Counter-intuitive strength of electric field induced second harmonic (EFISH) signals at the rear side of thin silicon membranes

Electric field induced second harmonic (EFISH) generation typically increases with laser intensity. Therefore, EFISH signals generated at the rear Si=SiO2 interface by a laser beam transmitted through a thin silicon membrane should be much weaker than the signals generated in reflection at the front surface. The counter-intuitive finding that the signal generated at the rear surface is stronger than the front surface signal [Appl. Phys. B 104, 735 (2011)] is nowconfirmed by detailed experiments applying 10 to 30 _mthin membranes.Using, in addition, the local laser beam intensity [Appl. Phys. B 123, 253 (2017)] leads to the hypothesis that the quasi-static electric field at the rear interface consists of two counteracting components.