The XPS and UPS spectra are presented in Figures and, respectively. The broadband XPS measurements showed that the binding energies derived from almost all the atomic orbitals of the constituent atoms could be observed in the HEA alloys (Table ). The spectra with an adjusted baseline showed very similar results to the average of the spectra of the five elements (Fig. ). However, shifts or disappearances of some peaks were observed. Therefore, a narrow-band analysis was performed to detail the observed formation of the alloy. It was found that almost all peaks shifted to the lower energies. In general, the shift to lower energies is due to the acceptance of electrons from outside . In other words, the pool of free electrons in the alloy mainly give electrons as bound electrons during the alloy formation. From the UPS measurements, the work functions were approximately 4.7 eV for the HEA and the constituent metal carriers (Fig. ). The values are close to those of the constituent elements. These results suggest that HEAs were formed in the present Au Ag Cu Pd Pt system using the sputtering method, and the nanostructures' optical properties should reflect these HEAs' properties. In the following sections, we summarise the experimental and FDTD simulation results for the resonance properties of the actual metasurfaces made out of noble HEA.