Definition of nonlinear media

[Niles]

Hi

From Wiki (http://en.wikipedia.org/wiki/Nonlinear_optics): "Nonlinear optics (NLO) is the branch of optics that describes the behaviour of light in nonlinear media, that is, media in which the dielectric polarization P responds nonlinearly to the electric field E of the light."

My question is: In principle, all media are nonlinear in the sense that we can Taylor expand the potential of the electrons, but whether a material behaves linearly or not depends only on the incoming field. Don't you agree?

[Dr Lots-o'watts]

Whether a medium is non-linear or not depends on the magnitude of the taylor series coefficients (chi(1), chi(2), chi(3) ...).

The induced polarization depends on both the material (the chi's) and the incident field.

Sure all media could be said to be non-linear, but when chi(x>1) are so close to zero as to be unmeasurable, the term becomes inappropriate, and we fall back in regular electromagnetism.

[shawl]

I agree with Dr Lots.

You may find the answer in the nonlinear physics books.