We present four molecular organic crystals which were recently proposed for applications in nonlinear optics (three-wave-mixing and electrooptic effects) : methyl - (2,4 - dinitrophenyl) - aminopropanoate (MAP), 2 - methyl - 4 - nitroaniline (MNA), 3 - methyl - 4 - nitropyridine - 1 - oxyde (POM) and urea. The nonlinear efficiency of molecular organic crystals is seen to depend on the molecular hyperpolarizability and on the crystalline geometry. Both features are discussed and shown to lead, for crystal point groups of lower symmetry, to new structural relations between crystalline non linear coefficients. Each of the four molecular crystals discussed here are shown to have specific properties connected to different types of applications.

New organic molecular materials for nonlinear optics

Chapter II-7 – Nonlinear Optical Properties of Guest–Host Polymer Structures

A new class of organic quinoid systems possessing intrinsically large, nonlinear second‐order optical susceptibilities has been developed based on theoretical calculations and dc‐induced second‐harmonic generation measurements of the molecular second‐order optical susceptibilities β. For one example, 2‐(4‐dicyanomethylenecyclohexa‐2, 5‐dienylidine)‐imidazolidine (DCNQI), βx = −240±60×10−30 cm5/esu. The unusually large magnitude and sign originate from certain charge‐correlated features in the quinoid ground and excited states.

Exceptional second-order nonlinear optical susceptibilities of quinoid systems

Chapter II-8 – Electro-Optic Organic Materials

The dc linear electro‐optic effect and the x‐ray crystal structure of the organic molecular solid 2‐methyl‐4‐nitroaniline (MNA) have been studied. One polar orientation was found to have an exceptionally large figure of merit for electro‐optic phase retardation ( 1/2 (n13r11−n33r31) = 270±50×10−12 m/V), which is traced to a single, large component of the corresponding microscopic molecular susceptibility. Comparison with second‐harmonic generation data demonstrates that the liner electro‐optic effect in MNA is primarily electronic in nature.

G. F. Lipscomb

Papers

First single-crystal polymers exhibiting phase-matched second-harmonic generation (A)