Spectral absorption control of femtosecond laser-treated metals and application in solar-thermal devices.

Direct femtosecond (fs) laser processing is a maskless fabrication technique that can effectively modify the optical, electrical, mechanical, and tribological properties of materials for a wide range of potential applications. However, the eventual implementation of fs-laser-treated surfaces in actual devices remains challenging because it is difficult to precisely control the surface properties. Previous studies of the morphological control of fs-laser-processed surfaces mostly focused on enhancing the uniformity of periodic microstructures. Here, guided by the plasmon hybridisation model, we control the morphology of surface nanostructures to obtain more control over spectral light absorption. We experimentally demonstrate spectral control of a variety of metals [copper (Cu), aluminium (Al), steel and tungsten (W)], resulting in the creation of broadband light absorbers and selective solar absorbers (SSAs). For the first time, we demonstrate that fs-laser-produced surfaces can be used as high-temperature SSAs. We show that a tungsten selective solar absorber (W-SSA) exhibits excellent performance as a high-temperature solar receiver. When integrated into a solar thermoelectric generation (TEG) device, W-SSA provides a 130% increase in solar TEG efficiency compared to untreated W, which is commonly used as an intrinsic selective light absorber.

Comparative study of femtosecond laser-induced structural colorization in water and air.

The study of femtosecond laser structural coloring has recently attracted a great amount of research interest. These studies, however, have only been carried out in air. At the same time, laser ablation has also been actively studied in liquids as they provide a unique environment for material processing. However, surprisingly, structural coloring has never been performed in liquids. In this work, we perform the first study of metal structural coloring in liquid and compare the results to metal structural coloring in air. Colors created in liquid are formed by nanoparticle-induced plasmonic absorption and result in a range of colors transitioning from purple to orange. Surface structures formed in liquid are less hierarchical and more uniform than those formed in air, producing a surface with a much higher reflectance due to reduced light trapping, resulting in a more vibrant color. However, colorization formed in water suffers from less uniform colorization due to turbulence at the air-water and water-target interfaces, resulting in slight changes to the laser beam's focus during processing. Finally, finite-difference-time-domain simulation based on the measured surface structures is used to understand the role of plasmonic resonance in colorization.

Robust mold fabricated by femtosecond laser pulses for continuous thermal imprinting of superhydrophobic surfaces.

Superhydrophobic surfaces rely on a large number of surface micro/nano structures to increase the roughness of a material. Producing such structures is possible through a multitude of relatively slow methods; however, economic and large scale production of superhydrophobic surfaces require using a fast process on a cheap substrate. Here, we used femtosecond laser processing to fabricate micro and nanostructures on tungsten carbide that we use as a mold to thermally imprint polypropylene sheets. The fabricated tungsten carbide mold was used to imprint more than twenty superhydrophobic polypropylene sheets before mold contamination reduces the surface contact angle below 150°.Using Toluene solution, the mold is subsequently capable of being cleaned of contamination from polypropylene residue and reused for further imprinting. Ninety thermoplastic imprints were conducted using a single tungsten carbide mold with only minimal structural degradation apparent on the micro/nano structured surface.

Femtosecond laser­induced herringbone patterns.

Femtosecond laser-induced herringbone patterns are formed on copper (Cu). These novel periodic structures are created following s-polarized, large incident angle, femtosecond laser pulses. Forming as slanted and axially symmetric laser-induced periodic surface structures along the side walls of ablated channels, the result is a series of v-shaped structures that resemble a herringbone pattern. Fluence mapping, incident angle studies, as well as polarization studies have been conducted and provide a clear understanding of this new structure.