ABSTRACT
Focused femtosecond laser pulses can induce permanent and localized refractive index modification of transparent materials under suitable conditions. Relative translation of the sample with respect to the laser beam allows one to literally draw optical waveguides, circuits or other complex structures in the substrate, fully exploiting the three dimensions. In this chapter, the main parameters involved in the fabrication process will be discussed. Several important devices will also be shown, which indeed exemplify the vast portfolio of applications reached nowadays by this technology by exploiting its unique capabilities. 9.1 IntroductionFocused femtosecond laser pulses yield peak intensities greater than 10 TW/cm2,which cause strong nonlinear absorption and localized
energy deposition in the bulk of transparent materials. After several picoseconds, the laser-excited electrons transfer their energy to the lattice, leading to a permanent material modification. Depending on the laser and material parameters, this laser modification may result in damaged and irregular scattering centres, or smooth structures with a positive refractive index alteration. Such smooth structures of positive refractive index change, suitable for waveguiding light at visible and infrared wavelengths by total internal reflection, may be formed along arbitrary three-dimensional paths by scanning the sample using computer-controlled motion stages. Additional advantages of femtosecond laser direct writing include maskless, single-step fabrication, and its ability to process many transparent materials including glasses, polymers and crystals. In this chapter, the relevant exposure parameters to produce low loss, high refractive index increase optical waveguides by femtosecond laser writing are reviewed and discussed. In particular, the chapter will focus on the possibility of using this microfabrication technology to produce important photonic devices such as directional couplers, Mach-Zehnder interferometers, Bragg grating waveguides, waveguide lasers and waveguide arrays. Exploiting the unique flexibility of the technology and its three dimensional capabilities these components can be further combined to produce more complex microsystems addressing such diverse application fields as optofluidics, optomechanics and integrated quantum optics. The chapter will review some of the most significant examples highlighting the advantages of this fabrication technology with respect to standard ones. 9.2 Waveguide Writing in Transparent Materials
9.2.1 Exposure Parameters and ConsiderationsAs illustrated in the previous chapters, the features induced in the substrate by the action of femtosecond laser pulses depend in a nontrivial way on many irradiation parameters, such as pulse energy, pulse duration and repetition rate, translation speed of the sample, focusing conditions, as well as on material characteristics such as bandgap, thermal characteristics and crystalline or amorphous properties. Thus, a careful experimental optimization
of the parameters is generally required for fabricating the optimum waveguide for a certain application. 9.2.1.1 Pulse energy and translation speed
Depending on the energy of the focused femtosecond laser pulses, three qualitatively different features have been observed in the bulk of transparent materials1-3:a smooth isotropic refractive index change for low pulse energies, a birefringent refractive index change for intermediate pulse energies and a void due to microexplosions for high pulse energies. The smooth isotropic refractive index change is the most relevant for waveguide writing in glasses.4 However, the precise microscopic mechanisms that lead to the refractive index increase are not yet fully understood and are still under investigation. Diverse interaction phenomena enter into play, including colour centre formation, densification, thermal diffusion and accumulation. The relative role of each of them is debated5-8 and depends on the material and the exposure conditions. In fused silica glass, for slightly higher pulse energies, the formation of birefringent nanostructures is observed, the axis of birefringence depending on the irradiation polarization.9-11 This peculiar feature has been explored to produce waveguides and photonic devices with polarization dependent characteristics, such as structures that are able to guide only one polarization11 and directional couplers with polarization dependent behaviour.12 For even higher pulse energies microexplosions are induced by the femtosecond pulses, which result in microvoid formation. This kind of modification does not present, in general, waveguiding properties. Interesting applications for this regime have been envisaged anyway in high-density three-dimensional data storage. Sub-micron size voxels (volume pixels) can be written in this way by femtosecond pulses and readout can be performed exploiting scattering or luminescence.13,14 For a given pulse energy, the translation speed controls the amount of total energy deposited per unit volume and typically influences the achieved refractive index change. However, since the interaction process is nonlinear, lowering (increasing) the speed with the same pulse energy is typically not equivalent to increasing (lowering) the pulse energy for a fixed speed.
