Topological-insulator-based terahertz modulator

TLDR

Three‑dimensional topological insulators possess an insulating bulk and metallic surface states, and their narrow band gaps make them attractive for terahertz optoelectronic applications. The authors experimentally realize an electronically tunable terahertz intensity modulator using a Bi₁.₅Sb₀.₅Te₁.₈Se₁.₂ single crystal. Modulation is achieved by applying a bias current that activates carrier absorption in the semiconducting bulk, enabling tunability of the terahertz response. The device delivers a frequency‑independent ~62 % modulation depth from 0.3 to 1.4 THz at room temperature with 100 mA, and its performance can be further enhanced at low temperatures in the low‑current regime, demonstrating a new terahertz application of topological insulators.

Abstract

Abstract Three dimensional topological insulators, as a new phase of quantum matters, are characterized by an insulating gap in the bulk and a metallic state on the surface. Particularly, most of the topological insulators have narrow band gaps, and hence have promising applications in the area of terahertz optoelectronics. In this work, we experimentally demonstrate an electronically-tunable terahertz intensity modulator based on Bi 1:5 Sb 0:5 Te 1:8 Se 1:2 single crystal, one of the most insulating topological insulators. A relative frequency-independent modulation depth of ~62% over a wide frequency range from 0.3 to 1.4 THz has been achieved at room temperature, by applying a bias current of 100 mA. The modulation in the low current regime can be further enhanced at low temperature. We propose that the extraordinarily large modulation is a consequence of thermally-activated carrier absorption in the semiconducting bulk states. Our work provides a new application of topological insulators for terahertz technology.