Highly Regular Laser-Induced Periodic Surface Structures on Titanium Thin Films for Photonics and Fiber Optics

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