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<p><a href="page.php?w=polarization_density">polarization density</a> <b>P</b> responds non-linearly to the <a href="page.php?w=electric_field">electric field</a> <b>E</b> of light. In order to obtain an electromagnetic field that is sufficiently intense, <a href="page.php?w=laser">laser</a> sources must be used. In nonlinear optics, the <a href="page.php?w=superposition_principle">superposition principle</a> no longer holds, and the polarization of the material is no longer linear in the electric field intensity. Instead, in the <a href="page.php?w=Perturbation_theory">perturbative limit</a>,</p><p>
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