Abstract-Ultrafast Terahertz Conductivity Probes of Topologically Enhanced Surface Transport Driven by Mid-Infrared Laser Pulses in Bi2Se3

L. Luo, X. Yang, X. Liu, Z. Liu, C. Vaswani, D. Cheng, M. Mootz, I. E. Perakis, M. Dobrowolska, J. K. Furdyna, J. Wang

https://arxiv.org/abs/1805.03540

The recent discovery of topology-protected charge transport of ultimate thinness on surfaces of three-dimensional topological insulators (TIs) are breaking new ground in fundamental quantum science and transformative technology. Yet a challenge remains on how to isolate and disentangle helical spin transport on the surface from bulk conduction. Here we show that selective midinfrared femtosecond photoexcitation of exclusive intraband electronic transitions at low temperature underpins topological enhancement of terahertz (THz) surface transport in doped Bi2Se3, with no complication from interband excitations or need for controlled doping. The unique, hot electron state is characterized by conserved populations of surface/bulk bands and by frequency-dependent hot carrier cooling times that directly distinguish the faster surface channel than the bulk. We determine the topological enhancement ratio between bulk and surface scattering rates, i.e., γBS/γSS∼3.80 in equilibrium. These behaviors are absent at elevated lattice temperatures and for high pumpphoton frequencies and uences. The selective, mid-infrared-induced THz conductivity provides a new paradigm to characterize TIs and may apply to emerging topological semimetals in order to separate the transport connected with the Weyl nodes from other bulk bands.

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.