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

Abstract

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 1 + jε 2 ) 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 only 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 1 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.

Plasmonics in the Ultraviolet with Aluminum, Gallium, Magnesium and Rhodium

“…Figure 1 shows the dimensionless Faraday number, Fa (or field-enhancement factor) [16] for different materials including all the above mentioned plus gold (Au), silver (Ag) and bismuth (Bi). Very recent results have presented Bi as a promising candidate for plasmonics in the UV range [17]. Fa quantifies and allows comparison of the ability of a material to enhance the electromagnetic field in the proximity of its surface.…”

“…[15]) and the shadowed region the exciting photon energies above 3 eV. The blue solid line represents the range of energies studied in each case (this depends on the optical constants available in the literature [17][18][19][20]). …”

Nanobismuth: Fabrication, Optical, and Plasmonic Properties—Emerging Applications

“…Especially, the optical dielectric function ε � ε 1 + iε 2 of bulk Bi was reported early by several authors, but in limited spectral regions, and shows a broad dispersion (related mostly with a limited sample quality) as seen in Figure 2. It is only recently that works providing a refined optical characterization of Bi crystals and films were reported [33][34][35]. Especially, a systematic spectroscopic ellipsometry characterization from the far-IR to the UV was performed on high quality Bi films to provide an artefact-free and Kramers-Kronig consistent dielectric function of the Bi material [19].…”

“…In this work, only the dielectric function standing for the in-plane response of the films (i.e., perpendicular to the trigonal axis) was determined. Yet, although the dielectric function of Bi crystals along the trigonal axis has been probed early in the near-to mid-IR showing no marked optical anisotropy in this region, it remains to be measured accurately in a broader spectral region (discussion and related references in [35]). …”

“…It has been shown in [35] that such negative ε 1 originates from the strong interband transitions peaking at 0.8 eV, whereas the negative ε 1 in the far-IR and THz originates from the freecarrier contribution. In sum, Bi nanostructures have the potential to display plasmonic effects in two spectral regions: (i) in the far-IR and THz due to the collective excitation of free carriers and (ii) in the UV-Vis-near-IR regions where, unlike the conventional plasmonic resonances of noble metals driven by the collective excitation of free electrons, the resonances in Bi nanostructures would be totally induced by interband transitions and called "interband plasmonic" resonances [19].…”