# Femtosecond Laser Textured Surfaces for Radiative Cooling: Black Metals
## 3 Results and Discussion
### Reflectivity of Laser Ablated Surfaces at IR Spectral Range  
The ablated area on W, Al, and Cu samples shows a black appearance in the visible spectral range. The reflectance of these black surfaces was tested at near-to-mid IR range 1.5-18 \(\upmu\)m shown in Figure 3 corresponding to the labeled areas on the samples (Figure 1). The IR reflectance shows a clear reduction in reflectance for all ablated metals: \(R<40\%\) (W), \(R<15\%\) (Al), \(R<55\%\) (Cu), compared with the non-ablated surfaces \(R\sim 98\%\) (W), \(R\sim 90\%\) (Al), and \(R\sim 95\%\) (Cu). The anti-reflectance effect is positively related to the energy deposited on metal surfaces, i.e., the higher energy deposition (higher fluence and smaller pulse spacing) produces surfaces with a smaller \(R\) at the IR window. The lowest reflectance for each sample was \(R<5\%\) (W-6), \(R<3\%\) (Al-1) and \(R<6\%\) (Cu-1) corresponding to a formidable reflectance decrease of around 90%. The formation of Al\({}_{2}\)O\({}_{3}\) is recognizable by its absorbance around \(\sim\)6 \(\upmu\)m in the reflectivity spectra. The so-called water absorption band associated with a 3-3.5 \(\upmu\)m window is due to Al-OH (a decrease in reflectivity due to absorption). Figure 4a,b summarizes the reflectance \(R\) over the measured \(\lambda\)= 1.5-18 \(\upmu\)m window for the \(4\times 5\) matrix of \(Spacing\times Fluence\) for Al and Cu. Apparently, the highest fluence at the strongest pulse-to-pulse overlap produced the lowest reflective surfaces for Al and Cu. This corresponded to the largest ablated volume as shown in Figure 4c. For the different \(\Delta_{x,y}\) pulse overlaps, a linear scaling of ablated volume \(V_{al}\propto F_{p}\) as well as the depth \(H_{ab}\propto F_{p}\) was observed. The surface morphology at different magnifications was inspected with SEM and shown in Figure 5 for specific fluence and pulse overlap conditions. Very different surface nano-micro topographies are observed from ripples with a period close to the laser wavelength \(\sim\)1 \(\upmu\)m to the structures with dimensions close to the focal spot size of \(\sim\)40 \(\upmu\)m. The most anti-reflective pattern on Al (in (b)) had the richest 3D topography. The conical inverted pyramidal structures are apparent as holes on the surface were clearly observed on Al, which has the lowest melting temperature of 660 \({}^{\circ}\)C compared with Cu (1084 \({}^{\circ}\)C) and W (3422 \({}^{\circ}\)C). Figure 3: The IR reflectance spectra at the wavelength of 1.5–18 \(\upmu\)m for (**a**) tungsten, ablated with different fluence for the largest pulse-to-pulse overlap \(\Delta_{x,y}=7\)\(\upmu\)m; (**b**) aluminum (ablated with different fluence for the largest pulse-to-pulse overlap \(\Delta_{x,y}=7\)\(\upmu\)m) and (**c**) different pulse spacing (at \(F_{p}=28.3\) J/cm\({}^{2}\)); copper ablated with (**d**) different fluence (for the largest pule-to-pulse overlap \(\Delta_{x,y}=7\)\(\upmu\)m) and (**e**) different pulse spacing at \(F_{p}=28.3\) J/cm\({}^{2}\). The most complex 3D textures were observed on Al, which has the lowest melting temperature among tested metals. The feature size of conical holes in Al is close to the full width at half-maximum (FWHM) measure of the focal spot size. The photon pressure exerted by a pulse of irradiance \(I_{p}=100\) TW/cm\({}^{2}\) or \(10^{18}\) W/m\({}^{2}\) is \(P_{ph}=I_{p}/c\approx\frac{1}{3}\times 10^{10}\) Pa = 3.3 GPa, where \(c\approx 3\times 10^{8}\) m/s is the speed of light. This pressure is applied when the fs-laser pulse energy is deposited (absorbed), while the reflection doubles it via the momentum transfer due to the reversal of the photon direction. Hence, the laser peening is present. The imparted pressure is a contributing factor to the formation of surface textures guided by an interplay of Al softening/melting together with fast thermal quenching due to high thermal conductivity.