Fabrication of internal diffraction gratings in planar fluoride glass using low-density plasma formation induced by a femtosecond laser

The fabrication of internal diffraction gratings with photo-induced refractive index modification in planar fluoride plates was demonstrated using low-density plasma formation excited by a high-intensity femtosecond (130 fs) Ti:sapphire laser (λp = 790 nm). The refractive index modifications with diameters ranging from 350 nm to 5 μm were photoinduced after plasma formation occurred upon irradiation with peak intensities of more than 1 × 1012 W/cm2. The graded refractive index profile was fabricated to be a symmetric around from the center of the point at which low-density plasma occurred. The maximum refractive index change (Δn) was estimated to be 1.3 × 10−2. The low-density plasma formation (nc < 1.79 × 1027 m−3]) causes the increase of the refractive index modification with fluoride glass.