## 3 Results and Discussion  
As shown in Figure 1a, a white LED source was used to illuminate Al nanodents from different angles in the range of 0\({}^{\circ}\) to 70\({}^{\circ}\). The divergence angles of LED sources ranged from 13\({}^{\circ}\) to 15\({}^{\circ}\). Through the lens collimation, the divergence angle was reduced to less than 5\({}^{\circ}\) in our case. The far-field light output of the LED was collected and characterized by an optical fiber spectrometer, keeping a distance of 2 cm from the sample, as shown in Figure 1a. Similar operations were used to measure the scattering spectra in the opposite direction. The source spectrum is presented in the inset in Figure 1a and shows two typical peaks around 450 and 550 nm. As shown in Figure 1b, Al nanodents exhibit blue scattering across the whole foil under vertical illumination of the white LED source. The morphology of Al nanodents was obtained with SEM and AFM, as shown in Figure 1c,d. The average size of these ordered hexagonal close-packed nanodents was about 300 nm, whereas the depth of the nanodents was relatively shallow, only about 30 nm. As shown in Figure 2b, strong blue light scattering was found on the surface of the Al nanodents illuminated with white LED. The blue light scattering became stronger with the increment of the incidence angle from 0\({}^{\circ}\) to 60\({}^{\circ}\). A simple schematic diagram was used to illustrate the position of the photos taken, as shown in Figure 2a. Obviously, light within the short wavelength range (blue) was first separated when the viewpoint ranged from 0\({}^{\circ}\) to 60\({}^{\circ}\). At large angles (\(>\)70\({}^{\circ}\)), weak light with longer wavelengths (green and yellow) were separated. However, the scattering intensity of light with longer wavelengths was very weak, therefore the scattering of Al nanodents was mainly localized in the short wavelength range. To further understand the selected scattering of Al nanodents, the light scattering spectra and the output spectrum of white LED were characterized simultaneously. The light peak around 450 nm in Figure 3a was found to grow higher as the incidence angle increased from 30\({}^{\circ}\) to 70\({}^{\circ}\). In contrast to the light peak around 450 nm, the light peak around 550 nm decreased with the increment of the incidence angle, which confirms the selected scattering in short wavelength ranges on Al nanodents. The spectrum of outputs also proves such selected scattering, as shown in Figure 3b. Significant reduction of the light intensity in the short wavelength range was clearly observed as the incidence angle increased from 30\({}^{\circ}\) to 70\({}^{\circ}\). In order to distinguish the peak in 550 nm, we added 0% to 20% (interval 5%) to the value of the intensity of light, with the incidence angle ranging from 30\({}^{\circ}\) to 70\({}^{\circ}\) in Figure 3b. With the increment of the incidence angle, a few intensity changes were observed at  
Figure 1: White Light Emitting Diodes (LED) scattering on Al nanodents. (**a**) A schematic diagram of optical measuring devices including the LED source, collimating lens group, optical fiber spectrometer, and Al nanodents; inset is the spectrum of the light source. (**b**) Optical image of the Al nanodents lighted by the LED source. (**c**) SEM image of the quasi-ordered hexagonal close-packed Al nanodents. (**d**) Depth of the Al nanodents obtained by atomic force microscope (AFM). Figure 2: Schematic diagram of the white LED light scattering on Al nanodents. (**a**) Schematic diagram of the scattering. (**b**) Image of the Al nanodents with normal illumination and white LED illumination with a 50\({}^{\circ}\) incidence angle from 30\({}^{\circ}\), 60\({}^{\circ}\), and 70\({}^{\circ}\) viewpoint angles (I, II, and III). 550 nm and a slight red shifts was observed to occur at the peak of 550 nm. The reason for the red shifts was that at very large incidence angles, the scattering of light with longer wavelengths is enhanced, as shown in Figure 2b.