The simplest configuration considered in Ref . 1 consists of an interface between an isotropic material with permittivity ? and a uniaxial crystal with permittivities ?o and ?e for the ordinary and the extraordinary waves respectively.
12.
If \ varepsilon _ { 2 } vanishes, then the tensor is diagonal but not proportional to the identity and the medium is said to be a uniaxial medium, which has similar properties to a uniaxial crystal.
13.
A uniaxial crystal exhibits two refractive indices, an " ordinary " index ( " n " o ) for light polarised in the x or y directions, and an " extraordinary " index ( " n " e ) for polarisation in the z direction.
14.
In a uniaxial crystal, there is only two orientations ( out of 6 ) in which birefringence can be seen, yes ? ( I had to guess from the hexagonal crystal structure of calcite-- the language isn't clear at all ! ) I couldn't figure out from the article
15.
Negative uniaxial crystals ( e . g . calcite CaCO 3, ruby Al 2 O 3 ) have " n e " o " so for these crystals, the extraordinary axis ( optic axis ) is the fast axis whereas for positive uniaxial crystals ( e . g . quartz SiO 2, sellaite ( magnesium fluoride ) MgF 2, rutile TiO 2 ), " n e " > " n o " and thus the extraordinary axis ( optic axis ) is the slow axis.
16.
Negative uniaxial crystals ( e . g . calcite CaCO 3, ruby Al 2 O 3 ) have " n e " o " so for these crystals, the extraordinary axis ( optic axis ) is the fast axis whereas for positive uniaxial crystals ( e . g . quartz SiO 2, sellaite ( magnesium fluoride ) MgF 2, rutile TiO 2 ), " n e " > " n o " and thus the extraordinary axis ( optic axis ) is the slow axis.
