= Self-defocusing
<Self-defocusing> is the spreading tendency caused when a beam lowers the <refractive index> most strongly where its own <intensity> is largest. In the local <optical Kerr effect> model $n=n_0+n_2I$, a beam brightest on its axis produces this behavior for $n_2<0$: its transverse <phase> profile resembles that of a diverging <optical lens>.
Compare the beam width after the same sample at several input powers while holding the input profile fixed. A power-dependent change can reveal nonlinear spreading, but ordinary <diffraction>, <optical absorption> and heating must also be considered. <Self-defocusing> modifies a beam's propagation rather than removing the ordinary <diffraction> it already experiences.
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