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

The fabrication of an internal diffraction grating with photoinduced refractive index modification in planar silica plates was demonstrated using low-density plasma formation excited by a high-intensity femtosecond (150 fs) Ti:sapphire laser (lambda(p) = 790 nm). The refractive index modifications with diameters ranging from 400 nm to 3 mum were photoinduced after plasma formation occurred upon irradiation with peak intensities of more than 2 x 10(13) W/cm(2). 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 (Deltan) was estimated to be 1.6 x 10(-2). The. low-density plasma formation (n(c) < 1.79 x 10(27) (m(-3))) causes the refractive index modification with a SiE' center defect. (C) 2002 Elsevier Science B.V. All rights reserved.