Abstract-Terahertz Second-Harmonic Generation from Lightwave Acceleration of Symmetry-Breaking Nonlinear Supercurrents

C. Vaswani, M. Mootz, C. Sundahl, D. H. Mudiyanselage, J. H. Kang, X. Yang, D. Cheng, C. Huang, R. H. J. Kim, Z. Liu, L. Luo, I. E. Perakis, C. B. Eom, and J. Wang

https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.124.207003

We report terahertz (THz) light-induced second harmonic generation, in superconductors with inversion symmetry that forbid even-order nonlinearities. The THz second harmonic emission vanishes above the superconductor critical temperature and arises from precession of twisted Anderson pseudospins at a multicycle, THz driving frequency that is not allowed by equilibrium symmetry. We explain the microscopic physics by a dynamical symmetry breaking principle at sub-THz-cycle by using quantum kinetic modeling of the interplay between strong THz-lightwave nonlinearity and pulse propagation. The resulting nonzero integrated pulse area inside the superconductor leads to light-induced nonlinear supercurrents due to subcycle Cooper pair acceleration, in contrast to dc-biased superconductors, which can be controlled by the band structure and THz driving field below the superconducting gap.

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Abstract-Single-Cycle Terahertz Driven Quantum Beats Reveal Symmetry-Selective Control of Excitonic Fine Structure in Perovskite

Z. Liu, C. Vaswani, X. Yang, X. Zhao, Y. Yao, Z. Song, D. Cheng, Y. Shi, L. Luo, D. H. Mudiyanselage, C. Huang, J.-M. Park, J. Zhao, Y. Yan, K.-M. Ho, J. Wang

https://arxiv.org/abs/1905.12373

Coherent time-frequency visualization reveals symmetry-selective vibronic (mixed exciton/lattice) quantum beats at cryogenic temperature after a single-cycle terahertz (THz) pumping in MAPbI3 perovskite. Above a critical threshold, a Raman phonon mode distinctly modulates the very {\em narrow}, middle region with {\em persistent} coherence for more than ten times longer than the two sides that predominately couple to infrared (IR) modes. Such spectral-temporal asymmetry and selectivity are inconsistent with a single exciton model, but in excellent agreement with a simulation of the Rashba-type, {\em three-fold} fine structure splitting of middle optically-forbidden, dark excitonic states and two bright ones, lying above and below. These hint ``Rashba engineering", i.e., periodic brightening and modulation of the spin-split excitons and Rashba parameters, by phonon symmetry and coherence.

Abstract-Ultrafast nonthermal terahertz electrodynamics and possible quantum energy transfer in the Nb 3 Sn superconductor

X. Yang, X. Zhao, C. Vaswani, C. Sundahl, B. Song, Y. Yao, D. Cheng, Z. Liu, P. P. Orth, M. Mootz, J. H. Kang, I. E. Perakis, C.-Z. Wang, K.-M. Ho, C. B. Eom, and J. Wang

https://journals.aps.org/prb/abstract/10.1103/PhysRevB.99.094504

We report terahertz (THz) electrodynamics of a moderately clean A15 superconductor (SC) following ultrafast excitation to manipulate quasiparticle (QP) transport. In the Martensitic normal state, we observe a photo enhancement in the THz conductivity using optical pulses, while the opposite is observed for the THz pump. This demonstrates wavelength-selective nonthermal control of conductivity distinct from sample heating. The photo enhancement persists up to an additional critical temperature, above the SC one, from a competing electronic order. In the SC state, the fluence dependence of pair-breaking kinetics together with an analytic model provides an implication for a “one photon to one Cooper pair” nonresonant energy transfer during the 35-fs laser pulse; i.e., the fitted photon energy $\hslash {\omega }^{}$absorption to create QPs set by $2{\Delta }_{SC}/\hslash \omega =0.33\%$. This is more than one order of magnitude smaller than in previously studied BCS SCs, which we attribute to strong electron-phonon coupling and possible influence of phonon condensation.

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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.