Demonstration of efficient nonlinear optical crystals with vanishing molecular dipole moment: Second-harmonic generation in 3-methyl-4-nitropyridine-1-oxide

Previous research toward more efficient nonlinear optical organic crystals has aimed at increasing the first-order hyperpolarizabilitiy of individual molecular units. We show that the general features derived from these studies (high conjugation, charge transfer, and molecular asymmetry) leave room for the cancellation of the molecular dipole moment. This can be usefully exploited in two directions: improving the crystal growth conditions and a possibility of acting on the crystal point group structure through otherwise hidden parameters. Pyridine-1-oxide, owing to the electronic ’’push–pull’’ property of the N–oxide bond, allows for substitution in the 4 position of donor as well as acceptor groups, among which the nitro group ensures dipole cancellation. The possibility of charge transfer from or toward the N–oxide bond according to the nature of the radical in the 4 position is investigated and confirmed by means of second-harmonic generation experiments in powder or solution (DCFISH) on a number of compounds. 3-methyl-4-nitropyridine-1-oxide is shown to be the best candidate, among related compounds, for crystal growth. A detailed study of its linear and first-order nonlinear properties is presented. Crystalline samples grown in solution are seen to be of excellent optical quality. Its efficiency compares to that of the best available phase- matchable organic crystals [d = (23±3)×10−9 esu]. Besides, the optical broadband phase-matching potential of the crystal could be useful in a number of applications like frequency doubling of ultrashort pulses in the near infrared.