Abstract
Recently, a novel technique for the direct writing of waveguides and photonic circuits in transparent glass substrates, exploiting nonlinear refractive index modifications induced by focused femtosecond pulses, has emerged [1]. The basic physical mechanisms underlying this process can be outlined as follows: when a femtosecond pulse is tightly focused in a transparent material, a nonlinear absorption mechanism, combining multiphoton and avalanche ionization, allows deposition of energy in a small volume around the focus, where the intensity is the highest [2]. The photogenerated hot electron plasma induces high temperatures and pressures that give rise to different phenomena such as densification, photostructural modifications and colour centers formation; under suitable conditions, the combination of these effects may lead to a local increase of refractive index over a micrometer-sized volume of the material. By moving the laser focus inside the substrate one can use the laser beam to define regions of increased index and thus directly produce three-dimensional light-guiding structures.
© 2009 IEEE
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