## 3 Results and Discussion  
Visual observations showed that the processed stainless steel sample demonstrated the brightest angle-dependent colors when irradiated by sun light. The copper brightness was next to the stainless steel and then followed aluminum, while the brass sample showed the least brightness. The observed different colors were attributed to the formation of LIPSS on the irradiated surfaces, as will be shown in the SEM analysis below. Color pallets of different metals are shown in Figure 2. Pallets (a-d) correspond to stainless steel, copper, aluminum, and brass samples, respectively. All samples were prepared at different scan speeds to optimize LIPSS for the observation of different colors. The optimal scanning speeds for stainless steel, brass, aluminum, and copper were 350, 400, 500, and 450 mm/s, respectively. Pictures of the samples were taken in sunlight at fixed incident position (\(\alpha\) = 20\({}^{\circ}\)). The angle of sample observation was changed from 0\({}^{\circ}\) to 40\({}^{\circ}\) when different colors were observed. The processing times for disks of 20 mm diameter of stainless steel, brass, aluminum, and copper were 15 s, 14 s, 11 s, and 12 s respectively. Scanning electron microscope (SEM, TESCAN Vega 3) images show that all angle-dependent colors were features of the periodic structures imprinted on the surface of each metal (Figure 3). The directions of all nanoripples were orthogonal to the polarization axis of the laser beam. The distance between  
Figure 1: **(a)** Experimental setup for metal surfaces treatment using a femtosecond laser at power 4.5 W, pulse duration 40 fs, central wavelength 1030 nm, and repetition rate 150 kHz coupled with a scan head (SH) containing an F-Theta objective lens. The sample was placed on a 3D motion stage (3 DMS). A half wave plate (HWP) and a thin film polarizer (TFP) were used to control the average power of laser pulses. **(b)** Experimental set up for the measurements of the angle-dependent color reflectance from the processed surfaces. Light (tungsten bulb) was shone on the processed samples at an angle \(\alpha\) = 20\({}^{\circ}\) with regard to the axis perpendicular to the sample; and the diffracted light was measured with a fiber spectrometer at variable angle (\(\beta\)). nanoripples was measured by SEM and was found to be approximately 1 \(\upmu\)m, which is very comparable to the wavelength of the used femtosecond laser). The measurements of the angle-dependent color reflectance from the surfaces were conducted using the setup shown in Figure 1b. The broad-bandwidth emission from the tungsten lamp was directed on the laser-treated area of the sample and the diffracted light was measured using the fiber spectrometer at different angles with respect to the normal of the processed surface. The diffraction equation \(\mathrm{m}\lambda=\mathrm{d}\) (\(\mathrm{sin}\alpha+\mathrm{sin}\beta\)) was used to determine the direction from which the different colors resulting from LIPSS were viewable. Here m is the diffraction order, \(\lambda\) is the wavelength, d is the LIPSS spacing, \(\alpha\) is the angle of incident light, and \(\beta\) is the angle of diffracted light. The spectral analyses of the treated aluminum, copper, and stainless steel surfaces are shown in Figure 4a-c. These graphs show a very consistent pattern for all metallic samples and hence serve as evidence of reflectance similarities due to the same LIPSS on different metallic surfaces. Meanwhile, in the case of non-treated surfaces (Figure 4d-f), the reflectance spectra remain the same at different angles of observation, contrary to the case of treated surfaces (Figure 4a-c). Figure 3: Scanning electron microscope (SEM) images of the processed (**a**) stainless steel, (**b**) copper, (**c**) aluminum, and (**d**) brass surfaces. The scanning speeds for stainless steel, copper, aluminum, and brass were 350, 400, 500, and 450 mm/s respectively. The laser beam direction is perpendicular to the Laser Induced Periodic Surface Structures (LIPSS) orientation.