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-Ultrafast terahertz response of multilayer graphene in the nonperturbative regime

P. Bowlan, E. Martinez-Moreno, K. Reimann, T. Elsaesser, and M. Woerner
Max-Born-Institut für Nichtlineare Optik und Kurzzeitspektroskopie, 12489 Berlin, Germany

 Published 27 January 2014; received 18 June 2013

The nonlinear dynamics of electrons in multilayer epitaxial graphene is investigated by time-resolved terahertz (THz) spectroscopy in a regime where the interaction of electrons with the external field dominates over scattering processes. The predominantly coherent electron response to the THz field involves both intra- and interband currents, leading to coherently driven interband transitions of carriers and to the generation of higher harmonics of the THz carrier frequency. The overall behavior of the graphene layers is always absorptive, even after generation of an initial electron-hole distribution by femtosecond midinfrared excitation. The results are in agreement with theoretical calculations of the nonperturbative THz response.

©2014 American Physical Society

URL:

http://link.aps.org/doi/10.1103/PhysRevB.89.041408

DOI:

10.1103/PhysRevB.89.041408

PACS:

72.80.Vp, 72.20.Ht, 73.22.Pr, 78.47.J-

Abstract-Ultrafast terahertz response of multilayer graphene in the nonperturbative regime

P. Bowlan, E. Martinez-Moreno, K. Reimann, T. Elsaesser, and M. Woerner
http://prb.aps.org/accepted/ed078Y8aSf212448242681137542a700e758b25c0
The nonlinear dynamics of electrons in multi-layer epitaxial graphene is investigated by time-resolved terahertz (THz) spectroscopy in a regime where the interaction of electrons with the external field dominates over scattering processes. The predominantly coherent electron response to the THz field involves both intra- and interband currents, leading to coherently driven interband transitions of carriers and to the generation of higher harmonics of the THz carrier frequency. The overall behavior of the graphene layers is always absorptive, even after generation of an initial electron-hole distribution by femtosecond mid-infrared excitation. The results are in agreement with theoretical calculations of the nonperturbative THz response.