Significance Numerous multi-scale surfaces are available in nature with special micro-nano structures, such as lotus leaves, rice leaves, rose petals, gecko toes, shark skin, butterfly wings, and insect compound eyes. Scientists confirm that, among these multi-scale structures, the micro-scale structures function to strengthen the mechanical stability for protecting the nanostructures, while the nanoscale structures exhibit 13 magical functions, including superhydrophobicity, superhydrophilicity, directional wetting, self-cleaning, drag reduction, reversible adhesion, directional adhesion, anti-reflection, structural color, high sensitivity, selective filtration, cyto-biocompatibility, and the regulation of cell behaviors. How to artificially fabricate these bionic multi-scale structures to achieve the goal of imitating and surpassing nature is a major topic in the fields of materials and manufacturing.

An ultrafast laser is a pulsed laser with a pulse duration ranging from tens of femtoseconds to 10 picoseconds. Owing to its extremely short pulse duration, ultrafast lasers have high instantaneous energy density and high pulse repetition frequency. This enables its photon energy to interact directly with the internal lattice and electronic structures of materials in a considerably short time (the magnitude of pulse width) through various phase transformation mechanisms, such as phase and Coulomb explosions. Therefore, ultrafast lasers can process materials rapidly and accurately. These lasers can significantly reduce the heat-affected zone in the ablated area and then achieve extremely high accuracy and resolution. Thus, such a laser is a reliable tool for fabricating various micro-nano structures with high flexibility. However, owing to the diffraction limit, its fabrication capability for nanostructures is far more restricted, along with its lower fabrication efficiency.

Progress For the past decade, the Laser Materials Processing Research Center, Tsinghua University, has been equipped with a new generation of high-power ultrafast lasers (a pulse duration of 400 fs--10 ps, a repetition frequency of 100 kHz--2 MHz, and an average power of 40--100 W), supported by various projects, such as the National Key Research and Development (R & D) Program of China, the 973 project, the National Natural Science Foundation of China, the major international cooperation projects, and the Tsinghua University Initiative Scientific Research Program. Based on the suitable equipment and projects, a research team led by Prof. Zhong Minlin has performed a systematic investigation to expand the ultrafast laser fabrication ability for micro and nano structures and explore the functionalization of these fabricated bionic micro-nano structures. This team developed a series of novel approaches for micro-nano structure fabrication and dual-scale precise modulation through ultrafast lasers. Moreover, the team discovered several innovative applications of micro-nano structured surfaces in relation to the aspects of superhydrophobicity, high anti-reflection, high sensitivity, and biomedical detection.

Specifically, they achieved an efficient fabrication of large-area superhydrophobic metal surfaces with a higher contact angle and lower rolling angle than those of lotus leaves (Fig. 2). The team fabricated novel surfaces comprising periodical micro-pillar arrays that were covered by dense nanograsses and dispersed microflowers, exhibiting the highest available Cassie state stability and lowest ice adhesion strength compared to the state-of-the-art superhydrophobic surfaces (Fig. 4). Inspired by cacti, beetles, and redbud leaves, they developed a patterned superhydrophobic/superhydrophilic surface using an ultra-contrasting wettability venation network with hierarchical micro-nano structures as the skeleton for the massive water collection with high efficiency (Fig. 6). The team proposed a unique oil-triggered surface (OTS) by combining the “lotus-leaf-like” superhydrophobicity, the “nepenthes-like” slippery liquid-infused surface hydrophobicity, and the “re-entrant structure-induced” superamphiphobicity to achieve a high throughput manipulation of droplets, avoiding pinning, droplet loss, and cross-contamination (Fig. 8). They fabricated two kinds of surfaces: micropillar arrays accompanying nanowires and microcolumns covered by nanoparticles to obtain ultra-low infrared reflectivity and visible reflectivity, respectively (Figs. 10 and 12). Further, the research team explored a new field of micro/nano-textured electrodes for efficient hydrogen and oxygen production through water splitting (Figs. 15--17). They produced a novel patterned surface-enhanced Raman scattering (SERS) platform consisting of a superhydrophilic central area surrounded by superhydrophobic structures with an extremely high Cassie-Baxter state stability. Based on the evaporation enrichment of water droplets on such a platform, they achieved atto-molar SERS detection (Fig. 20). Furthermore, the team addressed challenging issues in SERS research and applications, such as stability, uniformity, and manufacturability, thus to expand SERS applications to cancer diagnosis and food safety evaluation. Based on the above works, the research team has published more than 100 papers, granted over 20 patents, and developed more than 50 know-hows. More importantly, the technologies achieved significant applications in various critical fields.

Conclusions and Prospects Fabricating various micro-nano structures by ultrafast laser and realizing biomimetic functionalization is an attractive research area. However, challenges still remain, such as the fabrication of typical nanostructures with a size of 1--100 nm via the breaking of the diffraction limit, the design of novel diverse functional micro-nano structures and their free fabrication, and the efficient fabrication of large-area micro-nano structures. Facing these challenges, the authors summarize their research results in the past decade and select four representative research areas: the controllable fabrication of micro-nano structures for special wettings, the two-stage tuning of micro-nano structures for high anti-reflection, the nanostructures for electrocatalytic water splitting, and the laser-induced patterned surfaces for SERS. This paper is written for the special issue of the Journal of Chinese Lasers—Celebration of the 110th Anniversary of Tsinghua University—to summarize the past and address the future as well as to exchange, discuss, and promote R & D in the field. All comments are welcome.