Nonlinear polarization
= Nonlinear polarization
{title2=$P_i^{(2)}=\epsilon_0\sum_{jk}\chi^{(2)}_{ijk}E_jE_k$}
<Nonlinear polarization> is the part of an induced electric <polarization> nonlinear in the driving <electric field>. A quadratic response multiplies field components, creating sums and differences of their <frequencies>; <second-harmonic generation> is the equal-frequency sum case. Conversion strength also depends on <phase matching> and the material response.