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Second harmonic generation (SHG) is a nonlinear optical process in which two photons with the same frequency interact with a nonlinear material and generate a new photon with twice the energy of the initial photons due to the broken inversion symmetry of the media. At Semiconductor electrolyte interface (SEI), the second harmonic response contains three contributions: the second order response of the solid surface (χ(2)sur) and interfacial liquid (χ(2)liquid), and the bulk solid response at the semiconductor space charge layer region (χ(3)solid)*F_sc) induced by the presence of a DC field (F_sc). The latter is the electric field induced second harmonic response (EFISH). The EFISH response from semiconductor electrolyte interface (SEI) has been shown to be directly proportional to the electric field strength at the space charge layer region, which manifest itself in the extent of band bending of the semiconductor electrode. EFISH serves as a sensitive probe of the response of the semiconductor/liquid junction to optical illumination. Studying time-resolve EFISH from sub-picosecond to seconds at SEI can provide information of carrier dynamics during water oxidation/reduction in photoelectrochemical cell.
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