Tuning the plasmonic response of bimetallic films by laser irradiation

This work shows that surface treatment of multilayer Au and Ag thin films with single UV nanosecond laser pulses is a versatile and an easy-to-use tool for tuning their optical response or colour in a wide spectral range. As a laser parameter, we have varied the fluence. As film parameters, we have studied the film configuration by changing the deposition order, number of layers as well as the composition by varying the thickness of the Au layer while keeping that of Ag constant. The laser exposure produces melting of the layers followed by mixing of the two metals and dewetting, which lead to nanoparticle (NP) formation. The optical response is dominated by the surface plasmon resonance (SPR) associated to the NPs whose wavelength, intensity and bandwidth are consistent with the observed NP morphologies, the number density and the variation of optical constants with Au content. It is dominated by the balance between the interband transition onset of the alloy and the SPR of the nanoalloyed NPs for low fluences and by multipolar interactions for high fluences. While the Au content allows tuning the SPR in the range of ≈ 180 nm, the laser fluence allows a fine tuning of the SPR features and the film configuration has little impact on the optical response. All together this allows tuning the colour in ranges broader than 20 CIELAB units along the two chromatic axes.