Thickness dependence of second-harmonic generation in thin films fabricated from ionically self-assembled monolayers

An ionically self-assembled monolayer (ISAM) technique for thin-film deposition has been employed to fabricate materials possessing the noncentrosymmetry that is requisite for a second-order, chi((2)), nonlinear optical response. As a result of the ionic attraction between successive layers, the ISAM chi((2)) films self-assemble into a noncentrosymmetric structure that has exhibited no measurable decay of chi((2)) at room temperature over a period of more than one year. The second-harmonic intensity of the films exhibits the expected quadratic dependence on film thickness up to at least 100 bilayers, corresponding to a film thickness of 120 nm. The polarization dependence of the second-harmonic generation yields a value of 35 degrees for the average tilt angle of the nonlinear optical chromophores away from the surface normal. (C) 1999 American Institute of Physics; [S0003-6951(99)02904-6].