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Formation of Permanent Brown-colored Patterns in Transparent BK7 Glass upon Irradiation with a Tightly Focused Femtosecond Laser

Jin-Woo Jeon, Suk-Young Ji, Hoon-Young Kim, Wonsuk Choi, Young-Gwan Shin and Sung-Hak Cho

Transparent borosilicate glass (BK7) is extensively used in optics applications. Herein, we demonstrate that a brown-colored pattern is generated inside BK7 glass upon irradiation with a near-infrared Ti:sapphire femtosecond laser and reveal the importance of maintaining the laser power at values well below the BK7 damage threshold for color center formation. The use of low laser power prevents the occurrence of mechanical damage (e.g., micro-crack or thread formation) in the generated colored area, and Gaussian fitting of the absorbance spectrum of BK7 shows that this material exhibits a band gap of 4.21 eV with three absorption edges. Thus, the developed patterning method is expected to find numerous applications in the future color processing of lines and points in glass.