The growth and perfection of organic non-linear optical crystals: N-methyl urea (NMU) from methanol solution. III. The growth of large single crystals for optical examination

Abstract

Using growth conditions defined from previous studies of the kinetics of growth of this material, large crystals of N-methyl urea (NMU) have been prepared by seeded solution growth using the temperature lowering technique. The principal constraints to good growth are that crystals should be prepared using seeds cut from good quality crystals previously prepared by this technique and that they should be developed under conditions of low supersaturation (σ < 0.5−3%). Efficient stirring conditions should be used with flow reversal to minimise the effect of volume diffusion and macrostep formation which in turn leads to the inclusion of solvent in some sectors. The role of seed orientation in the definition of the development process is discussed. Under well-defined conditions of seed orientation and development crystals of dimensions 5 × 4 × 4 cm3 have been produced of high optical and structural perfection and with efficient non-linear optical performance. This material shows a figure of merit performance which improves on urea. For the doubling of light of wavelength 871.6 nm; d14 ≈ 2pm V−1, cf. urea d14 = 1.7pm V−1.