SUBMICRON PERIODIC STRUCTURES IN METAL OXIDE THINFILM VIA LASER ABLATION AND THERMAL OXIDATION

Periodic structures, featuring periodic modulation of optical constants, exhibit polarization control, spectral and angular selectivity of light, and thus were found extremely useful for laser radiation manipulation. However, fabrication technologies of periodic structures have to face strict requirements for laser optics, especially for high-power systems. Despite a sub-micrometer scale of periodicity required for visible and infrared spectral region applications, the next significant criteria are optical transparency and durability, which require a completely inorganic composition of all constituent materials. Polymers are quite easily processed and can be transparent in the target spectral regions, but they exhibit lower optical resistance compared with fully inorganic materials such as metal oxides [1]. Although direct patterning the latter while striving for sufficiently low surface roughness is a non-trivial task, especially if sub-micrometric periodicity is required. To get around a direct patterning of metal oxides, a metallic thin film deposited on a glass can be structured. Subsequent annealing at elevated temperatures promotes a metal oxidation reaction, resulting in a fully inorganic, transparent periodic structure. Several structuring techniques capable of submicron surface patterning have been described in the literature, including lithography, ion beam etching and ultrashort laser processing. Among these, only the latter can circumvent the complexities associated with other techniques, such as contamination during manufacturing, time-consuming and expensive large-scale production. Based on this, ultrashort laser processing, specifically Direct Laser Interference Patterning (DLIP), was chosen for this study.

In this work [2], we present a two-step method for the fabrication of submicron metal oxide periodic structure (Fig.1), avoiding complexities of direct structuring of dielectric materials. A linear surface pattern with a spatial periodicity of 750 nm was fabricated in pure tantalum metal thin film via the DLIP technique, followed by annealing to form the tantalum oxide periodic structure. To achieve this, prior investigation of non-patterned metallic thin films annealing was done in search of appropriate oxidation parameters. The fabricated structures were characterized in terms of composition, surface morphology, and optical properties, demonstrating the capabilities of the proposed method. This work was supported by the PerFIN project from the Research Council of Lithuania (LMTLT), agreement No S-MIP-22–80.