About polarization and electric field

[KFC]

In the text, it is said that the polarization is just the response of the input electric field so they have

P = \epsilon_0 \chi E
where P is the polarization and E is the input.

This makes sense to me. However, why we need \epsilon_0 sitting there? Since \epsilon_0 has no dimension, so even there is no \epsilon_0, the relation is still the relation b/w input and response. So what's the different if we have \epsilon_0 there?

And by the way, the book said above equation is given when considering linear case. So what is the relation b/w polarization and field when there exists nonlinearity?

Thanks.

 

[kanato]

\epsilon_0 has dimensions if you're not working in CGS units. It's a commonly used convention to write P = \epsilon E and then replace \epsilon with the value for the material at a given point in space, or \epsilon_0 for the vacuum, so \chi is the ratio of the permeability of the material to the permeability of space in any unit system.

There's nothing you can really say about the nonlinear case in general. There, P = f(E) where f is some function of the electric field. You could write it in the same form as above, like P = \epsilon_0 \chi(E) E but this is done without loss of generality, so there is nothing learned. In real materials, what usually happens is that there is some electric field where the polarization saturates, and for E greater than that field, the polarization will be basically constant. And if you go much higher than that, then you will eventually get breakdown of the material and it begins to conduct. The same thing happens in the magnetic case, except for the breakdown. The linearity assumption is good up to moderately large fields in most cases.

 

[tiny-tim]

This makes sense to me. However, why we need \epsilon_0 sitting there? Since \epsilon_0 has no dimension, so even there is no \epsilon_0, the relation is still the relation b/w input and response. So what's the different if we have \epsilon_0 there?

And by the way, the book said above equation is given when considering linear case. So what is the relation b/w polarization and field when there exists nonlinearity.

Hi KFC!

(have an epsilon: ε and a chi: χ )

In SI units, the permittivity ε₀ is measured in units of farad per metre.

The susceptibility χ is dimensionless.

Second-order susceptibility is a tensor, used in non-linear optics, with Pⁱ = ε₀χⁱ_jkE^jE^k

 

[KFC]

Thanks. I still have a question. I learn from a text that the so-called absorption coefficient is related to the imaginary part of \chi(\omgea), but in other books, they say the absorption coefficient should be

\frac{\omega}{c_0}\Im(\sqrt{1 + \chi_r + i\chi_{i}})

where \chi = \chi_r + i\chi_i, c_0 is the light speed in free space.

so which one is correct?