Tracking of Refractive Index variation of Polymethyl Methacrylate after irradiated by femtosecond Laser

Abstract

This research is aim to modify the subsurface of poly methyl methacrylate (PMMA) and create diffraction grating by means of ultra short UV laser(Ti:Saphire ) with pulse duration of 250 femto-second laser, wavelength of about 387nm, output beam from the system is 0.105 μJ, the grating writing (scanning) speed is 1.25 mm/s, the grating period is 40μm. The experimental result of interferometric patterns (with Mach-Zehnder interferometer) for the samples before and after radiation showed that the discontinuity of fringes pattern is due to the irradiated zone inside the sample because of photo- thermal and optical density changes in (PMMA) subsurface caused by laser radiation. It was found that when uses matlab programme for the two digitized interferograms images the refractive index map obtain, the variation in Δn is be clear along x-axis in this figure, the high peak of Δn occurs at midpoint of focusing Coherent Legend Ti: Sapphire laser to PMMA sample. The results above showed that laser direct writing technique is easy to change the optical characterizations of the materials, produce PMMA diffraction grating as well as high quality. This mechanism is strongly dependant on the optical properties of the material and is recommended to be studied experimentally and theoretically in the future.