[13a-C31-1~8] 3.7 Laser processing

[13a-C31-3] Inner surface roughness on fused silica micro-hole fabricated by NUV femtosecond laser pulses

〇Masahiko Shiraishi¹, Kenji Goya², Shoichi Kubodera¹, Kazuhiro Watanabe¹ (1.Soka Univ., 2.AIST)

Keywords:Femtosecond laser processing, Surface modification, Fused silica

In this report, the observation of inner surface roughness of micro-holes fabricated in a thin fused silica plate was achieved using SEM. We have proposed a new type of sensor with in-line/pico-liter micro-holes which can be fabricated into a fiber optic waveguide by using a second harmonic 400 nm femtosecond laser. The sensor has capability of measuring refractive index. Recently, many types of localized surface plasmon resonance (LSPR) sensors based on fiber optics have been reported. These sensors were designed to trigger interactions between transmitted light and metal nanoparticles (MNPs) with high sensitivity. The micro-holes designed in a fiber optic waveguide also meet the conditions necessary for inducing LSPR. The scattering of transmitted light at the boundary between the waveguide and sensing areas is influenced by the inner surface roughness. It is necessary to control the inner surface roughness. The femtosecond laser pulses at 400 nm of a 800 nm Ti:sapphire laser with a repetition rate of 1 kHz were focused on a thin fused silica plate by using an objective lens with an NA of 0.65. The pulse energy and pulse duration were 20 mJ and 350 fs, respectively. The number of laser pulse shots was changed from a single shot to 1,000 laser pulse shots. The whole views of the micro-holes were captured with 5,000 magnification using SEM. SEM photos show that the shapes of micro-holes were distorted because of diffraction of the focused laser beam. The close up images of the micro-hole side walls were also obtained with 50,000 magnification. After 50 laser pulse shots, the relatively smooth surface of micro-hole was obtained. After 100 laser pulse shots, the inner surface roughness was formed. It seems that the inner surface was melted by femtosecond laser ablation. Furthermore, the deposition of ablated fused silica particles occurred after 1000 laser pulse shots. In this report, the inner surface roughness of micro-holes fabricated by 400 nm femtosecond laser pulses were observed by using SEM. It was revealed that the condition of the inner surface depends on the number of laser pulse shots.