Thermo-optical properties of Bi nanoparticles embedded in germanate glasses and alumina thin films

Abstract

Materials, and particularly glasses, with a tailor-made spectral optical response that can be switched in a controlled way using external stimuli excitation are of interest for the development of active optical devices, such as active filters or all-optical switching components. Based on the dielectric functions of solid and liquid bismuth (Bi), it is expected that both solid and liquid Bi nanostructures will show optical resonances with relevant different features in the near ultraviolet to near infrared. In addition, Bi has a low melting point that enables the solid–liquid phase change under moderate heating. Profiting of these singular properties we present and review in this paper the main results that have been obtained in our laboratory concerning the thermo-optical response of bulk germanate glasses and amorphous Al2O3 thin films with embedded Bi nanoparticles (NPs). In both materials the optical transmission as a function of the temperature shows broad hysteresis loops. The optical contrast and width of these cycles depend on the Bi content and NP size. From the successful modelling of the optical properties of the Bi NPs-doped glasses, it is proposed that the observed material thermo-optical response arises from the change in the dielectric function of the Bi NPs upon melting, which induces relevant changes in the resonant optical behaviour of the Bi NPs. These results suggest that glasses with embedded Bi NPs are promising for the design of spectral-selective thermo-optical devices.