Photoinduced Structural Changes of Doped SiO2 Glasses using Ultraviolet Laser Pulses

There is a growing need for photonic band gap (PBG) devices in various applications including high-Q resonators, energy-efficient solar cells, microfluidic devices, microelectromechanical systems and so on. The PBG devices, which have artificial structures whose refractive index is periodically modulated, can reflect or diffract propagating light at a wavelength comparable to lattice spacing. The optical properties of the PBG devices can be controlled by changing lattice patterns, refractive index of lattices, duty ratio, interaction length and so on. Because of high potential for applications, the PBG effects have been intensively studied in the last decade. However, the fabrication of their fine structures, which can work at optical communication wavelengths and visible wavelengths, remains a challenging task. Most PBG devices are manufactured by the semiconductor fabrication technology including lithography and various etching techniques. Although this technology enables us to create precise structures with high spatial resolution, we must use complicated experimental equipments such as vacuum systems and clean-room environments. In particular, highly expensive electron beam lithography systems are necessary to obtain sub-wavelength photonic structures, which are important for practical applications. Direct laser writing technique of photonic structures is an attractive alternative. When optical materials are exposed to laser light below ablation threshold intensity, photochemical reactions such as chemical bond breakage and creation or annihilation of lattice defects occur in the laser irradiated regions, , thereby changing the refractive index and density. In 1978, Hill et al. firstly reported the direct laser writing of Bragg mirrors (one dimensional PBG devices) inside a Ge-doped SiO2 core optical fiber using Ar laser (Hill et al., 1978). This writing technique enables creation of photonic band gap devices with a simple setup on or in various glasses with sub-wavelength resolution (Svalgraad et al., 1994). Moreover, no debris or cracks are generated, although they often occur with laser ablation or mechanical machining. Especially, pulse laser, which has high peak intensity, is effective for such structural changes. From the point of view of practical applications, the structural changes of SiO2-based glass materials, which are used as optical fiber or waveguide cores, are more important. Therefore, so far, many researchers have reported the structural changes of glass materials using ultraviolet nanosecond lasers, near-infrared femtosecond laser and so on (Åslund, et al., 2000, Hirao, et al., 1997).