# Tunable Coloring via Post-Thermal Annealing of Laser-Processed Metal Surface
## 3 Results and Discussion
### Optical Properties
For OL1 substrates, the surface peak intensities increase substantially after annealing, although not at a rate proportional to the annealing temperature or time. At 300 \({}^{\circ}\)C, the spectral peak intensity of Ti substrate with OL1 annealed at 300 \({}^{\circ}\)C for 0.5 h (OL1\({}_{300^{\circ}\text{C},0.5\text{h}}\)) reaches 202% and increases to 233% for Ti substrate with OL1 annealed at 300 \({}^{\circ}\)C for 1.0 h (OL1\({}_{300^{\circ}\text{C},1.0\text{h}}\)). It drops back to 216% for Ti substrate with OL1 annealed at 300 \({}^{\circ}\)C for 1.5 h (OL1\({}_{300^{\circ}\text{C},1.5\text{h}}\)). At an annealing temperature of 400 \({}^{\circ}\)C, the peak intensity of Ti substrate with OL1 annealed at 400 \({}^{\circ}\)C for 0.5 h (OL1\({}_{400^{\circ}\text{C},0.5\text{h}}\)) increases to 200%, but drops to 192% for Ti substrate with OL1 annealed at 400 \({}^{\circ}\)C for 1.0 h (OL1\({}_{400^{\circ}\text{C},1.0\text{h}}\)), and to 158% for Ti substrate with OL1 annealed at 400 \({}^{\circ}\)C for 1.5 h (OL1\({}_{400^{\circ}\text{C},1.5\text{h}}\)). At an annealing temperature of 500 \({}^{\circ}\)C, the peak intensities of Ti substrate with OL1 annealed at 500 \({}^{\circ}\)C for 0.5 h (OL1\({}_{500^{\circ}\text{C},0.5\text{h}}\)) and Ti substrate with OL1 annealed at 500 \({}^{\circ}\)C for 1.5 h (OL1\({}_{500^{\circ}\text{C},1.5\text{h}}\)) increase to 138% and 193%, respectively, but drop to 111% for OLI\({}_{500^{\circ}\text{C},1.5\text{h}}\). The overall reflectance of all OL1 substrates increases after annealing, resulting in a brighter surface macroscopic color. As previously shown in Figure 1a, it is observed that the yellow color appears in the OL1 group with the macroscopic purple color. However, the light-yellow color may not significantly change the macroscopic color  
Figure 1: Macroscopic color and microscope images of **(a)** sample oxide layer 1 (OL1) and oxide layer 2 (OL2) before and after the thermal annealing at 300, 400, and 500 \({}^{\circ}\)C, duration of 0.5, 1.0, and 1.5 h, respectively. **(b)** SEM image of the oxide and substrate surface **(c)** FIB-SEM section image of the oxide thickness. (FIB-SEM: focused ion beam combined with scanning electron microscope)  
hue in the OL1 group. For example, while OL1 is annealed at 300 \({}^{\circ}\)C, it is found that the reflectance ratio of 500 nm (yellow) and 400 nm (purple) is approximately 0.6 (75:125) in the reference sample and increases to 0.76 (175:230) after thermal annealing. Hence, the yellow color actually affects the reflectance, but this may be not enough to change the color hue. For OL2 substrates, the surface maximum intensities of the spectra also increase after annealing. At 300 \({}^{\circ}\)C, the maximum intensities are 112%, 239%, and 151% for Ti substrate with OL2 annealed at 300 \({}^{\circ}\)C for 0.5 h (OL2300\({}^{\circ}\)C,0.5h), Ti substrate with OL2 annealed at 300 \({}^{\circ}\)C for 1.0 h (OL2300\({}^{\circ}\)C,1.0h), and Ti substrate with OL2 annealed at 300 \({}^{\circ}\)C for 1.5 h (OL2300\({}^{\circ}\)C,1.5h), respectively. No significant spectral shift can be observed. At 400 \({}^{\circ}\)C, the maximum intensity of Ti substrate with OL2 annealed at 400 \({}^{\circ}\)C for 0.5 h (OL2400\({}^{\circ}\)C,0.5h) increases to 112%, while its position remains at 800 nm. On the other hand, the maximum intensities for Ti substrate with OL2 annealed at 400 \({}^{\circ}\)C for 1.0 h (OL2400\({}^{\circ}\)C,1.0h), and Ti substrate with OL2 annealed at 400 \({}^{\circ}\)C for 1.5 h (OL2400\({}^{\circ}\)C,1.5h) not only increase (to 220% and 263% respectively) but also are significantly blue-shifted, which completely changes the hue. Similarly, the spectral maximum intensities of OL2 substrates annealed at 500 \({}^{\circ}\)C also increase (to 152% for OL2500\({}^{\circ}\)C,0.5h, 163% for OL2500\({}^{\circ}\)C,1.0h), and 216% for OL2500\({}^{\circ}\)C,1.5h), and are all blue-shifted from 800 nm to 400 nm.