Femtosecond‐Laser Direct Writing of Metallic Micro/Nanostructures: From Fabrication Strategies to Future Applications

DOI: 10.1002/smtd.201700413 self-assembly,[25,26] optical/electron-beam lithography,[27,28] focused ion beam,[29,30] and 3D printing,[31,32] have demonstrated distinct advantages and revealed great potential for the flexible nanostructuring of metals. However, all of these technologies suffer from insuperable problems in fabricating arbitrary 3D metallic micro/ nanostructures. For instance, chemical routes are versatile and powerful for preparing metallic nanostructures ranging from 0D nanoparticles to 3D features,[23,24] but metallic micro/nanostructures with complex configurations cannot be synthesized directly. Self-assembly approaches provide a controllable avenue to construct 2D and 3D micro/nanostructures by making full use of the interactions between individual nanobuilding-blocks,[25,26] but the assembly process cannot be predesigned freely. To construct metallic structures according to design, lithography techniques have been widely adopted for fabricating 2D metal patterns,[27,28] and the resolution can currently reach a few nanometers; nevertheless, lithography strategies are naturally 2D processing routes and are not capable of 3D fabrication. Recently, 3D-printing technology has made it possible to construct 3D metallic structures on a macroscopic scale.[31,32] However, it is difficult to improve the resolution to the microscale level, not to mention the nanoscale. Currently, the designable fabrication and integration of complex 3D metallic micro/nanostructures for the development of metallic micro/nanodevices remains a challenge. As a powerful 3D micro/nanofabrication technique, femtosecond-laser direct writing (FsLDW) has emerged as a nanoenabler for the designable fabrication of micro/nanostructures based on a wide range of materials.[33–42] Especially for metals, FsLDW technology is distinguished by its mask-free processing capability, arbitrary shape designability, high spatial resolution, ease of integration, and suitability for various nonplanar substrates. In the past decade, with the help of FsLDW technology, great success has been achieved in the fabrication and integration of metallic micro/nanostructures, promoting the rapid development of a series of scientific fields. Here, we summarize the recent advances of this powerful technology in the fabrication of metallic micro/nanostructures and highlight its contributions to metallic micro/nanostructure-enabled functional devices. The general concept of FsLDW strategies and future applications is illustrated in Figure 1. First, several The past decade has seen growing research interest in developing novel fabrication methodologies for metallic micro/nanostructures toward the rational design and integration of 0D to 3D metallic configurations. Among the various fabrication techniques, femtosecond-laser direct writing has emerged as a prominent approach to reach this goal and greatly facilitates the development of metallic micro/nanostructure-enabled applications. Here, a series of femtosecond-laser-mediated fabrication strategies, including the two-photon reduction of metal ions, template-assisted metal coating and metal filling, photodynamic assembly of metal nanoparticles, and selective nanoparticle sintering, are summarized. Additionally, abundant applications in microelectronics, metamaterials, near-field optics, microfluidics, micromechanics, and microsensors are introduced in detail. Current challenges and future perspectives in this field are also discussed. Femtosecond-Laser Direct Writing