Abstract-Terahertz tuning of Dirac plasmons in Bi 2 Se 3 topological insulator

P. Di Pietro, N. Adhlakha, F. Piccirilli, A. Di Gaspare, J. Moon, S. Oh, S. Di Mitri, S. Spampinati, A. Perucchi, and S. Lupi

https://journals.aps.org/prl/accepted/4a070KdfY0f1530f14789af239849412267149091

Light can be strongly confined in sub-wavelength spatial regions through the interaction with plasmons, the collective electronic modes appearing in metals and semiconductors. This confinement, which is particularly important in the terahertz spectral region, amplifies light-matter interaction and provides a powerful mechanism for efficiently generating non-linear optical phenomena. These effects are particularly relevant in graphene and Topological Insulators, where massless Dirac fermions show a naturally non-linear optical behavior in the terahertz range. The strong interaction scenario has been considered so far from the point of view of light. In this paper, we investigate instead the effect of strong interaction on the plasmon itself. In particular, we will show that Dirac plasmons in Bi${}_2$Se${}_3$ Topological Insulator are strongly renormalized when excited by high-intensity terahertz radiation by displaying a huge red-shift down to 60% of its characteristic frequency. This opens the road towards tunable terahertz non-linear optical devices based on Topological Insulators.

Abstract-Photoinduced terahertz dynamics in BizSes topological insulator

 F. Giorgianni, M. Shalaby,   C. Vicario,  C. P. Hauri,  S. Lupi

http://ieeexplore.ieee.org/document/8087694/

Plasmons are collective charge density waves in a conductive material, which show remarkable electromagnetic properties and have great potential for photonics from terahertz to the ultraviolet. Besides the conventional plasmonic excitations in metals and in semiconductors, 2-dimensional (2D) plasmons of massless Dirac electrons have been observed in graphene and at the surface of topological insulators (TIs). TIs are quantum electronic materials with an insulating gap in the bulk originated by a strong spin-orbit coupling and Dirac metallic states at their interfaces. Among the various TIs, Bi2Se3 is one of the most promising material due to its large band gap Eg~300 meV. Recently, it has been shown that the low energy single particle and collective (plasmons) electrodynamics response are dominated by surface Dirac electrons in Bi2Se3 in steady state regime [1]. The relaxation dynamics show that an excess of electrons are continuously injected from bulk to the surface through surface states-bulk phonon scattering channel [2,3]. In this work we study the time resolved photoinduced THz dynamics of plasmonic response in micro-ribbons patterned Bi2Se3 thin film by means of optical-pump/THz-probe spectroscopy.

Abstract-Light-induced electron localization in a quantum Hall system

• T. Arikawa,
• K. Hyodo,
• Y. Kadoya
• & K. Tanaka

http://www.nature.com/nphys/journal/vaop/ncurrent/full/nphys4078.html

An insulating bulk state is a prerequisite for the protection of topological edge states¹. In quantum Hall systems, the thermal excitation of delocalized electrons is the main route to breaking bulk insulation². In equilibrium, the only way to achieve a clear bulk gap is to use a high-quality crystal under high magnetic field at low temperature. However, bulk conduction could also be suppressed in a system driven out of equilibrium such that localized states in the Landau levels are selectively occupied. Here we report a transient suppression of bulk conduction induced by terahertz wave excitation between the Landau levels in a GaAs quantum Hall system. Strikingly, the Hall resistivity almost reaches the quantized value at a temperature where the exact quantization is normally disrupted by thermal fluctuations. The electron localization is realized by the long-range potential fluctuations, which are a unique and inherent feature of quantum Hall systems. Our results demonstrate a new means of effecting dynamical control of topology by manipulating bulk conduction using light.

Posted: Sep 14, 2016

New principle for highly efficient optical device verified

http://www.nanowerk.com/nanotechnology-news/newsid=44541.php

(Nanowerk News) A group of researchers at the University of Tokyo and their collaborators have demonstrated that when a topological insulator—a material that conducts electricity on the surface but not in the interior—with magnetic properties is exposed to light, the angles of the rotation of light polarization driven by the magneto-optical effect show the universal values determined by quantum mechanics (Nature Communications, "Terahertz spectroscopy on Faraday and Kerr rotations in a quantum anomalous Hall state").

Schematic rendering of the quantized magneto-optical effect on a topological insulator. When magnetic properties are embedded in a topological insulator by adding magnetic elements (blue arrow), an energy gap (mass gap) is created in the electrons on the surface. Then the quantum anomalous Hall effect, in which the Hall resistance shows a quantized value without application of magnetic field, appears. At this point, the Faraday and Kerr effects for terahertz light, with energy well below the mass gap, show rotation angles as defined by the fine structure constant. The image illustrates the polarization rotation generated by the Faraday effect. (Image: Ken N. Okada)

Magneto-optical effects, which comprise what are known as the Faraday and Kerr effects, refer to general optical phenomena in which the polarization of the light—namely, the direction of ac electric field of light—transmitted through magnetic materials or reflected from them rotates when the magnets are exposed to light. These magneto-optical effects serve as the principle behind optical communication devices and magneto-optical disks.

Once topological insulators are embedded with magnetic properties, an energy gap, called a mass gap, is created in surface electrons (see figure), resulting in what is referred to as a quantum anomalous Hall effect—a “quantized Hall effect” that appears in the absence of a magnetic field. The Faraday and Kerr effects occur when the surface of the topological insulator exhibits quantum anomalous Hall effect; scientists had predicted theoretically that the rotation angles of the light polarization would be the universal values determined by quantum mechanics.

Since this “quantized magneto-optical effect” exhibits a huge polarization-rotation efficiency in the absence of magnetic field without losing any energy, its observation had been anticipated.

The research group led by graduate student Ken Okada, Project Associate Professor Youtarou Takahashi, and Professor Yoshinori Tokura of the Graduate School of Engineering at the University of Tokyo, and their collaborators stabilized quantum anomalous Hall effect at a much higher temperature (around minus 270 degrees Celsius) than what had previously been attained, by developing a special technique for fabricating the topological insulator. The researchers then irradiated the topological insulator with terahertz light, a type of light that has about one-five hundredth the energy of visible light, which we can feel with our eyes. As expected through the theoretical prediction, they found that at low temperature in which the quantum anomalous Hall effect is well developed, the value determined solely from the Faraday and Kerr rotation angles converges to a physical constant, fine structure constant (~ 1/137).

In the current study the scientists observed a polarization rotation of 0.15 degree with an 8 nanometer-thick film, which translates into a rotation angle of 200,000 degrees per centimeter. This value is nearly two orders of magnitude as large as that of conventional polarization rotation devices. Since the quantum anomalous Hall effect produces high polarization rotation efficiency without any energy loss in the absence of magnetic field, this phenomenon may lead to energy-saving, high-efficiency optical devices in the terahertz range in the future.

“We were really excited when we observed the polarization rotations of terahertz light, as predicted in theory, in the lab,” says Takahashi. He continues, “The research on topological insulators, as a new group of material in condensed matter, is developing rapidly, and we expect a lot more discoveries of new quantum phenomena in the future.”

Read more: New principle for highly efficient optical device verified