# Tunable Coloring via Post-Thermal Annealing of Laser-Processed Metal Surface
## 3 Results and Discussion  
### Optical Properties  
Figure 1 shows the macroscopic color and microscope images of OL1 and OL2 with annealing treatment. The microscope images show that the oxide layer is formed with multiple color stripes. This texture is attributed to the Gaussian distribution of the laser pulse intensity and laser irradiation overlapping. Uneven energy distribution produces different thicknesses of oxide layers, which show different colors [15; 20]. Much like the principles behind the Red-Green-Blue (RGB) color model, the macroscopic color of the oxide surface is a combined result caused by multiple parts of the oxide layer. Figure 1a shows that the surface macroscopic color of the OL1\({}_{\text{Ref}}\) is purple. Microscope images show that the surface contains blue, yellow, and pink colors. While no significant changes are observed with post-annealing for most substrates, the macroscopic colors of Ti substrate with OL1 annealed at 500 \({}^{\circ}\)C for 1.0 h (OL1\({}_{500^{\circ}\text{C},1.0\text{h}}\)) and Ti substrate with OL1 annealed at 500 \({}^{\circ}\)C for 1.5 h (OL1\({}_{500^{\circ}\text{C},1.5\text{h}}\)) appear brighter. In addition, more yellow appears for Ti substrate with OL1 annealed at 400 \({}^{\circ}\)C for 1.5 h (OL1\({}_{400^{\circ}\text{C},1.5\text{h}}\)), OL1\({}_{500^{\circ}\text{C},1.0\text{h}}\), and OL1\({}_{500^{\circ}\text{C},1.5\text{h}}\). The macroscopic color of the OL2\({}_{\text{Ref}}\) surface is brown. Under the microscope, the surface appears to contain orange, red, and blue colors. OL2 with lower energy conditions can be further oxidized, showing the usual transformation from yellowish brown to blue. The amount of blue at the micro-level increases to replace the red and orange colors after post-annealing, with more significant change at higher annealing temperatures and longer treatment time. At the macro level, the macroscopic color begins to change significantly at annealing temperatures of 400 \({}^{\circ}\)C and above, from brown to brownish blue, and finally to dark blue. The surface turns blue with annealing at 500 \({}^{\circ}\)C for 1.0 h and does not change with a longer thermal treatment time of 1.5 h. An SEM image of the titanium oxide surface (OL1\({}_{\text{Ref}}\)) from the top view is shown in Figure 1b, indicating that the surface was quite rough and accompanied by evident cracks in the length scale of micrometer. However, the size of the cross-section is in the nanometer scale. Hence, the pores are not obviously visible here. After absorbing the laser energy, the surface changes its morphology by growing a film. Because the titanium surface cannot react with oxygen completely to form dense oxide in the laser pulse duration 1 ns, the surface is full of cracks and holes. This is also a result of thermal and growing stress during the elevated temperature exposure, indicating that the oxide layer is not dense, and that it contains oxygen. In addition, the oxide surface is bumpy, due to the Gaussian distribution of the laser pulse intensity. The thickness of the oxide layer (OL1\({}_{\text{Ref}}\)) created by laser is also measured with the FIB-SEM as shown in Figure 1c. The thickness of the oxide layer is around 196 nm, which can generate interference color. In summary, the thickness of the oxide layers is in a nanoscale level, while the surface of oxide is bumpy and contains defects. These oxide layers undergo color changes with post-annealing due to the increase in yellow and blue colors, respectively. These colors gradually spread across the oxide surfaces during the annealing treatment, and the rate and area of these color shifts generally increase with annealing temperature and treatment time. Figure 2a shows that the reflection spectral peak intensity of OLI\({}_{\text{Ref}}\) (around 400 nm) is 125% of that of the original Ti surface, while the spectral peak of OLI\({}_{\text{Ref}}\) (around 800 nm) is 73% of that of the original Ti surface.