12.1 Introduction

In 1994, Prof. Hirao of Kyoto University, Japan proposed a basic research idea of "induced structure". ^[1] He paid attention to the fact that glass is metastable from the viewpoint of thermodynamics. A metastable state of glass is easily changed to other states in an intensive external electromagnetic field. If we can control the external electromagnetic field induced structure and the concentration, variety and valence state of active ions in glass; in particular, if we can space-selectively control the induced microstructures or 3-dimensionally distributed electronic structure in glass, we expect that novel optical functions of the glass will be achieved. From the viewpoint of practical applications, we expect to obtain a glass with properties superior to corresponding single crystal. Based on this idea, we applied various electromagnetic fields such as X-ray, ultra-violet light, electron beam and laser to make microscopic modifications to glass structure, and observed many interesting phenomena and discussed the promising applications of the observed phenomena. ^{[2] [3] [4] [5] [6] [7] [8]}

We selected a femtosecond laser as a powerful tool to make microscopic modifications to glass structure. Compared with CW and long pulsed lasers, femtosecond laser has two apparent features: (1) elimination of the thermal effect due to extremely short energy deposition time, and (2) participation of various nonlinear processes enabled by highly localization of laser photons in both time and spatial domains. Due to the ultra-short light-matter interaction time and the high peak power, material processing with the femtosecond laser is generally...