Nonlinear optical materials play a pivotal role in the future evolution of nonlinear optics in general and in its impact in technology and industrial applications in particular. The progress in nonlinear optics has been tremendous since the first demonstration of an all-optical nonlinear effect in the early sixties, but until recently the main visible emphasis was on the physical aspects of the nonlinear radiation-matter interaction. In the last decade, however, this effort has also brought its fruits in applied aspects of nonlinear optics. This can be essentially traced to the improvement of the performances of the nonlinear optical materials. Our understanding of the nonlinear polarization mechanisms and their relation to the structural characteristics of the materials has been considerably improved, and, in addition, the new development of techniques for the fabrication and growth of artificial materials has dramatically contributed to this evolution.
The goal is to fInd and develop materials presenting large nonlinearities and satisfying at the same time all the technological requirements for applications such as wide transparency range, fast response, high damage threshold but also processability, adaptability and interfacing with other materials. These additional requirements are intrinsic to the fabrication of nonlinear integrated devices which, besides efficiently performing the expected nonlinear operation, must be miniaturized, compact, reliable and with precisely reproducible characteristics in large-scale low-cost production and long-term operation. A particularly important area is that of optical communications where devices at high speed and low optical power are needed to fully exploit the capabilities of optical-fibre transmission systems that are currently used and will be more so in the future.
For many years nonlinear optical materials have been selected mainly among inorganic crystals, especially ferroelectrics, and related crystals with oxygen polyedra. In recent years, not only new inorganic crystals have been discovered with even better performances, but also complete new families of materials have been studied and developed like the organic crystals, poled polymers, composites and artificial semiconductor microstructures. In the latter cases the electron confinement is being exploited to tailor the performances of the system in order to meet certain requirements. In the case of the organics the new molecular-engineering techniques have brought considerable improvement in their performances. All these improvements, besides rendering possible the implementation of nonlinear effects in devices, open the way to the study of new nonlinear optical effects and the introduction of new concepts.
The aim of the Course was to describe the new concepts which are emerging in the field of nonlinear optical materials, concentrating the attention on materials which seem more promising for applications in the technology of information transmission and processing. The School was attended by about 70 participants coming from all over the world. We are happy to present in this volume the final texts of nearly all the lectures and seminars presented at the Course. It should be mentioned that, during the Course, we had also several presentations of current activities made by the students and a number of informal sessions with very lively discussions.
We wish to thank the scientific secretary of the Course, Prof. G. P. Banfi, for his valuable contribution in all phases of the scientific organization, Mrs. E. Mazzi, from the Italian Physical Society, for her invaluable help before and during the Course, and Siemens S.p.A. for the generous financial support.