Nonlinear optical studies, characterization

Three new mainchain chromophoric polymers were synthesized and characterized. The physical properites of these polymers were compared to those of poly((4-N-ethylene-N-ethylamino)-a-cyanocinnamate). Films of the polymers can be made by either solution or melt casting of films. It was possible to obtain an oriented fibw of a polymer, namely poly(4 -[N-ethylene-N-(2-hydroxyethyl)amino] stilbene-4-formate, most likely because it has a melt transition temperature at around 210°C. Nonlinear optical studies of these polymers are in progress. 

One purpose of this tutorial paper on optical characterization is to provide a brief introduction for chemists to the concepts and methods involved in studies of the nonlinear optical properties of molecules and materials. The intent is to familiarize chemists with the range of commonly used techniques and their physical basis. An attempt is made to provide some background on macroscopic nonlinear optics, relating to what is actually measured, and the connection to molecular nonlinear optical properties. This paper is not intended to be a detailed or comprehensive review. The reader is referred to introductory (1, 2) and advanced (3-6) texts on nonlinear optics for more detailed or complete coverage of the subject. 

In this paper it has been attempted to provide an introductory overview of some of the various nonlinear optical characterization techniques that chemists are likely to encounter in studies of bulk materials and molecular structure-property relationships. It has also been attempted to provide a relatively more detailed coverage on one topic to provide some insight into the connection between the macroscopic quantities measured and the nonlinear polarization of molecules. It is hoped that chemists will find this tutorial useful in their efforts to conduct fruitful research on nonlinear optical materials. 

During the last 10-20 years, a large number of efficient theoretical methods for the calculation of linear and nonlinear optical properties have been developed— this development includes semi-empirical, highly correlated ab initio, and density functional theory methods. Many of these approaches will be reviewed in later chapters of this book, and applications will be given that illustrate the merits and limitations of theoretical studies of linear and nonlinear optical processes. It will become clear that theoretical studies today can provide valuable information in Are search for materials with specific nonlinear optical properties. First, there is the possibility to screen classes of materials based on cost and time effective calculations rather then labor intensive synthesis and characterization work. Second, there is Are possibility to obtain a microscopic understanding for the performance of the material—one can investigate the role of individual transition channels, dipole moments, etc., and perform systematic model Improvements by inclusion of the environment, relativistic effects, etc. 

The nonlinear optical properties in solution of selected functionalized PDAs, described herein, have also been evaluated by means of the z-scan technique. Off resonance studies (at 705 nm) show that nonlinear refraction is only comparable to that of the solvent for these dilute solutions, but that nonlinear absorption, characterized by p values, varies significantly, with the nature of the side-chains. O It can be inferred that if bulk films of these PDAs possess suitable nonresonant nonlinear refractive properties for optical devices, modification of side-chain structure can reduce the magnitude of undesirable two photon absorption. 

All these properties involve the low-lying vr electronic states of these molecules, that thus need to be properly characterized. This review deals with reliable methods of studying the low-lying electronic states and the linear and nonlinear optical responses of conjugated systems. 

The Polymer Data Handbook offers, in a standardized and readily accessible tabular format, concise information on the syntheses, structures, properties, and applications of the most important polymeric materials. Those included are currently in industrial use or they are under study for potential new applications in industry and in academic laboratories. Considerable thought was given to the criteria for selecting the polymers included in this volume. The first criterion was current commercial importance—the use of the polymer in conunercial materials—for example, as a thermoplastic, a thermoset, or an elastomer. The second criterion was novel applications—a polymer that is promising for one or more purposes but not yet of conunercial importance—for example, because of its electrical conductivities, its nonlinear optical properties, or its suitability as a preceramic polymer. The hope is that some readers wiU become interested enough in these newer materials to contribute to their further development and characterization. Finally, the handbook includes some polymers simply because they are unusually interesting—for example, those utilized in fundamental studies of the effects of chain stiffness, self-assembly, or biochemical processes. 

This work has been supported by AFOSR, NSF, a NSF/CNRS international program, and by the Center for Advanced Multifunctional Nonlinear Optical Polymers and Molecular Assemblies (CAMP) funded by ONR. The authors would like to thank CAMP collaborators for fruitful discussion, Dr. N. V. Kukhtarev from Alabama A M University for his contribution to the study of the non-Bragg orders, Dr. P. M. Allemand from Donnelly Corporation, Tucson, for his help during the impedance measurements, and Drs. A. Fort, M. Barzoukas, and C. Runser from IPCMS France for the EFISH characterization experiments. 

In order to fully use the potential of various structures prepared by this technique, an important step is a careful characterization of L-B films. In our laboratory, a number of spectroscopic and surface-sensitive techniques are used. Both second and third-order nonlinear optical processes have been observed using L-B films. The organization of this paper is as follows. First, some of the techniques used for the characterization of L-B films are discussed. Then some interesting examples of control of order and conformation in the L-B films are presented. This is followed by a subsection, which presents results of the study of both second and third-order nonlinear optical processes in L-B films. Finally, possible applications of L-B films in nonlinear optical devices are discussed. 

Synthesis and nonlinear optical characterization of a two-photon absorbing organic dye, trans-4-(dimethylamino)-4 -[N-ethyl-N-(2-hydroxyethyl)amino]stilbene (DMAHAS), were reported [49]. Linear absorption, single-photon-induced fluorescence, and two-photon-induced fluorescence of DMAHAS were experimentally studied. This dye showed a moderate two-photon absorption cross section of a2 = 0.91 X 10-46 cm s/photon at 532 nm as shown by an open aperture Z-scan technique. DMAHAS also showed strong two-photon-induced blue fluorescence of 432 nm when pumped with 800 nm laser irradiation. 

Liquid crystals are generally characterized by the strong correlation between molecules, which respond cooperatively to external perturbations. That strong molecular reorientation (or director reorientation) can be easily induced by a static electric or magnetic field is a well-known phenomenon. The same effect induced by optical fields was, however, only studied recently. " Unusually large nonlinear optical effects based on the optical-field-induced molecular reorientation have been observed in nematic liquid-crystal films under the illumination of one or more cw laser beams. In these cases, both the static and dynamical properties of this field-induced molecular motion are found to obey the Ericksen-Leslie continuum theory, which describe the collective molecular reorientation by the rotation of a director (average molecular orientation). 

---