Third-order optical nonlinearities in mesoionic compounds

Summary form only given.We describe the application of the well-known Z-scan technique to determine the nonlinear refractive index, n/sub 2/, of seven different mesoionic compounds. The samples were synthesized in our laboratory, based on quantum chemical AM1-TDHF calculated structures. Solutions in DMSO with concentrations around 10/sup 19/ molecules/cm/sup 3/ were prepared. Typical sample length varied between 0.2 mm and 5 mm. The linear absorption coefficient at the pump wavelength varies between 0.1 cm/sup -1/ and 10 cm/sup -1/ for all the samples. The observed peak-to-valley shape of the transmittance variation as a function of the linear displacement of the sample along the propagation direction, Z-axis, is characteristic of a self-defocusing (negative) nonlinearity. The measured values of n/sub 2/ are given. The macroscopic and microscopic third-order optical nonlinear coefficients can be related to each other through /spl chi//sup (3)/=Nf/sup 4//spl gamma/, where f is a local field correction factor given by f=[n/sub 0//sup 2/(/spl omega/)+2]/3, N is the number of molecules per cm/sup 3/, and n/sub 0/ is the linear index of refraction. Using this relation we have estimated the values of the second-order hyperpolarizability, /spl gamma/. Our results indicate that a structure like MI-7, which is planar, presents a better disposition of the donor and acceptor groups when compared to MI-2, a nonplanar structure. The facility to engineer and synthesize these organic materials may lead to the highest possible nonlinear susceptibilities for these structures, which may have potential applications to photonics.