Evaluation of a refractive index profile for a modification induced by focused femtosecond laser irradiation in the optical glasses

Abstract

We report on a refractive index modification (▵n) induced by femtosecond irradiation and evaluation of the profile for created lines inside the different types of optical glasses, i.e., silicate or borate glass containing the metal oxides such as BaO, TiO₂, or La₂O₃ and silica glass. The lines are fabricated by scanning a stage and focusing the femtosecond laser pulses, 800nm wavelength, a 250 kHz repetition rate and 200 fs pulse duration, from the Ti:sapphire regenerative amplifier system. The ▵n profiles of modification were obtained with Qualitative Phase Microscopy technique and presented systematically for a different input power and a variety of glasses. The ▵n profile changed with focusing condition using 10× (N.A.=0.3) or 40× (N.A.=0.85), and input power in a single glass. However, the ▵n and a trend of the sign was different depending on glass types. For example, silicate glass containing TiO₂, exhibited negative ▵n trend the ▵n became smaller in the modified region. Furthermore, the glass showed relatively large negative ▵n, < - 0.01 decrease of the ▵n, ▵n < -0.01, in the investigated power range. These results could be useful for a design or use of glasses for micro optics, such as grating, diffractive lens or lens array, produced by femtosecond laser fabrication.