Ultrafast Processes for Bulk Modification of Transparent Materials
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Ultrafast Processes for Bulk Modification of Transparent Materials Kazuyoshi Itoh, Wataru Watanabe, Stefan Nolte, and Chris B. Schaffer Abstract When a femtosecond laser pulse is focused inside a transparent material, the optical intensity in the focal volume can become high enough to induce permanent structural modifications such as a refractive index change or the formation of a small vacancy. Thus, one can micromachine structures inside the bulk of a transparent material in three dimensions. We review the mechanisms of and techniques for bulk modification of transparent materials using femtosecond laser pulses and discuss the fabrication of photonic and other structures in transparent materials, including waveguides, couplers, gratings, diffractive lenses, optical data storage, and microfluidic channels. Keywords: fluidics, laser, machining, optical.
Introduction Ultrafast lasers have extremely short pulse durations or, equivalently, extremely wide signal bandwidth. While the latter feature enables ultrafast photonic communications channels, the former promises the fabrication of integrated photonic communications and signal processing systems inside transparent materials. The extraordinarily high peak power of short-duration optical pulses provides a new approach for local modification of transparent materials through nonlinear optical processes. Recent demonstrations of threedimensional (3D) micromachining of glass using ultrafast laser pulses include the fabrication of waveguides, couplers, gratings, binary data storage devices, lenses, and channels. Thus, direct-writing of optical devices using ultrafast laser pulses has potential applications in the telecommunications and optical signal processing industries. The most important feature of this microfabrication technique is its ability to integrate 3D optical or photonic devices inside transparent materials by sequential direct-writing of individual
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devices. Although such a sequential approach is slow in comparison with conventional lithography, the new capability for 3D integration is priceless, in that it is quite difficult to achieve by other methods. Although mechanisms for ultrafastlaser-induced structural changes are not yet well understood, the current understanding will be reviewed here, followed by demonstrations of the fabrication of photonic devices and microfluidic channels. Recent advances, especially in writing waveguides using megahertz oscillators without the use of regenerative amplifiers, are discussed at the end of this article.
Mechanisms for FemtosecondLaser-Induced Structural Change When an intense femtosecond laser pulse is tightly focused inside the bulk of a transparent material (Figure 1a), the intensity in the focal volume can become high enough to initiate absorption through nonlinear field ionization (multiphoton absorption and tunneling ionization) and avalanche ionization, in which one electron gains energy from the laser
field and impact-ionizes an additional electron (Figure 1b).1–5 This nonlinear absorption results
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