REFRACTIVE-INDEXES OF THE OPTICALLY NONLINEAR ORGANIC-CRYSTAL -3-METHYL-5-NITRO-N-(1-PHENYLETHYL)-2-PYRIDINAMINE

The refractive indices of structurally well-characterized crystals of the new optically nonlinear organic crystal (S)-3-methyl-5-nitro-N-( I -phenylethyl)-2-pyridinamine have been measured from 480 to 660 nm and fitted to four-parameter Sellmeier equations with an accuracy of +/-3 x 10(-3) to +/-5 x 10(-3). The value of n(y) was accurately verified by the measurement of the external angle of incidence required, at the (101) plane, to observe type I phase matching with a fundamental wave length of 1064 nm. The phase matching locus has been established and i s of Hobden class eight. The values of the n(x), n(y), and n(z) refrac tive indices at 532 nm were found from the Sellmeier equations to be 1 .690, 1.901, and 1.725, respectively, and their relative magnitudes ar e correlated with the molecular arrangement in the crystal lattice. Th is material is a member of a series of pyridinamines that can exhibit the phenomenon of frequency-dependent orientation of their dielectric axes. Since this compound crystallizes in an orthorhombic structure, r otation of the dielectric axes is precluded for symmetry reasons.