that prove the viability of polymers in this application. The necessary switches, amplifiers, and modulators can be made today with inorganic materials, but there is some question whether these realizations can be combined and manufactured in large volume at sufficiently low cost. Organic thin film technologies may fit the economic as well as the technological requirements, but many advances will be required and the outcome is uncertain.

A nonlinear polymer in general has two components: the polymer itself and an optically nonlinear molecule (a chromophore) that is either chemically attached to the polymer or dissolved in it. In order for the polymer-chromophore system to be optically nonlinear, the chromophores must be aligned such that on average they are all pointing in the same direction within the polymer matrix. This alignment is accomplished through a process called poling. The polymer is poled by cooling it through the glass transition temperature while it is in a very strong electric field, and the order induced by the field is frozen in.

Poled polymeric systems have process and property advantages over their inorganic crystalline competitors. The polymers can be formed into thin films and lithographically patterned, and they can be chemically modified to tailor and improve bulk properties. There are disadvantages as well, in that the orientation in the poled polymer systems tends to decay with time, a problem that can probably be overcome.

Polymeric Light-emitting Diodes

Recently, light-emitting diodes (LEDs) based on conducting polymers have been achieved in a number of laboratories around the world. The active element is a thin film structure based on a modified poly(phenylene vinylene) (PPV), with a metal film as the electron injector and polyaniline as the hole injector. Various colors have been demonstrated, and the operating characteristics are competitive with inorganic LEDs. Highly flexible devices have been fabricated supported on a poly(ethylene terephthalate) base. The possibility of making large-area displays exists.

Much research and development remains to be done. For example, low-work-function metals are required, and they are difficult to passivate. However, the simplicity of fabrication of the laboratory devices, involving spin casting from solution, is promising if the problem of limited device lifetime can be solved.

Polymers for Electrophotography

One of the major applications of polymers with tailored electronic and optical properties has been in electrophotography for copier, duplicator, and printer applications. In this application an electroactive polymer is used as one component of the light-sensitive element used for creating the latent electrostatic image

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