Ultrafast dynamics and tunable coherent optical phonons via hybridization and gold layering in hybrid topological Bi2Se1.5Te1.5

Abstract

Topological insulators (TIs) are quantum materials that offer exotic optoelectronic, magnetic, and spintronic properties and lead to numerous applications such as quantum computing, Majorana Fermions, superconductivity, terahertz frequency generation, spintronics, thermoelectricity, and optoelectronics. TIs are inherently insulating in bulk, but, the topologically active surface states offer free electrons on the surfaces to enable them to be conducted on the surface. However, binary tetradymites (Bi₂Se₃, Bi₂Te₃, and Sb₂Te₃) are not fully insulating in bulk due to their semiconducting characteristics. To overcome this behaviour, the pure TIs can be converted to a new class of hybrid topological insulators (HTI) via doping. In this article, we present a detailed analysis of such kinds of HTI Bi₂Se_1.5Te_1.5 by examining their symmetry, band structure, charge carrier and phonon dynamics. Unlike pure TIs, this HTI possesses distinct optical properties, which are explored in this study. Additionally, a 15 nm gold film is deposited on TIs (Bi₂Se₃ and Bi₂Te₃) and HTI (Bi₂Se_1.5Te_1.5) crystal flakes to investigate the interaction between gold (Au) plasmonic coupling with these materials using transient reflectance spectroscopy. The main goal of Au is to tune the optical response of TIs through the plasmonic effect of Au. Besides carrier dynamics, the study also examines coherent acoustic and optical phonons in Bi₂Se₃, Bi₂Te₃ and Bi₂Se_1.5Te_1.5 as well as along with Au layered micro flakes. The hybrid Bi₂Se_1.5Te_1.5 topological material and gold layering introduce new opportunities for optoelectronics and spintronics, offering broad tunability in various optical and near-infrared regions.