Optical Properties of Bismuth Nanostructures Towards the Ultrathin Film Regime

TL;DR

This study investigates the optical properties of ultrathin bismuth films, revealing that continuous films below 15 nm retain bulk-like optical behavior, which is promising for nanophotonic applications.

Contribution

It provides the first detailed dielectric function measurements of ultrathin bismuth films and explains the effects of film continuity on optical properties.

Findings

Continuous films above 15 nm have dielectric functions similar to bulk bismuth.

Discontinuous ultrathin films show markedly different dielectric behavior.

Effective medium effects dominate in discontinuous ultrathin films.

Abstract

Bulk bismuth presents outstanding optical properties, such as a giant infrared refractive index (n near 10) and a negative ultraviolet visible permittivity induced by giant interband electronic transitions. Although such properties are very appealing for applications in nanophotonics, the dielectric function of bismuth nanostructures has been scarcely studied. Here, we determine by spectroscopic ellipsometry the far infrared to ultraviolet dielectric function of pulsed laser deposited bismuth thin films with nominal thickness tBi varied from near 10 nm to several tens of nm. For tBi above 15 nm, the films display a continuous structure and their dielectric function is comparable with that of bulk bismuth. For tBi below 15 nm, the film structure is discontinuous, and the dielectric function differs markedly from that of bulk bismuth. It is proposed from FDTD simulations that this marked difference arises mainly from effective medium effects induced by the discontinuous film structure, where quantum electronic confinement does not play a dominant role. This suggests that ultrathin and continuous bismuth films should present the same outstanding optical properties as bulk bismuth for high performance nanophotonic devices.