Quadratic nonlinear optical properties of a new organic crystal: N-methoxymethyl-4-nitroaniline (MMNA)

Abstract

It is known that p-nitroaniline (p-NA) shows no quadratic optical nonlinearity because of the centrosymmetry in the crystal, although it has large hyperpolarizability, ft If p-NA is chemically modified without decreasing its p value so as to form noncentrosymmetric crystals, it will change into strong quadratic nonlinear optical materials. We found that N-methoxymethyl-4-nitro-aniline(MMNA), prepared simply by reacting p-NA, formaldehyde and methanol with sodium hydroxide at room temperature, exhibited strong SHG activity. MMNA yielded large single crystals; 5-6 em in length, 1 em in width and 0.5 em in thickness, when crystallized in tetrahydrofuran solution for a few days. The crystal structure of MMNA evaluated by x-ray methods indicates that the crystal belongs to orthorhombic space group.P2 12 12 1 . The crystal data are a=11.236A ,b=17.550A , c=4.57~A, and 2=4. The calculated density of the crystal is 1.34 g/cm and the melting point is 115 °C. The refracti~e indexes at 1.06 ~m and 0.53 fm are n~=1.748, nbW=1.602 and nc w =1.635 respectively. Phase-matching in MMNA is obtained by illuminating laser beam with 3 incident angle to the crystallographic [110] direction, where the (110) plane is natural crystal growth surface. The deff obtained experimentally is 15 pm/V by comparing the phased-matched SH light with those of KTP and quartz. This value agrees well with the calculated one by using crystallographic data according to the theory of oriented gas model developed by Zyss. Since MMNA possesses various merits such as ease of preparation from p-NA, fast crystal growth rate, and high quadratic optical nonlinearity, it seems very promising for the future opto-electronic technology. 5