Abstract-Tracking Ultrafast Photocurrents in the Weyl Semimetal TaAs Using THz Emission Spectroscopy

N. Sirica, R. I. Tobey, L. X. Zhao, G. F. Chen, B. Xu, R. Yang, B. Shen, D. A. Yarotski, P. Bowlan, S. A. Trugman, J.-X. Zhu, Y. M. Dai, A. K. Azad, N. Ni, X. G. Qiu, A. J. Taylor, R. P. Prasankumar,



https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.122.197401

We investigate polarization-dependent ultrafast photocurrents in the Weyl semimetal TaAs using terahertz (THz) emission spectroscopy. Our results reveal that highly directional, transient photocurrents are generated along the noncentrosymmetric $c$ axis regardless of incident light polarization, while helicity-dependent photocurrents are excited within the $ab$ plane. This is consistent with earlier static photocurrent experiments, and demonstrates on the basis of both the physical constraints imposed by symmetry and the temporal dynamics intrinsic to current generation and decay that optically induced photocurrents in TaAs are inherent to the underlying crystal symmetry of the transition metal monopnictide family of Weyl semimetals.

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Abstract-Probing and controlling terahertz-driven structural dynamics with surface sensitivity

P. Bowlan, J. Bowlan, S. A. Trugman, R. Valdés Aguilar, J. Qi, X. Liu, J. Furdyna, M. Dobrowolska, A. J. Taylor, D. A. Yarotski, and R. P. Prasankumar

https://www.osapublishing.org/optica/abstract.cfm?uri=optica-4-3-383

Intense, single-cycle terahertz (THz) pulses are powerful tools to understand and control material properties through low-energy resonances, such as phonons. Combining this with optical second harmonic generation (SHG) makes it possible to observe the resulting ultrafast structural changes with surface sensitivity. This makes SHG an ideal method to probe phonon dynamics in topological insulators (TI), materials with unique surface transport properties. Here, we resonantly excite a phonon mode in the TI Bi2Se3${}_{\mathrm{}}{}_{\mathrm{}}$ with THz pulses and use SHG to separate the resulting symmetry changes at the surface from the bulk. Furthermore, we coherently control the lattice vibrations with a pair of THz pulses. Our work demonstrates a versatile, table-top tool to probe and control phonon dynamics in a range of systems, particularly at surfaces and interfaces.

© 2017 Optical Society of America

Abstract-Probing and controlling terahertz-driven structural dynamics with surface sensitivity

P. Bowlan, J. Bowlan, S. A. Trugman, R. Valdes Aguilar, J. Qi, X. Liu, J. Furdyna, M. Dobrowolska, A. J. Taylor, D. A. Yarotski, R. P. Prasankumar

(Submitted on 4 Oct 2016)

https://arxiv.org/abs/1610.00871

Intense, single-cycle terahertz (THz) pulses offer a promising approach for understanding and controlling the properties of a material on an ultrafast time scale. In particular, resonantly exciting phonons leads to a better understanding of how they couple to other degrees of freedom in the material (e.g., ferroelectricity, conductivity and magnetism) while enabling coherent control of lattice vibrations and the symmetry changes associated with them. However, an ultrafast method for observing the resulting structural changes at the atomic scale is essential for studying phonon dynamics. A simple approach for doing this is optical second harmonic generation (SHG), a technique with remarkable sensitivity to crystalline symmetry in the bulk of a material as well as at surfaces and interfaces. This makes SHG an ideal method for probing phonon dynamics in topological insulators (TI), materials with unique surface transport properties. Here, we resonantly excite a polar phonon mode in the canonical TI Bi2Se3 with intense THz pulses and probe the subsequent response with SHG. This enables us to separate the photoinduced lattice dynamics at the surface from transient inversion symmetry breaking in the bulk. Furthermore, we coherently control the phonon oscillations by varying the time delay between a pair of driving THz pulses. Our work thus demonstrates a versatile, table-top tool for probing and controlling ultrafast phonon dynamics in materials, particularly at surfaces and interfaces, such as that between a TI and a magnetic material, where exotic new states of matter are predicted to exist.

Abstract-Terahertz magneto-optical spectroscopy of two-dimensional hole and electron systems

N. Kamaraju, W. Pan, U. Ekenberg, D. M. Gvozdić, S. Boubanga-Tombet, P. C. Upadhya, J. Reno, A. J. Taylor, R. P. Prasankumar

http://arxiv-web3.library.cornell.edu/abs/1412.5058

We have used terahertz (THz) magneto-optical spectroscopy to investigate the cyclotron resonance in high mobility two-dimensional electron and hole systems. Our experiments reveal long-lived (~20 ps) coherent oscillations in the measured signal in the presence of a perpendicular magnetic field. The cyclotron frequency extracted from the oscillations varies linearly with magnetic field for a two-dimensional electron gas (2DEG), as expected. However, we find that the complex non-parabolic valence band structure in a two-dimensional hole gas (2DHG) causes the cyclotron frequency and effective mass to vary nonlinearly with the magnetic field, as verified by multiband Landau level calculations. This is the first time that THz magneto-optical spectroscopy has been used to study 2DHG, and we expect that these results will motivate further studies of these unique 2D nanosystems.

Abstract-An electrically driven terahertz metamaterial diffractive modulator with more than 20 dB of dynamic range

N. Karl¹, K. Reichel¹, H.-T. Chen², A. J. Taylor², I. Brener³, A. Benz³, J. L. Reno³, R. Mendis¹ and D. M. Mittleman¹

 HIDE AFFILIATIONS
1 Department of Electrical and Computer Engineering, Rice University, MS 378, Houston, Texas 77251-1892, USA
2 Center for Integrated Nanotechnologies, Los Alamos National Laboratory, P. O. Box 1663, MS K771, Los Alamos, New Mexico 87545, USA
3 Center for Integrated Nanotechnologies, Sandia National Laboratories, P. O. Box 5800, MS 1082, Albuquerque, New Mexico 87185, USA
Appl. Phys. Lett. 104, 091115 (2014); http://dx.doi.org/10.1063/1.4867276

http://scitation.aip.org/content/aip/journal/apl/104/9/10.1063/1.4867276

We design and experimentally demonstrate a switchable diffraction grating for terahertz modulation based on planar active metamaterials, where a Schottky gate structure is implemented to tune the metamaterial resonances in real-time via the application of an external voltage bias. The diffraction grating is formed by grouping the active split-ring resonators into an array of independent columns with alternate columns biased. We observe off-axis diffraction over a wide frequency band in contrast to the narrow-band resonances, which permits operation of the device as a relatively high-speed, wide-bandwidth, high-contrast modulator,with more than 20 dB of dynamic range.

© 2014 AIP Publishing LLC