It was more than ten years ago that an original version of this monograph was published with the title Quantum Optics in Japanese from Iwanami Shoten in Tokyo. Therefore, making the best use of this chance to translate the book into an English version, we have tried to include the exciting developments of the relevant subjects in these ten years, especially novel nonlinear optical responses of materials. The ?rst example of these nonlinear optical phen- ena is laser cooling and subsequent observation of Bose-Einstein and Fermi condensation of neutral atoms. Second, it is now possible to...

This is the Proceedings of the Taniguchi International Symposium on "Relaxation of Elementary Excitations" which was held October 12-16,1979, at Susono-shi (at the foot of f1t. Fuji) in Japan. The pleasant atmosphere of the Symposium is evidenced in the picture of the participants shown on the next page. The purpose of the symposium was to provide an opportunity for a limited number of active researchers to meet and to discuss relaxation processes and related phenomena not only of excitons and phonons in solids but also electronic and vibrational excitations in molecules and biological...