To experts in EM theory.

I’m fairly certain that for a biaxial material, conservation of energy restricts the imaginary parts of the the permittivity and permeability tensors.  Specifically, let us assume the exp{+jwt} time harmonic convention and, for a biaxial material, define the permittivity tensor by diag(exx,eyy,ezz) where exx = exx’ – j*exx’’, eyy = eyy’ – j*eyy’’, and ezz = ezz’ – j*ezz’’. Then, it’s probably well known that conservation of energy requires exx’’ >=0, eyy’’ >=0, and ezz’’ >=0.  Similarly for the permeability tensor.

More generally, I'm guessing that for general 3x3 permittivity and permeability tensors characterizing an anisotropic media, conservation of energy places the same restriction the imaginary parts of their eigenvalues.  Is this correct?

Most importantly, I'm looking for a reference that explicitly states what restrictions (like the above) on material parameter tensors result from physical considerations such as conservation of energy.  Hopefully, such a reference will verify my guess concerning eigenvalues of the material parameter tensors.

Thank you in advance for any light you can shed on this issue.

Greg.

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