (solid red) and imaginary (dashed red) parts of the principal components of $\widehat{ϵ}$ perpendicular to the h-BN optical axis. For the parallel component, the imaginary part is vanishingly small and only the real part is shown (blue). Inset illustrates the orientation of the tensor's principal axes with respect to the atomic sheets of h-BN. Left and right vertical dashed lines mark the TO and LO phonon frequencies, respectively