= Uniaxial optical medium
{title2=$\boldsymbol\varepsilon=\varepsilon_0\operatorname{diag}(n_o^2,n_o^2,n_e^2)$}
A transparent, nonmagnetic <linear anisotropic dielectric> is uniaxial when two principal <permittivities> agree. With the <optic axis> along $z$, its <permittivity> <tensor> has the displayed <diagonal matrix> form. Here $n_o$ and $n_e$ are principal <refractive indices> at a specified <frequency>; $n_e$ is not the <extraordinary wave>'s index in every direction.<Fundamentals of Photonics and Quantum Electronics lecture notes by Franz X. Kärtner>[§2.8.1, printed pp. 154–156, equations 2.406–2.413: uniaxial tensor and direction-dependent extraordinary index]{ref}
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