Design and Fabrication of Dual-Scale Broadband Antireflective Structures on Metal Surfaces by Using Nanosecond and Femtosecond Lasers.

Rui Lou,Guangying Li,Qing Liu,Guodong Zhang,Guanghua Cheng,Xuelong Li

doi:10.3390/mi11010020

Abstract

Antireflective surfaces, with their great potential applications, have attracted tremendous attention and have been the subject of extensive research in recent years. However, due to the significant optical impedance mismatch between a metal surface and free space, it is still a challenging issue to realize ultralow reflectance on a metal surface. To address this issue, we propose a two-step strategy for constructing antireflective structures on a Ti-6Al-4V (TC4) surface using nanosecond and femtosecond pulsed lasers in combination. By controlling the parameters of the nanosecond laser, microgrooves are first scratched on the TC4 surface to reduce the interface reflection. Then, the femtosecond laser is focused onto the sample surface with orthogonal scanning to induce deep air holes and nanoscale structures, which effectively enhances the broadband absorption. The antireflection mechanism of the dual-scale structures is discussed regarding morphological characterization and hemispherical reflectance measurements. Finally, the modified sample surface covered with micro-nano hybrid structures is characterized by an average reflectance of 3.1% over the wavelengths ranging from 250 nm to 2250 nm.

Highlights

Surface micro- and nanostructuring of metal, with potential applications in antireflection [1,2,3,4,5,6,7,8], transdermal enhancement [9], and wettability alternation [10,11], has gained great interest from researchers in recent years
Hierarchical micro- and nanostructures inspired by nature were recently designed and demonstrated to have multiple functions in metal surface modification, drawing significant attention
The hierarchical structures can effectively address the optical impedance mismatch between metal and free space, achieving ultralow reflection over the broadband spectrum on a metal surface

Summary

Introduction

Surface micro- and nanostructuring of metal, with potential applications in antireflection [1,2,3,4,5,6,7,8], transdermal enhancement [9], and wettability alternation [10,11], has gained great interest from researchers in recent years. ZnO substrates covered by biomimetic hierarchical structures exhibit low reflection over a wide spectral range [12] Due to their excellent performance in surface antireflection, such hierarchical structures have been applied in various fields, such as artificial blackbodies, photodetectors, infrared imaging, stray light shields, space-borne optics, etc. Several techniques for fabricating micro- or nanostructures have been proposed, such as chemical etching [17], coating [18], reactive ion etching [19], and pulsed laser processing [20,21]. Zheng et al used a hybrid method consisting of ultrafast-laser-assisted texturing and chemical fluorination treatment to fabricate a black titanium alloy by efficiently enhancing the surface broadband antireflection [30]. Microstructures and nanostructures were fabricated by the nanosecond and the femtosecond lasers, respectively, to enhance the metal surface broadband absorption. The morphology and formation mechanism of the hierarchical micro- and nanostructures were carefully studied

Materials

Preparation of the Micro-Nano Hybrid Structures