Third and fourth harmonic generation at Si-SiO2 interfaces and in Si-SiO2-Cr MOS structures

Third harmonic (TH) and fourth harmonic (FH) generation is systematically studied at unbiased buried Si(111)-SiO2 and Si(110)-SiO2 interfaces and DC-electric-field induced TH and FH generation is then observed in Si(111)-SiO2-Cr and Si(110)-SiO2-Cr MOS structures for the first time. A systematic phenomenological analysis of azimuthal anisotropy of TH and FH generation intensity is performed for (111) and (110) surfaces of O-h symmetric single crystals. A phenomenological model of electro-induced effects in TH and FH generation is then developed and the surface specificity and sensitivity of TH and FH generation are discussed. Optical interference of surface electro-induced and bulk bias-independent contributions to the effective third-order nonlinear polarization is proposed as the mechanism underlying surface sensitivity of electro-modulated TH probe.