Characterizing the Distribution of Space Charge in Poled Polymer Films
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voltage in the film. Such experiments are difficult to implement and to interpret - a functional form for the field profile must be assumed, and its predictions compared with the data. DC electrochromism has been used to estimate the molecular alignment and the electric field in a poled film. Page et al.9 used dc electrochromism as a diagnostic for the corona poling of a polymer film doped with nonlinear optical chromophores. However dc electrochromism, if not used in conjunction with an evanescent wave technique for depth- profiling, can only give an average field over the thickness of the sample. Furthermore, if the field is inhomogeneous, the assumptions used to separately calculate the field and alignment are invalid. AC electrochromism and the related technique, electroreflection, are easier to perform than TIRFS and have several advantages over dc electrochromism. Average fields, surface fields, and field nonuniformities can be measured or inferred from the synchronously detected absorbed or reflected signals. 10 The technique is more sensitive than dc electrochromism since information is contained in the 595 Mat. Res. Soc. Symp. Proc. Vol. 328. ©1994 Materials Research Society
amplitudes of the signals as well as in spectral shifts. Additional information is available from
the variation of the signal with the frequency of the applied field. Because electroreflection is a sensitive measure of the dc field it is anticipated that it will provide a valuable diagnostic for contact-poled films. In this work the electroreflection technique is described and used to make preliminary determinations of field uniformity in a series of doped polymethyl methacrylate
films. This is the first attempt to determine the electric field characteristics of poled polymer films for second order nonlinear optical applications using ac electrochromism. AC ELECTROCHROMISM AND ELECTROREFLECTION
In the band-shift approximation, where the effect of an electric field on an electronic transition is assumed to be a shift in the envelope of the transition in frequency, the change in the extinction coefficient with applied field for a perfectly oriented sample is:11 AE-K =- h(p I ."6)E(AIL.F)dS(v) _ - 1 -(p Kvh dv
- 6)2 (A oc:FEE dS(v) +
-
dv
Ih (p . 6)2 (AUd,_E)2d2SvE dv2
Sh2
Here AI is the change in dipole moment between the ground and excited states, Aa is the corresponding change in polarizability, F is the applied field, K is a constant, e is the polarization vector of the absorbed light, p is the transition moment, and S(v) is the lineshape function. In a rigid isotropic sample, where the permanent dipoles are randomly oriented and unable to realign with the field, the term linear in the field vanishes, and following orientational averaging, one obtains the familiar expression:
IB
+ 3-0hC d-2
A2
(2)
where
6)2 AL: FF> B=45 n1-2-
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