Chapter 21
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Stark Spectroscopy as a Tool for the Characterization of Poled Polymers for Nonlinear Optics 1
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M . I. Barnik , L. M . Blinov , T. Weyrauch , S. P. Palto , A. A. Tevosov , and W. Haase 3
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Organic Intermediates and Dyes Institute, 103787, B. Sadovaya 1-4 Moscow, Russia Institute of Crystallography, Russian Academy of Science, 117333, Leninsky pr. 59, Moscow, Russia Technische Hochschule Darmstadt, Institut für Physikalische Chemie, Peterenstrasse 20, 64287 Darmstadt, Germany 2
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The Stark spectroscopy (electroabsorption) technique was developed for the case of a cylindrically symmetric polar molecular ensemble. The quadratic-in-field electroabsorpton was used to evaluate the difference of the dipole moments of the ground and excited state and subsequently the linear-in-field electroabsorption was applied to study the polar order parameter of the chromophores induced by electricfieldsin thermal poling processes as well as by photoassisted poling at room temperature. The polar order has been measured for several systems including guest-host polymer-dye solutions and side chain polymers with chromophore pending groups. With the polar order parameter measured in a corona poled side chain polymer using the linear Stark effect, the second order nonlinear optical susceptibility has been calculated, which agrees with the same value measured directly by the second harmonic generation (SHG) technique. It is shown, that the quadratic-in-field effect allows one to distinguish between soft and rigid polymeric matrices.
Due to their low cost, easy processibility and high performance characteristics polymers possessing nonlinear optical properties are of current scientific and technological interest (7). In certain cases, a glassy polymer may have a polar structure, e. g., when a polymeric ferroelectric liquid crystal is cooled downfromthe melt. Amorphous polymers may be prepared in the form of nonequilibrium but long living electrets when they are cooled downfromthe melt with a d.c. electric field applied (2,5). Such nonlinear optical materials may possess a rather high second order susceptibility and exhibit larger electro-optic coefficients than e. g. lithium niobate. Poled (polar) polymers containing nonlinear optical chromophores aligned by 4
Corresponding author 0O97-6156/95/0601-O288$lZ00/0 © 1995 American Chemical Society In Polymers for Second-Order Nonlinear Optics; Lindsay, G., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1995.
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BARNIK ET AL.
Stark Spectroscopy To Characterize Poled Polymers
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an electric d.c. field are of great interest as media generating the optical second harmonic (4-6). The main advantage of such materials is that they can be easily prepared in the form of thinfilmsof large area and good optical quality. There are many problems relevant both to the physical mechanism and to the optimization of the poling process still to be solved. Among them of great importance is the problem of a quantitative description of the degree of the polar order and its relaxation with time for the chromophores responsible for the nonlinear properties of a material irrespective of the type of poling used. The measurement of the macroscopic polarization (the dipole moment of a unit volume) using the thermally stimulated depolarization current technique is strongly influenced by any charges trapped in the sample and is, in principle, a destructive technique since it requires heating of the sample. The intensity of the second harmonic generation (SHG) itself depends on many factors and needs a special calibration. With an electrooptic technique (e. g. the Pockels effect used in (7,8)) thefieldinduced birefringence is measured, which is only indirectly related to the polar order parameter. In addition, a special geometry of the sample is necessary for electrooptic measurements (a MachZehnder interferometer (9), reflecting prisms (7), etc.). The Stark spectroscopy, i. e. the measurement of thefieldinduced changes in the absorption spectrum (electroabsorption), allows the determination of the polar order parameters of dipolar chromophores. Its application to Langmuir-Blodgett films (10,11) has already been reported. Some preliminary results on Stark spectroscopy application to electrically poled polymers have also been published (12). The idea is to measurefirstthe magnitude of the dipole moment difference Apt for a relevant dye on an isotropic polymer solutionfromthe quadratic-in-field Stark effect and then to measure polar order parameter for the same polymer poled in a d.c. electricfieldfrom the linear-in-field Stark effect characteristic of noncentrosymmetric (polar) media. The paper is organized as follows: First a theory for the quadratic- and-linear-infield electroabsorption is presented for arigidand nonrigid molecular ensembles including polar ones. Then the simpliest case of two model isotropic solid solutions (guest-host systems) of the same dye in polycarbonate (PC,rigidensemble) and polymethylmethacrylate) (PMMA, nonrigid ensemble) are experimentally studied. After this, two other model systems representing polar molecular ensembles are studied: A dye solution in PMMA poledfromelectrodes and a side chain polymer poled by the corona discharge, both on coolingfromthe melt. The value of the polar order parameter for the corona poled sample is used for the calculation of %() to be compared with the same value measured in a SHG experiment. Finally, an example of the polar structure prepared by photoassisted poling is discussed. The latter was suggested originally in (7,8). 2
Quadratic- and Linear-In-Field Electroabsorption The geometry of the Stark spectroscopy experiments is shown in Figure 1. A polymer film is sandwiched between two optically transparent electrodes (shaded areas), the direction of the poling and the measuringfieldE coincides with the wave vector of light (the z-axis). The light polarization vector e is always located in the x,j>-plane. For rod-like molecules of dyes the direction of the dipole moment of the ground state of the chromophore [i usually coincides with its long molecular axis and the transition dipole moment \i of the longest wavelength absorption band. In typical cases (e. g., for azo- or stilbene chromophores) the direction of the dipole moment of the excited g
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In Polymers for Second-Order Nonlinear Optics; Lindsay, G., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1995.
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POLYMERS FOR SECOND-ORDER NONLINEAR OPTICS
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Figure 1. The geometry of a sandwich cell (E: d.c. electric field; e\ arbitrarily chosen electric vector of nonpolarized light; p: chromophore dipole moments) and the sketch of the set-up for electroabsorption measurements (M: monochromator; D: photomultiplier; V: d.c. voltmeter; LA: lock-in amplifier; FG: function generator; C: computer).
state \i for the transition mentioned also coincides with the longitudinal axis. Thus all of the vectors \i , Pg, \x and Ap=p p are collinear (direction p in the Figure). Let us consider first an orientationally isotropic ensemble consisting of molecules with only two energy levels (i), corresponding to the ground and an electronic excited states. The two molecular states are characterized by their own dipole moments and polarizabilities. The corresponding energy levels of a moleculefixedrigidly in space are shifted in an electricfieldE by e
e
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