Unveiling the Far Infrared-to-Ultraviolet Optical Properties of Bismuth for Applications in Plasmonics and Nanophotonics

For years bismuth (Bi) has appealed to a broad community of scientists due to its peculiar electronic, optical, and more recently plasmonic and photocatalytic properties, which enable both the understanding of basic science phenomena and the development of a wide range of applications. In spite of this interest, a comprehensive spectral analysis of the dielectric function (ε = ε<sub>1</sub> + jε<sub>2</sub>) of bulk Bi from the far infrared (IR) to the ultraviolet (UV) region is not available. So far, the data have been reported in limited spectral ranges and show a wide dispersion that is especially notorious for the IR region. In this work we report ε for Bi in a wide spectral range from 0.05 to 4.7 eV (24.8 to 0.3 μm, far IR to UV). ε is extracted from spectroscopic ellipsometry measurements of excellent quality (dense and smooth) Bi films by using the transfer matrix formalism and Kramers–Kronig consistent analysis. The higher quality and accuracy of the obtained ε compared with the literature data is demonstrated. The analysis and use of this reference bulk dielectric function provides crucial information for the exploration and understanding of the optical, plasmonic, and photocatalytic properties of Bi nanostructures. From its analysis, it is evidenced that the optical properties of Bi in the mid wave IR-to-UV are driven <i>only</i> by interband transitions, which are responsible for the dominant absorption band peaking at about 0.8 eV. Therefore, the plasmonic behavior and the photocatalytic performance of Bi nanostructures in the visible and UV are likely driven by these interband transitions that make ε<sub>1</sub> turn negative in this region without the need of exciting free carriers. Furthermore, classical electrodynamic simulations using the obtained ε show a strong size dependence for the optical extinction of Bi nanospheres in the far IR-to-near IR with Mie-like resonances broadly tunable across this region.