Optical characterization of PMMA phase gratings written by a 387 nm femtosecond laser

In most cases authors are permitted to post their version of the article (e.g. in Word or Tex form) to their personal website or institutional repository. Authors requiring further information regarding Elsevier's archiving and manuscript policies are encouraged to visit: a b s t r a c t a r t i c l e i n f o We report on the fabrication of optical Bragg type phase gratings in polymethyl methacrylate substrates by a femptosecond Ti: Sapphire laser. As for their optical characterization, a spatially resolved microscopy interferometric technique is used to investigate the two-dimensional distribution of the refractive index change produced by the irradiation process. The technique gives a direct and quantitative two-dimensional profile of the index of refraction in irradiated PMMA, providing information on how the fabrication process depends on the laser irradiation. Recently, there has been much interest in applying focused femto-second laser pulses at high repetition rate to induce various localized microstructures near the focal point of laser beam in transparent materials. Focused femto-second laser pulses induce structural changes inside the transparent materials, such as glass and polymers, due to nonlinear absorption at high peak powers. The induced refractive index change can be used to fabricate the three-dimensional microstructures and optical devices [1]. Several photonic structures have been realized in glass crystals [2–8]. Structures induced by the irradiation with near infrared femto-second laser pulses have been investigated in various polymer materials with different glass transition temperatures [9]. Waveguide fabrication within polymethylmethacrylate (PMMA) by femto-second laser pulse irradiation has been reported [10,11]. Diffractive optical elements using various polymeric materials have also been fabricated by translating the laser focus area [12–14]. These experiments have shown that the refractive index in the laser focus area increases as compared with that before the irradiation. Different mechanisms have been proposed for explaining these, as changes in polymer density, or structural changes induced by photodecomposition processes or also by tensile stress. Direct observation of femto-second light-induced refractive index changes is thus necessary for understanding and controlling, namely, scanning speed, laser energy and laser pulse duration [15]. These parameters can be changed independently but their effects are unavoidably superposed upon each other. Phase contrast microscopy has been used for optical analysis of phase grating structures [16]. In this work we report on quantitative spatially resolved interferometric measurement of Bragg type phase gratings inside PMMA produced by femto-second laser irradiation. Spatially resolved …