Abstract-A review on terahertz photogalvanic spectroscopy of Bi2Te3- and Sb2Te3-based three dimensional topological insulators

Helene Plank, Sergey D.Ganichev,

https://www.sciencedirect.com/science/article/pii/S0038110118302478

The paper overviews experimental and theoretical studies of photogalvanic effects induced in BiSbTe-based three dimensional topological insulators by polarized terahertz radiation. We present the state-of-the-art of this subject, including most recent and well-established results. We discuss a phenomenological theory based on symmetry arguments and models illustrating the photocurrents origin. We give a brief glimpse of the underlying microscopic theory, as well as an overview of the main experimental results.

Abstract-Ultrasensitive Room‐Temperature Terahertz Direct Detection Based on a Bismuth Selenide Topological Insulator

Weiwei Tang,   Antonio Politano,  Cheng Guo,   Wanlong,  Guo,   Changlong Liu,   Lin Wang,  Xiaoshuang Chen,  Wei Lu,

https://onlinelibrary.wiley.com/doi/abs/10.1002/adfm.201801786

Despite their huge application capabilities, millimeter‐ and terahertz‐wave photodetectors still face challenges in the detection scheme. Topological insulators (TIs) are predicted to be promising candidates for long‐wavelength photodetection, due to the presence of Dirac fermions in their topologically protected surface states. However, photodetection based on TIs is usually hindered by the large dark current, originating from the mixing of bulk states with topological surface states (TSSs) in most realistic samples of TIs. Here millimeter and terahertz detectors based on a subwavelength metal–TI–metal (MTM) heterostructure are demonstrated. The achieved photoresponse stems from the asymmetric scattering of TSS, driven by the localized surface plasmon‐induced terahertz field, which ultimately produces direct photocarriers beyond the interband limit. The device enables high responsivity in both the self‐powered and bias modes even at room temperature. The achieved responsivity is over 75 A/W, with response time shorter than 60 ms in the self‐powered mode. Remarkably, the responsivity increases by several orders of magnitude in the biased configuration, with the noise‐equivalent power (NEP) of 3.6 × 10−13 W Hz−1/2 and a detectivity of 2.17 × 1011cm Hz−1/2 W−1 at room temperature. The detection performances open a way toward realistic exploitation of TIs for large‐area, real‐time imaging within long‐wavelength optoelectronics.

Abstract-Nonvolatile Solid-State Charged-Polymer Gating of Topological Insulators into the Topological Insulating Regime

R. M. Ireland, Liang Wu, M. Salehi, S. Oh, N. P. Armitage, and H. E. Katz

https://journals.aps.org/prapplied/abstract/10.1103/PhysRevApplied.9.044003

We demonstrate the ability to reduce the carrier concentration of thin films of the topological insulator (TI) ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ by utilizing a nonvolatile electrostatic gating via corona charging of electret polymers. Sufficient electric field can be imparted to a polymer-TI bilayer to result in significant electron density depletion, even without the continuous connection of a gate electrode or the chemical modification of the TI. We show that the Fermi level of ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ is shifted toward the Dirac point with this method. Using terahertz spectroscopy, we find that the surface chemical potential is lowered into the bulk band gap (approximately 50 meV above the Dirac point and 170 meV below the conduction-band minimum), and it is stabilized in the intrinsic regime while enhancing electron mobility. The mobility of surface state electrons is enhanced to a value as high as approximately $1600{\mathrm{cm}}^2/Vs$ at 5 K.

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Researchers bridge quantum and classical physics using THz bismuth-selenium topological insulators

ALEXANDRU MICU

http://www.zmescience.com/science/news-science/quantum-classical-physics/

Scientists may have found a substance that allows them to finally link the opposing models of quantum and classical physics. In time, this finding could allow them to understand why the classical model breaks down at an quantum level, why quantum physics doesn’t seem to work at visible scales, and how the two can be reconciled.

We know these two models of understanding the physical world — the quantum for really small bits and the classical for larger bits — don’t mix well together. Usually when one is in charge, the other is completely absent. Most of the rules of classical physics break down at the quantum level — gravity, for example, doesn’t seem to be doing much on the atomic level even as it’s literally holding the universe together overall. There’s nothing in the rules of classical physics that can explain quantum entanglement, either.

Scientists know that there must be something tying the two models together, but we’ve yet to find even a clue of what that is. Now, thanks to a newly developed material, scientists have a chance to see quantum mechanics in action on a scale visible to the naked eye — offering hope of finding a bridge between the two models.

“We found a particular material that is straddling these two regimes,” says team leader N. Peter Armitage, from Johns Hopkins University.

“Usually we think of quantum mechanics as a theory of small things, but in this system quantum mechanics is appearing on macroscopic length scales. The experiments are made possible by unique instrumentation developed in my laboratory.”

