Resonance wavelengths of silver–gold compound metal island films

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Abstract
The resonance wavelength of a silver–gold compound metal island (CMI) film can be easily defined by the control of the mixture ratio of silver and gold. We have investigated its mechanism by observing the film microstructure with a scanning electron microscope. The shift of the resonance wavelength induced by the adjustment of the mixture ratio is attributed to two important factors: (1) the changes of the island shape and (2) the changes of the dielectric constant of the silver–gold alloy. For silver-rich CMI films, the island shape slightly changes according to the mixture ratio and becomes near spherical or prolate ellipsoidal. On the other hand, for gold-rich CMI films, the island shape becomes more oblate as the gold becomes richer. Therefore the resonance wavelength shifts are considered to be caused mainly by the changes of the dielectric constant and the island shape for the the silver-rich and the gold-rich CMI films, respectively. The resonance wavelengths calculated theoretically from the scanning electron microscope observations are similar to the measured ones.