FABRICATION OF POLYUREA THIN-FILMS FOR OPTICAL 2ND-HARMONIC GENERATION BY VAPOR-DEPOSITION POLYMERIZATION - EFFECTS OF POLING FIELD AND MONOMER STRUCTURE ON REACTIVITY AND SHG PERFORMANCE

Aromatic PV (polyurea) thin films were fabricated for second-order non linear optical application by vapor deposition polymerization using MD I (4,4-diphenylmethane diisocyanate) and DDM (4,4-diaminodiphenylmetha ne) or DNB (p-diamino nitro benzene). The former was abbreviated to PV (DDM) and the latter to PU (DNB). In PU (DDM) fabricated by VDP (vapo r deposition polymerization), the SH (second-harmonic) intensity showe d orientation stability up to 200 degrees C due to polymerization and simultaneous orientation under the de field (the poling process). In P V (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 degrees C. The decay was attribut ed to hindrance of the polymerization by the de field, suggesting that the strong poling field reduces reactivity of some monomers. In PU (D NB) fabricated by EVDP (VDP under electric field), SHG (second-harmoni c generation) was observed. Applying a higher electric field was effec tive 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 fa brication. We attributed this to the increase in the number of urea gr oups. The use of appropriate monomers that polymerize under high elect ric fields would result in improvement of SHG characteristics.