Nonlinear optics (χ⁽²⁾, χ⁽³⁾)

Layer 1 — Physicsin the optics subtree

Intensity-dependent polarization: SHG, SFG, OPA (χ⁽²⁾); self-phase modulation, Kerr, FWM (χ⁽³⁾). Enables frequency conversion, squeezed light, entangled photons.