On the Variation with Wavelength of the Optical Constants of Thin Metallic Films

Abstract

Assuming that a thin metallic layer may be characterized, from the optical point of view, by its thickness d, its refractive index ν, and its index of extinction κ, the curves representing 2νκd and (ν2 − κ2 − ∊a)d, where ∊a is a constant, are plotted against the wavelength for very thin layers of gold, silver, copper, iron, nickel, and cobalt, thermically evaporated and investigated in a vacuum of some 2 to 3 × 10−6 Torr. The analogy which the curves relating to the thin layers of the three first named of these metals exhibit with the characteristic curves of a resonator having a well-defined natural frequency and large damping is pointed out. The experimentally determined variations of the optical constants of these thin films with the wavelength of light can be interpreted by use of the discontinuous-structure microcrystallite-assembly description in the theory of David. Agreement is satisfactory with the supposition that the microcrystallites have the form of ellipsoids of revolution or even of spheres. In the case of gold and copper, the phenomena are perturbed by a maximum of the characteristic absorption caused by an electronic transition that is located in the visible portion of the spectrum.