Evaluation of nonlinear optical susceptibility of polydiacetylenes by third harmonic generation

Abstract

Abstract Nonlinear optical susceptibility χ (3) of polydiacetylenes has been evaluated by third harmonic generation. First, in order to obtain the χ (3) tensor component along the polymer main chain, thin single crystals of two representative polydiacetylenes, poly‐PTS and poly‐DCHD were made by utilization of sampling technique in microscopy, i.e., the microtome cutting of single crystal embedded in plastic resin. The THG intensity observed was proportional to cos 8 θ, where θ was the angle between the polymer main chain and the polarization of laser light. The χ (3) value of poly‐DCHD was found to be rather higher than that of poly‐PTS. At resonant wavelength of 1.97 μm, the χ (3) of poly‐DCHD attained 8 x 10 −10 esu. Furthermore, it was confirmed that when geometrical correction were properly made, the χ (3) obtained from polycrystalline thin film of poly‐PTS agreed well with that from thin single crystals. Secondary, regarding poly‐diphenyldiacetylene derivatives, it was found that the π‐conjugation between the polymer main chain and aromatic substituents was effective on the improvement of χ (3) values. The χ (3) magnitudes of poly‐BTFP and poly‐DFMP reflect well the dihedral angles between polymer main chain and the phenyl substituents (58° for poly‐BTFP and 67° for poly‐DFMP) as a measure of π‐conjugation. Especially, at nonresonant region of 2.1 μm the χ (3) of poly‐BTFP is about 5 times greater than that of poly‐PTS.