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Dipartimento di Fisica - Politecnico di Milano

Fabrication by Infrared laser Radiation Exposure (FIRE)

Femtosecond laser micromachining (FLM) of transparent materials is a research field that experienced a great expansion since its first demonstration, at the end of the nineties. This technology evolved from a pure research activity to a consolidated and versatile alternative to standard microfabrication technologies. Moreover, FLM has the unique capacity to realize three dimensional structures, opening the possibility to new architectures and devices.
The basic idea underneath this technology is the non-linear absorption. The peak intensity that is reached in the focal volume, when a femtosecond laser pule is focused inside the material, is so high to induce non-linear phenomena, such as multiphoton absorption or tunnel ionization, followed by avalanche ionization. These phenomena induce an energy transfer and a material modification localized only inside the focal volume, without affecting the surroundings. By moving the sample with respect to the beam focal point, is then possible to obtain three dimensional modifications with arbitrary geometry.
The basic components that can be realized, inside transparent materials, with this technique are optical waveguides and fluidic microchannels (the latter need a chemical attack as secondary fabrication step). Further operations that can be performed with the same setup are welding, cutting, surface treatment, etc. The combinations of all these possibilities results very useful in the fabrication of integrated devices like optofluidic or optomechanical chips, that are then characterized in other laboratories of the research group.


This laboratory is equipped with instrumentations for femtosecond laser micromachining performed inside the substrate volume (in case of transparent materials), or surface structuring of any king of material.

LASER SOURCES

Two different laser sources are available. The first one is an Yb:KYW laser, working in mode-locking regime with cavity dumping. It provides pulsed with a duration of 300 fs and energy of 1.5 μJ with variable repetition rate, from 10 kHz to 1.1 MHz. This system has been developed in the framework of a collaboration between Max-Planck Institut di Heidelberg and HighQLaser GmbH, taking into consideration the target needs of FLM fabrication process. The second source is a commercial laser HighQLaser FemtoREGEN. It is based on an Ytterbium mode-locked oscillator with a pulse duration of around 400 fs, with pulse energy of 16 μJ and variable repetition rate, from 1 kHz to 1 MHz.

FABRICATION EQUIPMENT

Two different fabrication lines are present, and thanks to a system of mirrors, the two laser sources can be used on both the lines. The two lines are equipped with LBO crystal-based frequency duplication stage, so that it is possible to fabricate using either the fundamental laser frequency or their second harmonic. The first line is provided with an air cushion-based motion stage (Aerotech FiberGLIDE 3D), with a positioning precision of 50 nm and a maximum speed of 300 mm/s. The second line has a mechanical-air cushion hybrid motion stage with 100 nm precision and the possibility to use an SLM for multi-foci laser writing. Several microscope objectives are present, with different NA, from 0.3 to 1.4 to focus the laser beam into the substrate.



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