THE GROWTH AND PERFECTION OF ORGANIC NONLINEAR-OPTICAL CRYSTALS - N-METHYL UREA (NMU) FROM METHANOL SOLUTION .3. THE GROWTH OF LARGE SINGLE-CRYSTALS FOR OPTICAL EXAMINATION

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 lowerin g technique. The principal constraints to good growth are that crystal s should be prepared using seeds cut from good quality crystals previo usly prepared by this technique and that they should be developed unde r conditions of low supersaturation (sigma < 0.5-3%). Efficient stirri ng conditions should be used with flow reversal to minimise the effect of volume diffusion and macrostep formation which in turn leads to th e inclusion of solvent in some sectors. The role of seed orientation i n the definition of the development process is discussed. Under well-d efined conditions of seed orientation and development crystals of dime nsions 5x4x4 cm(3) have been produced of high optical and structural p erfection and with efficient non-linear optical performance. This mate rial shows a figure of merit performance which improves on urea. For t he doubling of light of wavelength 871.6 nm; d(14) approximate to 2 pm V-1, cf. urea d(14) = 1.7 pm V-1.