<title>Linear and nonlinear optical properties of the organic nonlinear material 4-nitro-4'-methylbenzylidene aniline (NMBA)</title>

Abstract

The organic crystal 4-nitro-4'-methylbenzylidene aniline was identified as a promising nonlinear material by the powder technique. The material gave a second harmonic intensity 16 times that of urea. Large single crystals of dimensions 5 X 3 X 1 cm<SUP>3</SUP> were grown by temperature lowering a supersaturated ethyl acetate solution. An orthoscopic examination revealed the positions of the principal dielectric axes. The refractive index dispersions were determined to an accuracy of +/- 0.0015 using the minimum deviation technique and Maker fringe spacings. These dispersion curves were fitted with a Sellmeier equation which allowed the indices to be determined to +/- 0.0006. The nonlinear d- coefficients d<SUB>11</SUB>, d<SUB>33</SUB>, d<SUB>31</SUB> and d<SUB>13</SUB> were evaluated at 1000, 1064 and 1300 nm using the Maker fringe technique. The coefficient d<SUB>11</SUB> was over 200 times larger than KDP d<SUB>36</SUB>. In addition, the non-diagonal coefficient d<SUB>31</SUB> was comparable to the phase- matching coefficient in the organic material DAN. Critically phase-matched second harmonic signals were observed at all fundamental wavelengths. There was excellent agreement between the experimentally determined and theoretical phase-matched incidence angles. This agreement is presented. Noncritical phase-matched conditions have been calculated and are reported. Both angle and wavelength noncritical phase-matching is possible with this crystal.