Formation of Elemental Distribution in Glass using Thermal Accumulation with Femtosecond Laser Irradiation
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1230-MM07-04
Formation of Elemental Distribution in Glass using Thermal Accumulation with Femtosecond Laser Irradiation M. Shimizu1, K. Miura1, N.Yasuda1, M. Sakakura2, S. Kanehira2, M. Nishi1, Y. Shimotsuma2, and K. Hirao1 1 Department of Material Chemistry, Graduate School of Engineering, Kyoto University, Kyoto 615-8510, Japan 2
Innovative Collaboration Center of Kyoto University, Kyoto 615-8520, Japan
ABSTRACT Elemental migration inside a glass was induced space-selectively and microscopically by high-repetition femtosecond(fs) laser irradiation. The tendency of the elemental migration depended on the strength of the bond between cations and oxygen ions:strongly bonded ions like Si or Al migrated to the center of the irradiated spot, whereas weekly bonded ions such as Ca migrated to the outside. Judged from analyzed temperature distribution, this phenomenon may be due to the thermomigration(Soret effect). The refractive index distribution was modified locally by controlling elemental distribution and optical waveguide was formed in phosphate and borate glasses.
INTRODUCTION The fs laser has been recognized as an effective tool for microscopically and threedimensionally modifying transparent materials such as glass[1-3]. In fs laser processing, the heat generated by laser irradiation has often been thought of as a disadvantage that contributes to low accuracy in processing. However, a processing method that aggressively uses the heat accumulation during laser irradiation at a high repetition rate has attracted considerable attention because it is effective for the writing of low-loss optical waveguides[4,5] and for space-selective precipitation of functional crystals inside the glass[6-8]. It is important for optical application to control the distribution of the refractive index. About ten years ago, our group discovered that refractive index changes on the order of 10-2 to 10-3 could be induced within various types of glasses[1,2]. However, because the increase of refractive index is related to local densification in the glass, the maximum laser-induced refractive index difference was limited to ~10-2. In this research, we report on the formation of elemental distribution by focusing a lot of laser pulses at a high repetition rate (more than several hundred kHz) in a glass. Elemental distribution has a potential to induce a larger refractive index change than that induced by the densification mentioned above. We will discuss about the distribution behavior of each element for various types of multicomponent glasses and about the temperature distribution during the laser irradiation to clarify the mechanism of migration. In addition, we will show the application of this phenomenon to fabrication of an optical waveguide.
EXPERIMENT Regeneratively amplified fs laser pulses were used to induce the elemental migration in glasses. The 800 nm laser pulses were generated by a Ti-sapphire laser oscillator and amplified at 250 kHz (Coherent Inc., Mira-RegA9000). In the typical condition, the pulse duration was 70 f
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