Electro-Optic and Nonlinear Optical Properties of 2, 6-Dibromo-N-Methyl-4-Nitroaniline (Dbnmna) Crystals
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ELECTRO-OPTIC AND NONLINEAR OPTICAL PROPERTIES OF 2,6-DIBRCOt4-N-METHYL-4-NITROANILINE (DBNMNA) CRYSTALS
AJAY NAHATA, KEITH A. HORN, AND JAMES T. YARDLEY Allied-Signal, Inc., Engineered Materials Sector Columbia Turnpike, Morristown, New Jersey 07962
ABSTRACT We report a new organic nonlinear optical crystal, 2,6-dibromo-N-methyl-4nitroaniline (DBNMNA). The refractive indices of DBNMNA were measured at several wavelengths between 500 and 1100 rnm and fit to a single oscillator Sellmeier equation. Second harmonic Maker fringe data yielded d 3 3 values through comparison with y-cut crystalline quartz. We also report measurements of the electro-optic coefficients, (na rl 3 -nc r 3 3 ), r 4 2 , and r 5 l at several wavelengths. We find good agreement between the observed dispersion and that predicted by the simple two level dispersion model. However, the second order susceptibility is not well described by a single tensor component hyperpolarizability.
INTRODUCTION The intrinsically large second order nonlinearities and low dielectric constants associated with organic materials have generated significant interest in their use for parametric frequency generation and high speed electro-optic devices [1]. In this work, we present the structure, refractive indices, nonlinear optical properties and linear and quadratic electro-optic properties of single crystal DBNMNA, a new organic nonlinear crystal. The electro-optic coefficients are modeled by a fit to the two level dispersion equation,
from which a non-resonant X(2) is
obtained.
CRYSTAL GRCOTH AND STRUCTURE DBNMNA was prepared by the reaction of bromine with N,N-dimethyl-4-nitroaniline methanol at reflux. Clear yellow rhombohedral b-platelet single crystals were grown with typical dimensions of 2 x 2 x 0.2 mm3. Single crystal X-ray diffraction showed that DBNMNA belongs to the orthorhcmbic space group Fdd2, point group mam2, with unit cell parameters of a=11.745(l) A, b-29.640(2) A, c=10.807(2) A, and Z=16. Powder X-ray diffraction patterns showed that crystals grown from acetonitrile, ethanol, and methylene chloride all belong to the same space group. The unit cell contains four identical groups of four molecules defined by C2 v symmetry, each related to the other by simple translations. The approximate charge transfer axis, amino to nitro, of each molecule is 71" off the c-axis in the a-c plane, yielding a net polar vector along the c-axis. Figure 1 shows an ORTEP plot and a projection along the crystallographic a-axis. Further details of the crystal structure are given in Reference (2]. in
ABSORPTION AND REFRACTIVE INDEX MEASUREMENTS The optical absorption spectrum of single crystal DBNMNAwas measured from 200 nim to 2500 rnm. The absorption spectrum was recorded at room temperature using unpolarized light propagating along the b-axis. These data indicate that the title compound is essentially transparent from 500 nm to 1500 rnm with weak infrared overtone bands occurring in the near infrared. Using a thin polycrystalline sample, the peak of the charge transfe
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