Cylindrically Focused Nonablative Femtosecond Laser Processing of Long-Range Uniform Periodic Surface Structures with Tunable Diffraction Efficiency

Periodic surface structures are core components for controlling the dispersion and steering characteristics of light. Here, a mask-free approach using nonablative femtosecond laser processing is proposed and demonstrated to fabricate extremely long-range uniform periodic surface structures on silicon with tunable diffraction efficiency. First, a cylindrically focused femtosecond laser scans over silicon substrates to efficiently produce large-area periodic modified stripes in a nonablation regime. Second, the modified stripes act as fine etch stops to generate the desired structures on sample surfaces during the subsequent chemical etching process. The structures produced by the method achieve optimal long-range uniformity compared to the reported laser-induced periodic surface structures, which possess a minimum divergence of structure orientation angles of <5 degrees. In addition, the optical characteristics of the prepared structures are measured experimentally. Distinguishable polychromatic diffraction patterns can be clearly observed by broadband light irradiation. Significantly, the chemical etching process endues the structures with ingenious morphology controllability, so that the diffraction efficiency of the incident light can be flexibly tuned, which exhibits a near-linear function of the etching duration. Such morphology-controllable periodic surface structures may facilitate applications in broad fields, such as optical communications and optical sensors.