Real Time Probes of Dye Doped Polymer Systems
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REAL TIME PROBES OF DYE DOPED POLYMER SYSTEMS
J.B. HALPERN, P. AMUZU-WILLIAMS AND L. HOUSE. Department of Chemistry & Materials Research Center of Excellence, Howard University, Washington, DC 20059. L.A. LEE, Lester A. Lee Associates, Ft. Washington, MD. INTRODUCTION Recently, advanced photonic devices have been fabricated in the laboratory and are becoming commercially available. Thus, there is considerable interest in inexpensive but efficient non-linear optical (NLO) materials that are simple to make and work with. In the last three years a large number of publications and patents have appeared describing NLO properties of organic materials, usually dyes, incorporated into or synthetically attached to polymers [1]. Such materials must be oriented before they have second-order NLO activity. Two methods have been used. In one, contact poling [2-5], two electrodes are formed on or in the material and an electric field is placed between them. In the other, corona poling, a discharge deposits charge on the polymer, which creates a strong orienting field [6-8]. One could generalize that contact poling is (more) difficult to do, but the results are easy to understand, while corona poling is simple to do, but the results are (more) difficult to understand. This paper describes a set of corona poling experiments. While most investigators have worked hard to discover polymer-dye systems which will retain their orientation over a long period of time, the experiments described here use a systems which relaxes quickly in order to increase the rate at which data can be taken. Many of the experiments which will be described below have characteristic decay times, and one should be able to scale the results to stiffer systems with longer decay times. Guest-host systems often have shorter relaxation times than side chain polymers or organic glasses. We looked at thin films of 2 methyl-4-nitroaniline (2MNA) in polymethyl methacrylate (PMMA) [9], where the free volume available to the dye molecule is considerable, especially if the film is not completely baked to remove all of the excess solvent after spin coating.
EXPERIMENTAL Corona poling was done at room temperature. Films were coated from saturated solutions of 2MNA in PMMA dissolved in methyl isobutylketone onto glass plates with grounded transparent conducting films. Simultaneous (0.1 second interval) measurements were made of second harmonic generation (SHG) at 1064 nm, the electrostatic voltage (ESV) on the film,
Mat. Res. Soc. Symp. Proc. Vol. 228.,c 1992 Materials Research Society
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laser pulse energy, corona voltage and corona current. The Trek high voltage power supply used for exciting the corona discharge was bipolar and could be operated as a voltage or transconductance amplifier, driven by a function generator to provide controlled time varying poling voltages or currents. The electrostatic voltmeter was a Trek Model 344. The sensing head is a 5 mm cube with a small hole in one of the faces. The average voltage'is measured on any surface placed in front of
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