Flowing Afterglow Synthesis of Polymer Films with Nonlinear Optical Properties
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PETER HAALAND AND HAO JIANG Wright Laboratory Materials Directorate, WL/MLPJ, WPAFB, OH 45433
ABSTRACT Extensively conjugated polymer films with excellent morphology, homogeneity, robustness, and nonlinear optical coefficients have been synthesized in the flowing afterglow of a lowpressure argon plasma. Polymers of benzene, thiophene, furan, and napthalene are among the systems studied. The polymers, which grow by accretion of positive ions through a low-voltage sheath, have bulk and surface textures which scale systematically with film thickness. The hopes and perils of employing these new materials as active elements in gradient-index reflectors and optical switches will be described, as will the link between morphology and deposition mechanism. INTRODUCTION Polymer films with substantial optical nonlinearities have shown promise for optical computing, including very recent reports of teraflop parallel autocorrelation calculations. 1 Optical bistability may result whenever elements of an optical path have significant third order nonlinear optical susceptibilities, X(3). Electronic nonlinearities, such as are found in many conjugated molecules, can have sub-picosecond risetimes 2, making them useful for fast optical logic elements. The basis for such logic elements is the intensity dependent refractive index n2,
which induces a phase shift 3: A0=n,I(2irFl/,)
(1)
and thus a change in the optical admittance with light intensity. In this expression 1, F, 1, and A are the intensity, finesse, interaction length, and wavelength, respectively. Several optical designs which exploit this phase shift are reviewed by Tutt and Bogess. 4 The nonlinear effect can have refractive (+/- Re(n 2)) and absorptive (mIr(n2)) components. A simple optical limiter based on a gradient index mirror with X(3) elements might, for example, induce a transient shift in reflectance spectrum beyond the pass band of a detector. (Figure 1). Whether contemplated for use in optical logic or intensity dependent reflectors, materials for optical limiting must have uniform morphology to avoid light scattering. Low linear absorption to prevent thermal damage at intermediate intensities is also desirable. A large nonlinear susceptibility (X(3)) is necessary, but not sufficient, for building a useful optical limiter.
"123 Mat. Res. Soc. Symp. Proc. Vol. 374 01995 Materials Research Society
Detector
R
f
low I
lambda
R
n
high I
lambda Figure 1: Schematic illustration of an optical limiter employing X(3) elements. Variation of n(x) for a gradient-index reflector shifts the reflectance band beyond the pass-band of the detector. Nonlinear spectroscopy of conjugated polymers has been reported on many materials, from semiconductors 5 to fullerenes 6 to india ink.7 There is a plausible mechanistic basis for expecting large (X(3)) in conjugated polymers. Polyacetylene is prototypical, though extensive work has also been reported on polymers of aromatic ring compounds. Conventional syntheses based on Grignard coupling of aromatic halides using a metal
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