The material Armitage developed is a topological insulator, a class of material first theoretically predicted in the 1980s, and first produced in 2007. Topological insulators are conductive on their outer layer while being insulators on the internal one. This causes the electrons flowing along the material to do some pretty weird stuff. For example, Armitage and his team found that a beam of terahertz radiation (sometimes called THz or T-rays – an invisible spectrum of light) passing through their bismuth-selenium topological insulators can be made to rotate slightly — an effect only observed at the atomic scale up to now.

This rotation could be predicted with the same mathematical systems that govern quantum theory — making this the first time researchers have witnessed quantum mechanics occurring on the macro scale. It could form the basis on which the quantum and classical models can be linked, the ‘theory of everything’ that scientists have been trying to find for decades.

The experiment is definitely “a piece of the puzzle” but according to Armitage, there’s still a lot of work to be done before this link is fully understood. He hopes that one day we’ll have a completed picture of physics, and new materials like the team’s topological insulator might be the way we get there.

The full paper “Quantized Faraday and Kerr rotation and axion electrodynamics of a 3D topological insulator” has been published in the journal Science.

Abstract-Robust Topological Terahertz Circuits using Semiconductors

Babak Bahari, Ricardo Tellez-Limon, Boubacar Kanté

(Submitted on 10 Jul 2016)

http://arxiv.org/abs/1607.02697

Topological Insulator-based devices can transport electrons/photons at the surfaces of materials without any back reflections, even in the presence of obstacles. Topological properties have recently been studied using non-reciprocal materials such as gyromagnetics or using bianisotropy. However, these effects usually saturate at optical frequencies and limit our ability to scale down devices. In order to implement topological devices that we introduce in this paper for the terahertz range, we show that semiconductors can be utilized via their cyclotron resonance in combination with small magnetic fields. We propose novel terahertz operating devices such as the topological tunable power splitter and the topological circulator. This work opens new perspectives in the design of terahertz integrated devices and circuits with high functionality.

Abstract-Coherent control of injection currents in high-quality films of Bi2Se3

D. A. Bas, K. Vargas-Velez, S. Babakiray, T. A. Johnson, P. Borisov, T. D. Stanescu, D. Lederman, A. D. Bristow

http://arxiv.org/abs/1412.7747

(Submitted on 24 Dec 2014 (v1), last revised 28 Jan 2015 (this version, v2))

Films of the topological insulator Bi2Se3 are grown by molecular beam epitaxy with in-situ reflection high-energy electron diffraction. The films are shown to be high-quality by X-ray reflectivity and diffraction and atomic-force microscopy. Quantum interference control of photocurrents is observed by excitation with harmonically related pulses and detected by terahertz radiation. The injection current obeys the expected excitation irradiance dependence, showing linear dependence on the fundamental pulse irradiance and square-root irradiance dependence of the frequency-doubled optical pulses. The injection current also follows a sinusoidal relative-phase dependence between the two excitation pulses. These results confirm the third-order nonlinear optical origins of the coherently controlled injection current. Experiments are compared to a tight-binding band structure to illustrate the possible optical transitions that occur in creating the injection current.

Comments:	11 pages, 3 figure, journal article
Subjects:	Optics (physics.optics); Mesoscale and Nanoscale Physics (cond-mat.mes-hall)
Journal reference:	Appl. Phys. Lett. 106, 041109 (2015)
DOI:	10.1063/1.4907004
Cite as:	arXiv:1412.7747 [physics.optics]
	(or arXiv:1412.7747v2 [physics.optics] for this version)

Abstract-Warping effect-induced optical absorbance increment of topological insulator films for THz photodetectors with high signal-to-noise ratio

http://pubs.rsc.org/en/Content/ArticleLanding/2014/NR/C3NR06506E#!divAbstract

J. M. Shao,^a   H. Li^a and   G. W. Yang*^a  

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Corresponding authors

^a

State Key Laboratory of Optoelectronic Materials and Technologies, Institute of Optoelectronic and Functional Composite Materials, Nanotechnology Research Center, School of Physics & Engineering, Sun Yat-sen University, Guangzhou 510275, P.R. China
E-mail: stsygw@mail.sysu.edu.cn

Nanoscale, 2014, Advance Article

DOI: 10.1039/C3NR06506E
Received 08 Dec 2013, Accepted 13 Jan 2014
First published online 27 Feb 2014

Strong optical absorbance makes topological insulator (TI) surfaces a promising high-performance photodetector in the terahertz (THz) to infrared frequency range. Here, we study the optical absorbance of more realistic TI films with hexagonal warping effect using the Fermi's golden rules. It was found that when the warping term is λ ≠ 0, the absorbance is no longer a universal value as that of graphene or ideal Dirac cone, but increases monotonously with the photon energy. The increment is positively correlated with the parameter λ/v_F³ where v_F is the Fermi velocity. The relative signal-to-noise ratio (SNR) of the TI film working as a photoresistor-type photodetector is significantly enhanced by the warping effect-induced absorbance increment. These investigations provide useful information for developing TI-based photodetectors with high SNR in the range of THz to infrared frequency.