Due to delocalized π-electrons, organic materials exhibit very large non-resonant nonlinear optical effects and extensive studies have been motivated by their wide range of potential applications such as second harmonic generation(SHG), frequency mixing, electro-optic modulation, optical parametric oscillation and optical bistability.
3-methyl-4-methoxy-4'-nitrostilbene(MMONS) is a very promising organic material for nonlinear optical application, but the difficulties in the growth of crystals of large size and good quality posed the grave limitation of utilizing MMONS crystals. Large single crystals of MMONS of typical dimensions ~40×40×30㎣ were successfully grown by the low temperature solution growth method. The crystal structure of MMONS was determined by single crystal X-ray diffraction method. The overall morphology of grown crystals are represented by polar morphology, which is characterized by unidirectional growth along one direction of the polar c-axis. The perfection of the crystal was characterized by synchrotron X-ray topography.
Optical transmission spectra revealed that the UV cutoff wavelength was 510 nm for the polarization along the polar c-axis and 500 nm for the polarization along the a- and b- axes. The laser damage threshold was 56 MW/㎠ for 8ns pulse, while the threshold went up to as high as 17 GW/㎠ for the beam of 30 ps pulse width. From the linear refractive index data, the phase matching angles for type Ⅰ and type Ⅱ and the corresponding walk-off angles were calculated. The type Ⅱ phase matching at (θ, φ) = 73.13°, 0°) gave the largest value of the effective second-order nonlinear coefficient, $d_eef$=39 pm/V$. The full coefficients of the second-order nonlinear susceptibility tensor were measured by using the Maker fringe method and $d_31$ component was determined for the first time. The second harmonic generations of type Ⅰ and type Ⅱ collinear phase matchings were exploited by utilizing the birefringence of the crystal. Maximum SHG conversion efficiencies of 2.5 % for type Ⅰ and 13% for type Ⅱ were obtained at 1064nm. Nonlinear refractive indices and absorption coefficients were measured by using Z-scan technique at 532 nm and 1064nm for the polarizations parallel to the crystallographic b- and c-axes. The sign of the second-order nonlinear refractive index was found to be positive and self-focusing was observed as a consequence. The nonlinear refractive index values showed highly anisotropic behaviour along the b- and c-axes, which is caused by the delocalized π-electrons distributed mainly along the charge transfer c-axis. Large nonlinear phase change due to the cascaded second-order nonlinear process was obtained for the detuning angle of ±0.1° from the type Ⅱ phase-matching angle (θ, φ) = (73.13°, 0°). From the observed nonlinear phase change the effective nonlinear refractive index $n_2^eff$ was calculated to be $1.7×10^{-12}㎠/W$, which is the largest among the reported values. The observed large phase shift was compared with the value obtained by computer simulation and was discussed in association with the reported value of KTP.
