Femtosecond Laser Micromachining Photonic and Microfluidic Devices i
Femtosecond laser micromachining of transparent material is a powerful and versatile technology. In fact, it can be applied to several materials. It is a maskless technology that allows rapid device prototyping, has intrinsic three-dimensional capabilitie
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Fundamentals of Femtosecond Laser Modification of Bulk Dielectrics Shane M. Eaton, Giulio Cerullo, and Roberto Osellame
Abstract Femtosecond laser pulses focused beneath the surface of a dielectric are absorbed through nonlinear photoionization mechanisms, giving rise to a permanent structural modification with dimensions on the order of a micrometer. At low pulse energies, the modification in many glasses is a smooth refractive index change, enabling photonic device fabrication. At higher pulse energies, the laser-induced modification may contain birefringent, periodic nanoplanes which align themselves orthogonally to the laser polarization. These nanogratings are not ideal for most waveguide devices but when the sample is exposed to hydrofluoric acid after writing, preferential chemical etching along the direction of the nanoplanes forms several millimeter-long buried microchannels which are useful for microfluidic applications. At even higher pulse energies, ultrahigh pressures within the focal volume lead to microexplosions causing empty voids which can be used for threedimensional photonic bandgap devices and memories. In addition to pulse energy, other parameters have been shown to strongly influence the resulting morphology after femtosecond laser exposure including repetition rate, scan speed, focusing condition, polarization, pulse duration, depth, and direction.
1.1 Introduction In 1996, Hirao’s group showed that by focusing subpicosecond pulses in the bulk of transparent glass, the modification induced beneath the sample surface could be tailored to produce a permanent refractive index increase [1]. Because of the nonlinear nature of the interaction, absorption is confined to the focal volume inside
S.M. Eaton () G. Cerullo R. Osellame Istituto di Fotonica e Nanotecnologie - Consiglio Nazionale delle Ricerche (IFN-CNR), and Department of Physics - Politecnico di Milano, Piazza Leonardo da Vinci 32, 20133, Milan, Italy e-mail: [email protected]; [email protected]; [email protected] R. Osellame et al. (eds.), Femtosecond Laser Micromachining, Topics in Applied Physics 123, DOI 10.1007/978-3-642-23366-1 1, © Springer-Verlag Berlin Heidelberg 2012
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the bulk material. By scanning the sample relative to the laser focus with computercontrolled motion stages, a region of increased refractive index could be formed along an arbitrary three-dimensional path, unlike traditional photolithography, which is limited to fabricating devices in-plane. In this chapter, the current understanding of the femtosecond laser–material interaction physics in the bulk of dielectrics is discussed and the important exposure conditions influencing the resulting waveguide properties are reviewed.
1.2 Femtosecond Laser–Material Interaction Peak intensities on the order of 10 TW/cm2 can be readily produced by focused femtosecond laser pulses from today’s commercial laser systems. Such intensities result in strong nonlinear absorption, allowing for localized energy deposition in
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