Fabrication of TiO2 Thin Film Based Fresnel Zone Plates by Nanosecond Laser Direct Writing

Fresnel zone plates (FZPs) have been gaining a significant attention by industry due to their compact design and light weight. Different fabrication methods have been reported and used for their manufacture but they are relatively expensive. This research proposes a new low-cost one-step fabrication method that utilises nanosecond laser selective oxidation of titanium coatings on glass substrates and thus to form TiO2 nano-scale films with different thicknesses by controlling the laser fluence and the scanning speed. In this way phase-shifting FZPs was manufactured where the TiO2 thin-films acted as a phase shifter for the reflected light while the gain in phase depended on the film thickness. A model was created to analyse the performance of such FZPs based on the scalar theory. Finally, phase-shifting FZPs were fabricated for different operating wavelengths by varying the film thickness and a measurement setup was built to compare experimental and theoretical results. A good agreement between these results was achieved and an FZP efficiency of 5.5% to 20.9% was obtained when varying the wavelength and the oxide thicknesses of the zones.