Femtosecond laser techniques for fabrication of QTF and tailoring of its optical properties for gas sensing applications

This Ph.D. thesis reports on the development of femtosecond (fs) laser machining processes fabrication of QTF and tailoring of its optical properties for gas sensing applications . Pursuing such a strategy would allow for the exploitation of many advantages of fs-laser processing such high precision and quality of processing, low environmental impact and flexible production processes. The issue related to the transparency of quartz in the 1 to 5 μm range has been addressed by developing a blackening approach for quartz crystals. This strategy consists in the laser surface texturing of the target material, in order to enhance its optical absorption in a particular wavelength range. In particular, a femtosecond pulsed laser was used to create matrices-like patterns on the surfaces of quartz crystal wafers. These matrix patterns were designed to reduce and flatten the quartz transmittance across the wavelength range of interest. Finally, a proof of concept was demonstrated by implementing two laser-textured QTF as photodetector in a LITES setup for detection of two water vapor absorption features at 1.39 μm and 7.38 μm. An environmental-sustainable method for QTF prototype production was developed through direct laser cut of quartz wafers exploiting femtosecond laser ablation. This activity was performed in collaboration with the FOLAS Lab research group in their facilities at the Faculty of Mechanical Engineering at University of Ljubljana. This method exploited a milling channel approach to cut through the complete depth of the target wafers. This process was characterized as function of the femtosecond laser working parameters and was then employed for the cut of actual QTFs. The resonant properties of the laser-cut devices were evaluated experimentally by means of photoacoustic excitation and they were found to be comparable both to their simulated behavior and to the performances of standard QTFs, thus confirming the validity of this manufacturing technique.