Femtosecond laser-induced crystallization in glasses is of great scientific interest for its potential applications in the fabrication of optical and photonic devices.Compared with other fabrication techniques, the most unique advantage of the femtosecond laser technique is its nonlinear absorption nature, which makes it possible to precipitate various crystals with geometrically complex structures inside glasses

Researchers at Shanghai Institute of Optics and Fines Mechanics (SIOM/China) and Zhejiang University and Xi’an Jiaotong University and South China University of Technology have report on direct-writing Ge crystals in three dimensions inside a borosilicate glass sample by using 250 kHz femtosecond laser irradiation. [Optics Letters 36 (2), 262, 2011]

Crystalline Ge was induced space selectively inside a borosilicate glass by 800 nm, 250 kHz femtosecond laser irradiation. Micro-Raman spectra and x-ray diffraction analysis confirmed that the laser-induced crystals were cubic Ge. A periodic structure consisting of Ge crystalline lines was inscribed in the glass sample by continuously moving the focal point of the laser beam. Large third-order nonlinear optical properties and ultrafast response time were observed from the crystallization region owing to highly optical nonlinearity of Ge crystals. These results may find some applications in fabrication of functional optical and photonic devices, such as optical circuits.