# Femtosecond Laser Textured Surfaces for Radiative Cooling: Black Metals
## 2 Experimental: Materials and Methods  
### Femtosecond Machining Using Modular Station  
An fs-laser setup for large-area 3D machining was assembled by Quoba Systems, delivering 80 W average power at repetition rates up to 2 MHz (1030/515/343 nm, 200 fs) with translational/rotational stages, scanners with theta lens, and fabrication control software.  
The laser machining setup, beam delivery, and focusing have been described elsewhere [27]. Such fs-fab stations can be assembled and integrated from separate components, as demonstrated in Ref. [27]. Two high-precision rotational axes are available for a full 5-axis fabrication capability. Integration of high-\(NA\) objectives for resolution down to sub-\(\upmu\)m fabrication is possible along with the setup of a Bessel beam for high-speed cutting of transparent materials and patterning on non-flat surfaces. An external pulse compressor (n\({}^{2}\) Photonics GmbH, Hamburg, Germany) for 50 fs and 10 fs pulses in one or two stages, respectively, can be used with this setup. The information required to determine processing conditions is repeated in a shortened form to facilitate discussion.  
The multi-axis fs-laser CNC micro-machining system is based on an fs-laser, high-speed galvanometer scanner, 5-axis sample positioning stages, system control, and software. The 80 W Carbide (Light Conversion) laser has \(\lambda=1030,515,343\) nm wavelengths, pulse duration \(t_{p}=200\) fs, maximum pulse energy of 0.8 mJ, and repetition rate up to \(f=2\) MHz with tunable GHz and MHz bursts. The implemented polarizer can be switched between linear and circular polarizations.  
A telecentric fused-silica F-theta lens (Jenoptic, JENar) with a \(F=160\) mm focal length was used with a galvanometer scanner (Raylase, SUPERSCAN IV). The laser beam diameter \(D=5\) mm. This is defined by the F-number \(F_{\texttt{F}}=F/D=32\) related to the numerical aperture \(NA=1/(2F_{\texttt{F}})=0.0156\). The theoretical diameter at focus \(2r=\frac{43}{\pi}F_{\texttt{F}}=42\)\(\upmu\)m while the depth-of-focus is \(DoF=\frac{83}{\pi}F_{\texttt{F}}^{2}=2.69\) mm for \(\lambda=1030\) nm. The axial extent of the pulse defined by the pulse duration \(ct_{p}=60\)\(\upmu\)m was much shorter than the axial extent of the focal region. The laser beam quality factor was \(M^{2}=1.2\) defining the beam diameter at focus 50.5 \(\upmu\)m and \(DoF=3.87\) mm. The F-theta lens focusing produced a beam with diameter \(2r=60\)\(\upmu\)m for the entire scanning field of 50 \(\times\) 50 mm\({}^{2}\). This value was used for the calculation of fluence and intensity. The focal position was fixed without an axial position change during the ablation of deeper patterns.  
High precision, direct drive stages (Standa) with non-contact optical encoders ensured high positioning accuracy: 0.5 \(\upmu\)m bidirectional repeatability, 1 \(\upmu\)m absolute accuracy over the whole travel area of 400 \(\times\) 400 mm\({}^{2}\). The acceleration and speed could reach 2 g and 2 m/s, respectively. System controllers (Polaris) were used to synchronize all axes with the laser providing an infinite field of view (IFOV) with split motion tasks between stages and scanners to achieve optimized scanning speed for accuracy and minimal stitching errors.  
For fs-laser fabrication on metals, a \(1.1\times 0.5\) cm\({}^{2}\) W sheet, and \(1\times 1\) cm\({}^{2}\) metal coupons of Al and Cu were used for surface texturing experiments. All samples were not polished. W samples were ablated at a constant fluence per pulse of 24.76 J/cm\({}^{2}\) with various pulse-to-pulse spacing (pulse spacing, 10.0, 1.0, 0.5, 0.25 \(\upmu\)m) and number of passes \((1,5)\) shown in Figure 1a. Al and Cu coupons were fabricated with a \(5\times 4\) matrix labeled as 1-16 as shown in Figure 1b,c, respectively. Each fabricated area was \(1.4\times 1.4\) mm\({}^{2}\) with corresponding conditions of fluence per pulse (28.3, 22.6, 17.0, 11.3, 5.7 J/cm\({}^{2}\)) and pulse spacing (7, 14, 28, 16 \(\upmu\)m).