Orthogonality of eigenmodes

When light propagates in a uniform anisotropic medium, there are two eigenmodes, represented 

The electric displacement of the eigenmodes, Di (/ = 1,2), ir perpendicular to the propagation direction. 

Therefore the propagation direction s and the electric displacement Di are orthogonal to each other. 

Because E is the eigenmode 1 with the eigen wavevector ki, and E2 is the eigenmode 2 with the eigen wavevector k2, 

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