Femtosecond laser-induced refractive index change and phase-type Fresnel zone plate in fluorotellurite glass

We report a phase-type Fresnel zone plate (FZP) fabricated by femtosecond (fs) laser-induced refractive index change (RIC), which was embedded in a newly developed fluorotellurite glass. The effect of laser direct-writing parameters on the morphology of the laser-induced structures was studied and negative RIC was observed. A maximum RIC of 4.42×10-3 was achieved at laser inscription energy of 0.13 µJ. Additionally, structure change from TeO4 units to TeO3- units triggered by fs laser irradiation is verified by Raman spectra measurements for the first time, to the best of our knowledge, and demonstrated to be a vital factor for the negative RIC. On this basis, we firstly fabricated a two-level phase-type FZP designed to work at 1340 nm inside such fluorotellurite glass. The phase-type FZP exhibits strong IR beam focusing and excellent imaging capabilities with a resolution of 128 line pairs per millimeter (lp/mm). This study reveals the facts that such novel fluorotellurite glass has good processibility with fs laser direct writing and made it a potential alternative material for micro-optics in NIR applications.