Consider a metal such as Al. How do we look up or determine the relative permittivity of the metal? Suppose we have some known incident frequency, say about c/500nm for visible light. We can look up the bulk plasma frequency (3.7 PHz for Al), can it be calculated from that? For context, what I want to calculate is the desired hole diameter and geometry as a function of the desired filter centre wavelength, in a nanoscale plasmonic hole-grid structure like this. https://www.intechopen.com/books/pl...ce-plasmon-polaritons-in-cmos-digital-imaging We know the metal thickness, want to design the appropriate hole spacing and diameter, know the metal chosen, and know the permittivity of the substrate (glass etc) material.