Fabrication of polyurea thin films for optical second-harmonic generation by vapor deposition polymerization. Effects of poling field and monomer structure on reactivity and SHG performance

Aromatic PU (polyurea) thin films were fabricated for second-order nonlinear optical application by vapor deposition polymerization using MDI (4,4-diphenylmethane diisocyanate) and DDM (4,4-diaminodiphenylmethane) or DNB (p-diamino nitro benzene). The former was abbreviated to PU (DDM) and the latter to PU (DNB). In PU (DDM) fabricated by VDP (vapor deposition polymerization), the SH (second-harmonic) intensity showed orientation stability up to 200°C due to polymerization and simultaneous orientation under the dc field (the poling process). In PU (DNB) fabricated by VDP, although the enhanced optical nonlinearity due to the introduction of the NO₂ group was achieved, a drastic decay of SH intensity was observed at 200°C. The decay was attributed to hindrance of the polymerization by the dc field, suggesting that the strong poling field reduces reactivity of some monomers. In PU (DNB) fabricated by EVDP (VDP under electric field), SHG (second-harmonic generation) was observed. Applying a higher electric field was effective in improving the optical nonlinearity. Film fabrication at higher temperature was also effective in improving the orientation stability and the optical nonlinearity, due to further polymerization during fabrication. We attributed this to the increase in the number of urea groups. The use of appropriate monomers that polymerize under high electric fields would result in improvement of SHG characteristics