Organic Materials for Nonlinear Optics
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the "blue" for use in inexpensive printers u s i n g older x e r o g r a p h i c photoreceptor technology. Known crystals were insufficiently nonlinear, unless used in waveguides where the high intensity levels maintained over long propagation distances could increase conversion. LiNb03:Ti waveguides were the most appropriate and advanced for this application, but suffered optical damage at short wavelengths and were not reliably manufacturable. A spatial-light-modulator image-bar could also be used in Xerographie printers. A row of imaged pixels would eliminate the need for a füll Scanner. Again LiNb0 3 was the leading material, having a usefully large electrooptic r-coefficient and being commercially available. However, achieving intimate contact between electrodes on the driver and the electro-optic crystal was difficult. Since the dielectric constant of LiNb0 3 is large, the electrodes' fringing fields are partially repelled into regions of lower e, reducing moduiation efficiency and increasing nonuniformity of pixels. It seemed that larger nonlinearities, new materials, and more fabricationflexibilitywere needed. Since one would expect improvements in these directions to foster new device concepts, a program to characterize, understand and develop organic NLO materials was begun. Pioneering works had shown a high potential for organics due to the large, nonresonant, nonlinear polarizabilities which aecompany certain molecular structures.1'3"6,8"10 Research and (longterm) technological opportunities have been recognized by many, evidenced by numerous academic, industrial, and government sponsored research programs which have since come into being. A program in organic NLO materials
could be logically supported1,3"6'8"10 by the following points: 1. "Nonlinear activity" can be large in delocalized pi-electron Systems; 2. Molecular strueture-property relationships are partly known, can be predicted through quantum-mechanical calculations and modeis, and can be tested through straightforward experimentation with fluids; 3. Many highly active molecules can be obtained, and even tailored, because of the maturity of chemical science; 4. A p p r o x i m a t e m i c r o s c o p i c macroscopic relationships exist which allow a description of nonlinear polarization in molecular terms; 5. Various molecular assemblies may be created, potentially with properties to meet the requirements of specific application; 6. Processing of organic materials is very diverse and flexible, and can often be done inexpensively and routinely; 7. Many secondary properties are advantageous in applications (e.g., low refractive indices and dielectric constants). More recently, with increased interest in nonlinear refractive index effects, a broader interest in resonant processes of organic materials has developed. 56 Research in this area is very different, verging on nonlinear spectroscopy and photochemistry. Responses are highly dispersive with details depending on properties of specific excited states and their evolutionary dynamics. The latt
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