# Tunable Coloring via Post-Thermal Annealing of Laser-Processed Metal Surface
## 1 Introduction  
Color generation on metals and semiconductors is currently of high interest. For example, recently, material scientists from Toyota proposed using a type of structural blue paint with a 15 \(\upmu\)m layer for vehicle surface coating. Its wide range of applications includes imprinting identification codes, decorative artwork, coating, and other types of useful information storage [1; 2; 3; 4; 5]. Key techniques, such as plasmonic nanostructures, anodizing, and laser coloration, possess their own unique features [6; 7; 8]. Among these methods, laser coloration has gained much attention due to its ability to achieve good spatial resolution through non-complex and speedy processes with low demands on critical experimental conditions [9; 10; 11; 12]. This technique involves surface irradiation, which causes the substrate material to react with different gases, usually oxygen, to generate multiple distinguishable colors. There are two key routes through which color is generated. The first is through the creation of periodic micro/nanostructures, which exhibit angle-dependent color properties in a macro scale. This requires the use of an ultrafast laser, such as a femtosecond laser [13]. The second generates angle-independent color properties via laser oxidation. This is commonly described as the growth of transparent oxide, in which substrate species are heated via laser light absorption [14]. Numerous types of metal and alloy substrates can undergo laser oxidation to form transparent oxide layers. Through the interference effect, the oxide layer allows the substrate to display a variety of colors [9; 15; 16]. Of thetwo methods, laser oxidation has been gaining popularity, as it is comparatively less complex and low cost, and therefore potentially more practical.  
Many research works have shown that the specific color produced through laser oxidation on metal or alloy surfaces depends mainly on laser irradiation parameters. In order to produce richer and more vivid colors, much effort has been put into investigating the effects of these parameters, which include laser wavelength, laser fluence, pulse duration, and number of scanning passes [10; 14; 17; 18]. However, limitations still exist. The display of color is caused by the light interference at the oxide layer, although the thickness of this layer is under 400 nm, which is key to determining the specific color produced [15]. However, it is difficult to precisely control the thickness of the oxide layer in the nano scale in order to traverse all colors on the color palette. Thus, researchers have supplemented existing methods with additional steps to expand the range and vibrancy of colors generated. These include pre-heating the substrate before coloration, locating the samples off the focal plane [7], and performing the laser irradiation process in different gas environments or underwater [16; 19].  
In this paper, thermal annealing is used to modify the oxide layer formed on the surface of the titanium substrate, as an effective means for finely tuning within a wider range of laser-generated colors. Substrates with a laser-processed titanium surface are exposed to various annealing temperatures for different lengths of time. The reflectance spectra are further measured to study observable color variation. X-ray photoelectron spectroscopy (XPS) tests are performed to explain the mechanism of color change. Experimental results demonstrate the promising potential of thermal annealing post-treatment as a viable method to achieve richer, laser-generated colors on metal surfaces, which can be used for decoration, marking, or identification purposes.