ABSTRACT
Femtosecond laser (fs) has been widely used in materials microprocessing when high accuracy and small structure size are required. We demonstrate what effect the irradiation from focusing the laser pulse has on the structural changes in the laser-irradiated region and a few characteristics of the fs laser processing in transparent materials. Various interesting phenomena, e.g. space-selective refractive index change, valence state change, formation of elemental distribution, growth of nonlinear crystal, precipitation of silicon, in glass that can be accomplished with an fs laser are introduced. The photo-induced phenomena in transparent materials
using an fs laser from the viewpoint of photonic application are also described. 8.1 IntroductionMaterials processing technology by using femtosecond (fs) laser irradiation has attracted tremendous interest from both scientific and technological communities. Femtosecond laser has been widely used in materials microscopic processing when high accuracy and small structure size are required. Local structural change induced inside transparent materials using tightly focused fs laser pulses enables us to fabricate various kinds of three-dimensional (3D) structures inside glasses, polymers and inorganic crystals; this microfabrication technique with an fs laser has been used to produce small optical devices such as optical waveguides, photonic crystals, 3D optical memory and so on [1-8]. A key advantage of using fs laser, as opposed to longer pulses, for direct writing is that such pulses can rapidly and precisely deposit energy in transparent materials. When a transparent material such as glass is irradiated by a tightly focused fs laser, a photo-induced reaction should occur only near the focused part of the laser beam due to various nonlinear physicochemical reactions. In the past several years, a lot of research efforts have been devoted to the field of 3D microscopic modifications to transparent materials by using ultrafast laser. In this chapter, we focused on the characteristics of the fs laser in transparent materials processing and various types of photo-induced phenomena with the fs laser irradiation. 8.2 Characteristics of Ultrafast Laser ProcessingOne of possible origins of the fs-laser-induced refractive index change is a pressure wave, which is generated as a result of the rapid relaxation of thermoelastic stress. Because a pressure wave compresses materials, the refractive index increase in an fs laser focal region might be the result of the densification by the pressure wave. Another possible origin is a rapid temperature elevation and rapid cooling. A number of studies showed that rapid cooling of a silica glass induces a glass structure of a higher fictive temperature, which has a higher density [9, 10].
