Highly Regular Laser-Induced\nPeriodic Surface Structures\non Titanium Thin Films for Photonics and Fiber Optics

Modern photonic devices demand low-cost, scalable methods\nfor creating\nperiodic patterns over diverse surfaces including nonplanar and tipped\nones, the examples of which can be readily found in fiber optics.\nLaser-induced periodic surface structures (LIPSS) offer an attractive\nroute for fabricating such patterns in a single-step straightforward\nprocedure, where the temporal and spatial locality of the self-interference\neffects ensure robustness against variations of the laser processing\nparameters. In this work, we show the LIPSS-assisted oxidation of\nthin titanium films by near-IR femtosecond laser pulses as a promising\ntechnology for the production of regular gratings consisting of rutile\nridges. The self-terminating nature of the oxidation process allows\nthe predefinition of the grating relief by the initial thickness of\nthe titanium film, rendering the fabrication process with reproducibility\nand stability to variations of the laser parameters. LIPSS were found\nto act as gratings providing a diffraction efficiency above 7%. Such\ngratings were recorded over sidewalls of the optical fibers as well\nas their endfaces, allowing free space light coupling into the guided\nmode at the incidence angles up to 65° exceeding fiber acceptance\nangles. Additionally, gratings recorded over the polished side of\nthe D-shaped fiber were found to act as a Bragg grating manifesting\nitself through a narrow resonance (<1 nm) in the reflection spectrum\nof fiber-guided light and suggesting possible applications in sensing\nand light management. Finally, by exploiting the high regularity of\nthe formed gratings, we also demonstrated resonant coupling of the\nincident to the surface plasmon waves on the silver-coated LIPSS.\nWith the large resonance amplitude and the Q-factor as large as 150,\nLIPSS were justified as an easy-to-fabricate template for plasmonic\ngrating production with high potential for plasmonic biosensing and\nnonlinear optics.