# Femtosecond Laser Textured Surfaces for Radiative Cooling: Black Metals
## 4 Conclusions and Outlook  
Laser ablation of the metals W, Al, and Cu by ultra-short 200 fs laser pulses at high 20-120 TW/cm\({}^{2}\) average intensity/irradiance with a large focal area (\(\sim\)60 \(\upmu\)m diameter) produced complex hierarchical 3D surface textures with features ranging from ripples of wavelength \(\sim\)1 \(\upmu\)m to inverted pyramidal cones (most typical on Al) with openings of tens-of-micrometers. The most complex 3D textures with the largest depth changing were on Al, which has the lowest melting temperature among the metals investigated here. The primary chemical change observed in the outer surface layers through XPS was the formation of oxides, which correlates with the reflectance reduction due to the lower refractive index of metal oxides. Interestingly, the ablation, which starts as an electron removal from the surface after energy deposition, is synonymous with oxidation, which is the loss of electrons by definition. Ablation at an energy deposition up to 100\(\times\) times larger than the threshold fluence/intensity facilitates a high local temperature, material flow driven by strong thermal gradients, oxidation, and subsequent thermal quenching. This process results in an ablation depth of 10\({}^{1}\)-10\({}^{3}\)\(\upmu\)m, producing complex 3D inverted pyramidal-shaped oxides that contribute to approximately a 55-95% reduction in reflectance compared to the original metal surfaces.  
\begin{table}
\begin{tabular}{l l l} \hline \hline
**Sample** & **Metal Atomic Percentage** & **Non-Metal Atomic Percentage** \\ \hline W – no ablation & W: 100.00\% & O: 0\% \\ W – 4 & W: 70.57\% & O: 29.43\% \\ Al – no ablation & Al: 93.09\% & O: 6.47\% \\ Al – 1 & Al: 52.92\% & O: 46.2\% \\ Cu – no ablation & Cu: 38.63\% & O: 8.68\% C: 38.63\% \\ Cu – 1 & Cu: 72.22\% & O: 27.28\% C: 0\% \\ \hline \hline \end{tabular}
\end{table}
Table 4: Elemental analysis using EDS, according to the K\({}_{a}\) lines.  
In the context of radiative cooling, the oxidation and passivation of metal surfaces are expected to enhance thermal emissivity, which is structure and morphology-dependent. Metals are naturally reflective with low emissivity, but the formation of oxide layers and increased surface roughness, which render them anti-reflective, are favorable for radiative cooling, especially at elevated temperatures relevant to industrial and space applications. This work has demonstrated that laser ablation with an industrial fs-laser of 80 W power at a fast scan rate can be used to create low-reflectivity surfaces across the 2-20 \(\upmu\)m near- to mid-IR spectral window.  
Conceptualization, R.B., H.-H.H. and S.J.; methodology, N.Z., R.B., H.-H.H. and T.K.; software, R.B., M.K. and T.K.; validation, N.Z., D.S., S.P., D.M.Z., L.G. and P.R.S.; formal analysis, N.Z. and R.B.; investigation, N.Z.; resources, P.R.S.; data curation, R.B., N.Z. and Y.N.; writing--original draft preparation, S.J., N.Z. and H.-H.H.; writing--review and editing, all the authors; visualization, N.Z. and S.J.; supervision, R.B.; project administration, R.B.; funding acquisition, R.B. and P.R.S. All authors have read and agreed to the published version of the manuscript.  
This research was partly funded by the Australian Research Council Linkage LP220100153 and Discovery DP240103231 grants.  
All the data are presented within this manuscript.  
We acknowledge support by Swinburne University of Technology for lab allocation to test 80 W fs-laser machining system. We are grateful to [PERSON] for radiation safety discussions.  
Author [PERSON] was employed by the company Quoba Systems Pty. Ltd. All authors declare that the research was conducted without any commercial or financial relationships that could be construed as a potential conflict of interest.