FIG 3 (IMAGE)
Caption
Fig. 3: Distinct mechanisms of EFISH, CISH, and QCSE. (a, b) In centrosymmetric materials, symmetric electron distribution in momentum space (blue area), corresponding to an even potentials in real space, leads to mutually cancelling second-harmonic polarization, thereby forbidding SHG. (c) Application of an external electric field breaks centrosymmetry by inducing asymmetric electron potentials between momentum states at ±k (EFISH). (d) In the material with high carrier density, electrostatic field is screen and carriers movement (current) breaks symmetry through asymmetric electron distributions in momentum space (CISH). (e) Both EFISH and CISH borken the inversion symmetry enabling non-even potential function giving rise to effective second-order nonlinear susceptibility χ(2) with dispersion. (f, g) Formation of excitons or IST leads to resonant enhancement in χ(2) . Application of a electrostatic field modifies the electronic band structure via the Stark effect, thereby modulating this resonant χ(2) .
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