Efficient Second Harmonic Generation in Hydrazone, Derivatives of Substituted Aromatic Aldehydes
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EFFICIENT SECOND HARMONIC GENERATION IN HYDRAZONE, DERIVATIVES OF SUBSTITUTED AROMATIC ALDEHYDES RICHARD S. POTEMBER, ROBERT C. HOFFMAN, AND KAREN A. STETYICK The Johns Hopkins University, Applied Physics, Johns Hopkins Road, Laurel, MD 20723-6099 ABSTRACT Hydrazone, 1,1-dimethylhydrazone, methylhydrazone, phenyihydrazone and p-nitrophenyl hydrazone derivatives of substituted aromatic aldehydes were prepared and screened for second harmonic generation using the Kurtz powder technique. One compound, 4-nitro-3-methoxybenzaldehyde hydrazone exhibited a second harmonic signal up to 32 times that of ammonium dihydrogen phosphate (ADP) and 4-nitrobenzaldehyde hydrazone exhibited a second harmonic signal five times higher than previously reported, up to 40 times that of ADP. 3-methyl-4nitrobenzaldehyde hydrazone, 4-nitrobenzaldehyde phenylhydrazone, 1naphthaldehyde phenyihydrazone, 1-pyrenecarboxaldehyde phenyihydrazone exhibited second harmonic signals 25, 2.5, 5 and 20 times that of an ADP standard. INTRODUCTION Second harmonic generation in organic materials, such as MNA (2methyl-4-nitroaniline), has been studied for many years [1]. Second harmonic generation in those materials arises from the large nonlinear polarizabilty of the x-bond structure and the asymmetrical charge distribution associated with donors and acceptors on the molecule. The polarizability for any substance is P =cOX1E +EOX2E 2 + OZ2E + ...
(1)
where eoXIE is the linear polarizability term associated with linear refraction. The second term is the second-order nonlinear polarizability responsible for second harmonic generation in anisotropic crystals, a term which vanishes in isotropic materials. The third-order nonlinear polarizability is responsible for four-wave mixing effects, photorefraction, and optical bistability. P is the macroscopic polarization, co is the permittivity of free space, E is the electric field, and X is the dielectric suscepibility. For second harmonic generation, Xis a third rank tensor of the form xijk(2(o). The second-order nonlinear term then takes the form Pi(2o•) = roXijk(2o)Ej(2w)Ek(2o)
(2)
ij,k = 1,2,3. The numerical values of the tensor coefficients are usually represented by the 2 symbol d [2] and are functions of frequency and temperature. The units of Xijk( 0) are different from that of the first order linear susceptibility term Xij(wO). The first order term is a dimensionless ratio, whereas the second order susceptibility tensor has units of inverse electric field, or m/V in the inks system [2]. Organic molecules have the advantage of being amenable to molecular engineering. For example, 2-methyl-4-nitroaniline (MNA) consists of a nitro acceptor group and an amino donor group separated by a conjugated ring system. This molecule is very polarizable, has a high degree of charge transfer and thus is an excellent material for second order harmonic generation [1].
Mat. Res. Soc. Symp. Proc. Vol. 173. 01990 Materials Research Society
698
It has been shown that the second harmonic generation effect observed in org
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