I am writing regarding a statement on the invited paper of Prof. Boardman Allan and Dr. Xie Ming, appeared in SPIE proceedings 2001, and also in the book "Introduction to Complex Mediums for Optics and Electromagnetics" chapter of complex magnetic materials, which I could not understand, or find an answer.

The statement is: "it is clear that (g × E) is normal to E, so that the overall dielectric property tensor of a magneto-optic material must have off-diagonal elements."

My questions are:

1) How can one relate / understand  the off-diagonal elements in the permittivity tensor are due to the fact that g x E is normal to E (are there any mathematical aspects hidden beneath that)?

2) And, why the other off-diagonal elements equal to zero ? (is tehre any coordiantes' sytem transformation)

Thank you inadavance for your answers !

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