# Femtosecond Laser Textured Surfaces for Radiative Cooling: Black Metals
## 1 Introduction
### Towards Large Area Patterning  
All current solar cells trap light using random micro-textured surfaces; in Si, this is achieved through a KOH-etched pattern of random pyramids. With appropriate coating, that random texture contributes to anti-reflection properties. However, the best-performing limit of such light trapping (ray optics) cannot surpass the Lambertian 4\(n^{2}\) criterion, where \(n\) is the refractive index of the solar cell. We have developed fabrication protocols based on electron beam lithography, projection stepper lithography, and DLW for patterning different mask layers: Cr, Si\({}_{3}\)N\({}_{4}\), and AlO\({}_{x}\) used for wet and dry plasma etching of Si and Si-on-insulator surfaces for light trapping beyond the Lambertian limit [18]. The dry plasma etching is the most promising approach when the mask is patterned by fs-DLW using Bessel-like beams which have a long linear focal region [22]. Plasma etching was simplified to achieve fast and isotropic etching with a reduced directional bombardment of the surface. After passivation etching with the removal of \(\sim\)300 nm of the surface layer, the minority carrier lifetime becomes acceptable (a few milliseconds) for actual Si solar cells. In 2023, 23.1% efficiency of light-to-electrical power conversion was achieved using Si solar cells with photonic crystal (PhC) light trapping patterns defined by using Gaussian pulses, which are now changed to Bessel beam fs-DLW [22]. The PhC pattern fabrication for light trapping harnesses the wave nature of light to enhance sunlight absorption through interference.The method is amenable for fabrication over large practical areas up to the panel size of \(\sim\)1 m\({}^{2}\).  
The large area fs-laser patterning to control surface wetting and anti-icing properties on carbon-fiber-reinforced polymer (CFRP) is another promising field where texturing can be made without damaging fibers [23]. By using the two-liquid (water and ethanol) Owens-Wendt method, it was possible to determine the fs-laser ablated structure with the highest water-repelling property, a super-hydrophobic \(\theta_{c}\sim 159^{\circ}\) contact angle, for a square pattern (60 \(\upmu\)m period with 45 \(\upmu\)m width) [24]. The fraction of the textured surface in contact with a liquid can be determined by a graphical method using the Cassie-Baxter equation [25]. The high precision nature of fs-laser ablation is revealed in the ablation of the graphene placed on a sapphire wafer substrate for creating mesh patterns that guide epitaxial lateral overgrowth, facilitating a reduction in dislocations in GaN growth [26].  
Here, we reveal the anti-reflective performance of fs-laser patterned metals, including W, Al, and Cu over the near-to-mid IR spectral range. Laser ablation at fluences up to 28 J/cm\({}^{2}\)/pulse was orders of magnitude larger than the metal ablation at \(\sim\)20 \(\times\) 10\({}^{-3}\) J/cm\({}^{2}\) with a focal spot of a few tens of micrometers. At these conditions, which can be used for large industrial lasers, surface texturing produces nano to micro roughness over several orders of magnitude, from classical ripples of wavelength size to tens of micrometers. This hierarchical nature of surface structures was the key to the observed low reflectivity \(R<2\%\) over a very broad 1-10 \(\upmu\)m IR spectral range. Given [PERSON]'s scaling for emissivity \(E=1-R\) (in the absence of transmittance \(T\)), such surfaces can be used for radiative cooling in the 8-13 \(\upmu\)m atmospheric window as well as for heat exchangers, anti-icing surfaces, and the backside of solar cells including space applications. Such surfaces can be made on different metals and alloys by simple direct laser writing.