Topological-insulator-based terahertz modulator

X B Wang,Y Wu,L Cheng,Hyunsoo Yang,Elbert E M Chia,D P Zhu,Jian-Xin Zhu,L Wang

doi:10.1038/s41598-017-13701-9

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 Bi1:5Sb0:5Te1:8Se1:2 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.

Highlights

Terahertz (THz) technology has been well developed in the past several decades with applications spanning from time-domain spectroscopy[1], to public security[2], medical imaging[3], and high speed communications[4]
The existence of surface states at room temperature has been confirmed by angle-resolved photoemission spectroscopy (ARPES) results[29,30,31], the surface states are always contaminated by the residual conductivity in the bulk arising from the presence of intrinsic impurities[32,33]
Bi1.5Sb0.5Te1.8Se1.2 (BSTS), one of the most insulating topological insulators has been characterized by Terahertz Time-Domain Spectroscopy (THz-TDS), which indicated the presence of an impurity band about 30 meV below the Fermi level[38]

Summary

Introduction

Terahertz (THz) technology has been well developed in the past several decades with applications spanning from time-domain spectroscopy[1], to public security[2], medical imaging[3], and high speed communications[4]. THz modulators based on semiconductors and metamaterials have been demonstrated to control the carrier concentration and the optical response of semiconductors by electrical or optical doping[8,9,10,11,12]. It was found that graphene-based modulators have superior performances due to its special band structure with linear dispersion and density of states close to the Fermi energy[17,18,19,20]. Bi1.5Sb0.5Te1.8Se1.2 (BSTS), one of the most insulating topological insulators has been characterized by Terahertz Time-Domain Spectroscopy (THz-TDS), which indicated the presence of an impurity band about 30 meV below the Fermi level[38]. We confirm that the large modulation arises from the thermal-activated free carriers in the semiconducting bulk state

Methods

Results

Conclusion