Structural investigations and optical properties of chiralN-(4-nitrophenyl)-(l)-prolinol gel-grown crystals

A gel-growth technique is used to grow highly nonlinear N-(4-nitrophenyl)-(l)-prolinol (NPP) single crystals of larger size with improved optical quality over those produced by other methods, such as solution or melt growth. High-quality samples are obtained by lowering the temperature of a NPP saturated water-acetonitrile mixture in a tetramethoxysilane gel. X-ray characterization gives evidence of the occurrence of twins, in agreement with the results of nonlinear optical experiments under phase-matched conditions. A close agreement between the effective interaction length with the actual physical thickness of the samples attests to their optical quality. The high quadratic nonlinear efficiency of NPP (25 times that of currently available 3-methyl-4-nitropyridine-1-oxide crystals) is confirmed. The low, monoclinic P$2_1$ symmetry of NPP does not fully determine the orientation of the principal dielectric framework. A further inspection of the orientation within the crystal unit cell of the chiral prolinol groups, together with an oriented-gas description of molecular and crystalline polarizabilities, complements the partial crystallographic information, allowing us to account fully for the orientation of the ‘‘symmetry’’-free dielectric axes.