Stability and Performance Issues for Nonlinear Optical Chromophores, Polymers and Devices
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ABSTRACT In the course of development of any new technology the issues of device stability and reliability are inevitably encountered and must be summarily dealt with. The development of organic and polymeric nonlinear optical media has been no exception and has now evolved to a stage wherein solutions to a few performance issues are of crucial importance. Discussions here focus on just two of these critical problems. First, we describe a preliminary study intended to identify some of the variables involved in optical damage. The goal here is to identify suitable photochemically stable chromophores and polymers and to elucidate any processing and environmental influences which contribute to damage. Second, the thermal stability of nonlinear optical chromophores has already attracted much attention and significant progress has been made in enhancing the thermal stability of many systems. Our most recent effort has concentrated on chromophores with a variety of different configurations and combinations of nitrile acceptor groups, some of which possess both large nonlinearities and excellent thermal stability. Some newer aspects of a thermal stability screening protocol which are particularly useful for these thermally stable chromophores and polymers are also described. INTRODUCTION Researchers have grappled with a host of issues pertinent to the optimization of organic nonlinear materials. 1 "4 There is nothing more effective to obviate the shortcomings of a NLO chromophore or polymer than to attempt to implement these materials into a functional device ... both the obvious and unanticipated deficiencies immediately manifest themselves. A brief review of some important prior work motivated by these device demands on optical and thermal stability is in order. The extent of indifference to optical damage of electrooptical polymer NLO waveguides is really rather remarkable given the scope of work on other aspects of these systems. This oversight is also in contrast to the numerous studies of damage in single crystal (usually inorganic) systems. The solitary reported studies of optical damage in poled polymer nonlinear optical waveguides appear to be from the group at Hoechst-Celenese. In an initial study inverted rib waveguides composed of a methacrylate copolymer containing an aminonitrostilbene chromophore were fabri15 Mat. Res. Soc. Symp. Proc. Vol. 392 * 1995 Materials Research Society
cated and their optical power handling characteristics at near-IR wavelengths were characterized. It was found that the optical transmission of the guides decayed within hours as they were exposed to light and the rate of degradation was markedly enhanced in the presence of oxygen. 5 In a subsequent study a waveguide containing an alkoxynitrobiphenyl derivative showed little degradation in contrast to the earlier results with the aminonitrostilbene. 6 The implication here is that damage may be correlated to chemical structure and hence possibly amenable to control by structure modifications guided by some knowledge of the chemical
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