Characterizing the NLO Chromophore Orientation of Polymeric Film by Electroabsorption Spectroscopy
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The dispersion of third-order nonlinear coefficients
(3)
(3) o he
ifrn
X113 3 and X3333 of three different
NLO(nonlinear optical) polymer films were determined by electroabsorption spectroscopy. The first material investigated is an epoxy-based polymer BP-2A-NT, with azobenzene NLO chromophore 4-[((4-nitrophenyl)(azo)phenyl)azo)]aniline in its side chain. The other materials are two polydiacetylenes, poly(BPOD) and poly(4-BCMU), in which the(3) delocalized polymer chains contribute to the third-order nonlinearity. The complex spectrum of X333 3 of each material is very similar in shape to corresponding X3(3)spectrum. The ratio of X 33) to X (3) is 3.2 for BP-2ANT, 1.5 for both poly(BPOD) and poly(4-BCMU). These ratios indicate that the distribution of the side-chain NLO chromophores of BP-2A-NT is very close to three-dimensional isotropy, and the distribution of the main-chain chromophores of poly(BPOD) and poly(4-BCMU) is concentrated on the film plane. I. Introduction Nonlinear optical (NLO) polymers have received a great deal of attention as promising electro-optic materials over the past decade[I-3]. Various of NLO polymers have been developed as candidate materials for application in high frequency optical modulators and integrated semiconductor-NLO polymer circuits[4-7]. Among these applications, not only the nonlinearity of the material, but also the orientation of the NLO chromophore is of great concern, since a practical application needs to employ a particular or a combination of NLO tensor elements which are sensitive to the orientation of the NLO chromophores.
Electroabsorption spectroscopy can determine the dispersion of each X(3) component and can provide information about chromophore orientation in the material[8-10]. The normal incidence of electroabsorption can determine the imaginary part of the change of refractive index •Kwhich is due to the applied electric field and is associated with the component X,(33 By using Kramers-Kronig relation , the(3) real part of the change of the refractive index 8n can be determined. The complex value of X113 3 is subsequently determined. Similarly, electroabsorption an (3) (3),(3) at tilted incidence can determine the complex value X,(3), which is composed of 1133 and 3333 (3)3 Tu (3) Thus 3333 can be uniquely determined from these measurements. The obtained ratio of X333 to which can be can be utilized to offer information about the arrangement of the chromophores XH(3)1133 (3) (3)islsthn3 3 treated as a microscopic one-dimensional rod like unit. If the ratio of X(333 to x11 3 is less than 3, the chromophores are inclined to lie along the film plane. If the ratio of X(3)3
to X(33
the chromophores are isotropically oriented in three-dimensional space. If the ratio of (3)3
equals 3, 3
X• 33
to
is larger than 3, the chromophores are inclined to maintain perpendicular to the film plane. In this paper, we apply this technique to two polydiacetylenes with main-chain chromophore[ 11], and an epoxy-based polymer BP-2A-NT[12], with side-chain chromophore, to
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