Abstract-Topological-insulator-based terahertz modulator

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

https://www.nature.com/articles/s41598-017-13701-9

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

Abstract-Terahertz Conductivity of Twisted Bilayer Graphene

http://prl.aps.org/abstract/PRL/v110/i6/e067401
Xingquan Zou¹, Jingzhi Shang¹, Jianing Leaw¹, Zhiqiang Luo¹, Liyan Luo¹, Chan La-o-vorakiat¹, Liang Cheng¹, S. A. Cheong¹, Haibin Su², Jian-Xin Zhu³, Yanpeng Liu⁴, Kian Ping Loh⁴, A. H. Castro Neto⁵, Ting Yu¹, and Elbert E. M. Chia^1
1Division of Physics and Applied Physics, School of Physical and Mathematical Sciences, Nanyang Technological University, 637371 Singapore
²Division of Materials Science, School of Materials Science and Engineering, Nanyang Technological University, 639798 Singapore
³Theoretical Division and Center for Integrated Nanotechnologies, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA
⁴Department of Chemistry, National University of Singapore, 3 Science Drive 3, 117543 Singapore
⁵Graphene Research Centre and Physics Department, National University of Singapore, 6 Science Drive 2, 117546 Singapore

Received 9 April 2012; published 7 February 2013

Using terahertz time-domain spectroscopy, the real part of optical conductivity [σ₁(ω)] of twisted bilayer graphene was obtained at different temperatures (10–300 K) in the frequency range 0.3–3 THz. On top of a Drude-like response, we see a strong peak in σ₁(ω) at ∼2.7  THz. We analyze the overall Drude-like response using a disorder-dependent (unitary scattering) model, then attribute the peak at 2.7 THz to an enhanced density of states at that energy, which is caused by the presence of a van Hove singularity arising from a commensurate twisting of the two graphene layers.

© 2013 American Physical Society