ABSTRACT
N onlinear optics is the basis of all the fledgling photonics technologies, 1 •2 where light works with, or even replaces, electrons in. applications traditionally carried out by microelectronics. Over the past decade, the study of nonlinear optical properties in organic and polymer systems has enjoyed rapid and sustained growth.3-7 Because of their large optical nonlinearities and mechanical, chemical, thermal and photo stabilities, organic nonlinear optical materials are the leading practical materials for fabricating optoelectronic devices. They also have proven to be excellent subjects in which to study the physics of many-body electron correlation effects, excitation dynamics and materials relaxations, and exotic electronic excitations such as exiton strings. 8
In the search for understanding physical mechanisms that determine the nonlinear optical responses in organic structures and for new materials that may qualify for device applications, we are constantly required to know accurately the values of microscopic and macroscopic nonlinear optical properties of the system under consideration. As a result of an extraordinary expansion in the study of nonlinear optical responses of organic systems, different conventions, reference standards, and units have been used, making it difficult to compare theoretical results with experimental values, or to evaluate nonlinear optical properties of different systems or sometimes even to understand the results on the same system from different research groups. The best example is perhaps the value of the molecular second order susceptibility f3x (- 2w; w, w) for the heavily studied system, paranitroaniline (PNA).39.47 A8 A careful survey shows that at the same wavelength, different research groups using different conventions obtained approximately the same value, while in fact they should differ by a factor of 3/2. This case is one of several major findings that will be made more clear later in this Chapter.
