Published work on organic materials for photonic device applications has tended to emphasize enhancements in nonlinear optical properties. Yet the linear properties of these materials are just as critical for determination of whether working devices are feasible. Simple calculations are presented to demonstrate the interplay between index of refraction, optical loss, and nonlinearity in device design. The implications of the result are discussed.
Although photorefractive materials are those that exhibit both appreciable electro-optic and photoconductive behavior, the chromophore parameters that lead to large electro-optic coefficients have not so far led to similar photorefractive performance. Recently, it has been shown that the electro- optic contribution is one of two principal contributions to the overall photorefractive behavior, and that it is usually the smaller of the two. The larger factor, first referred to in the literature as an 'orientational enhancement,' is due to the birefringence of the chromophore: the difference in polarizability parallel to the dipole moment and perpendicular to it. A figure of merit incorporating both effects has been derived, although its determination is more difficult than that for purely electro-optic materials. The polarizability anisotropy is a function of all three components of the diagonalized polarizability tensor. Two expressions for these three parameters can be obtained by index of refraction and light scattering measurements on chromophores in solution. The third must be estimated. Results from this new characterization protocol are presented and compared with the results of photorefractive experiments on materials containing the tested chromophores. Another class of compounds has also been found to be of potential use in holographic storage, in this case write- once applications. These chromophores undergo an irreversible photochromic reaction that is triplet sensitized. A mechanism is proposed for this reaction and supporting data provided.
Materials to be used for optoelectronic, photorefractive, or frequency doubling applications must have high nonlinearities, good thermal stabilities, and low optical loss (high transparency). Organic compounds synthesized for incorporation into poled nonlinear polymers typically exhibit tradeoffs between nonlinearity and each of the other two qualities. By judicious use of arylamino donor groups and cyano-containing acceptor groups, a small set of chromophores that are both highly nonlinear and stable at high temperatures has been prepared. By selecting delocalized bridging moieties that are either tuned for optimum hyperpolarizability or exhibit two charge-transfer excited states, highly transparent chromopohores with excellent nonlinearities can be prepared. The results suggest that thermal stability and nonlinearity are jointly achieved by modifying donor and acceptor groups, while transparency and nonlinearity are jointly achieved by modifying bridging groups.
Two experimental studies of new photorefractive polymer materials are described. First, an examination of the fabrication, stability, and photorefractive behavior of a recently-reported high-efficiency polymeric composite 33%PVK:50%DMNPAA:16%ECZ:1%TNF (K. Merrholz et at., Nature 371, 497) shows that this material is metastable, crystallizing on time scales from hours to days depending upon preparation. The grating phase shift also varies considerably from sample to sample. In the second part of this paper, first measurements and analysis of the properties of a new photorefractive polymer composite which contains a liquid nonlinear optical chromophore are presented. This new composite, which is immune to crystallization even at high chromophore loading levels, shows a useful diffraction efficiency of several percent at high fields, but the index of refraction grating is shown to consist of two components; a smaller photorefractive index grating and a grating which is not photorefractive in origin as shown by grating translation measurements.
Recently, substantial progress has been made in molecular design of nonlinear optical chromophores toward the goal of large nonlinearities while retaining thermal stability. Modifications of donor groups, delocalized (pi) -systems, and acceptor groups have all been made, and examples of each are described. The utility of each modification for preparation of electro-optic polymers is discussed. Resistance to thermal decomposition is not sufficient for practical applications; NLO polymers must also be resistant to thermal depoling and to sublimation of chromophores out of the polymers. It is therefore concluded that chemical tethering is required. Synthetic strategies for tethering our better chromophores to thermally stable polyimides are described, and the results of materials prepared to date are given.
Among the many physical properties of organic nonlinear optical (NLO) chromophores that require optimization, their generally inadequate thermal stability has recently attracted significant attention. Here we describe protocols for the evaluation of the thermal stability of chromophores that may be employed prior to their incorporation into a device environment. These protocols involve a variety of conventional analytical techniques including thermal analysis (TGA, DSC) and electrochemistry (CV) that have led to the identification of the most facile thermal decomposition process of the widely utilized family of 4-amino-4'- nitroazobenzene NLO dyes. Knowledge of the decomposition mechanism has facilitated implementation of structural modifications in the amine donor of these dyes that deliver enhanced thermal stability. It is anticipated that these protocols and structure modifications are of general relevance.
Due to the requirements for thermal stability in the fabrication and operation of electro-optic devices, the quest for suitable NLO chromophores and polymers involves more than the search for highly nonlinear materials. We describe here the preparation and characterization of aryl substituted azole derivatives as a class of thermally stable nonlinear chromophores and their incorporation into high temperature thermoplastics as a host-guest array. Criteria for predicting the thermal stability of NLO chromophores are discussed as are the polar order decay properties of various poled polymer systems as a function of temperature.
Experimental measurements of molecular hyperpolarizabilities can be useful in two ways. First, they allow comparison between different chromophores, so that structure-function relationships can be understood and increasingly nonlinear compounds synthesized. This application requires only good relative measurements. The second and ultimately more important application is prediction of the macroscopic nonlinear optical properties of custom materials. The latter application requires accurate absolute values for the molecular parameters. Examples of both types of comparisons are discussed below. We describe experimental measurements on several types of nonlinear optical chromophores, and the choice of conventions and reference standards that leads to accurate predictions of electro- optic coefficients in poled polymers. The degree to which nonlinearity and thermal stability are correlated is also discussed.
The realization of practical electro-optical devices based on the poled polymer waveguide approach is contingent on the simultaneous solution of a number of outstanding materials and fabrication issues. We have prepared and studied the molecular properties of a number of heterocycle containing chromophores with structures derived from high temperature polymer materials and further evaluated many of them as guests in polyimide polymer hosts. Correlations between the structure of these chromophores and their physical properties including optical nonlinearity, absorption, dipole moment and thermal stability have been made. Furthermore, a correlation between thermal stability and oxidation potential for some of these heterocyclic chromophores has been found and extension of this correlation to other types of nonlinear chromophores is suggested.
Several classes of organic compounds have been synthesized at IBM's Almaden Research Center, and their nonlinear optical properties measured. Within each class, substitution of different donor and acceptor groups has led to definitive conclusions regarding the relative merits of particular moieties.
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