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-Lightwave-driven gapless superconductivity and forbidden quantum beats by terahertz symmetry breaking

X. Yang, C. Vaswani, C. Sundahl, M. Mootz, L. Luo, J. H. Kang, I. E. Perakis, C. B. Eom,  J. Wang, 

Fig. 1: Pseudo-spin coherent oscillations forbidden by equilibrium symmetry and strong HH generation nonlinearities.

https://www.nature.com/articles/s41566-019-0470-y

Light-induced supercurrents chart a path forward for the electromagnetic design of emergent materials phases and collective modes for quantum engineering applications. However, controlled spatial–temporal modulation of the complex order parameter characterizing such non-equilibrium macroscopic quantum states remains elusive. Such ultrafast phase-amplitude modulation can manifest via high harmonic modes beyond those allowed by equilibrium symmetries. Here, we drive moving condensate states via subcycle dynamical symmetry breaking achieved with nonlinear oscillating terahertz photocurrents. These non-equilibrium macroscopic quantum states with broken inversion symmetry are controlled via Cooper pair acceleration by asymmetric and multi-cycle terahertz photoexcitations. The observed supercurrent-carrying states evolve during a lightwave cycle and exhibit three distinguishing features: Anderson pseudo-spin precessions forbidden by equilibrium symmetry, strong high harmonic coherent oscillations assisted by pairing and long-lived gapless superfluidity with minimal condensate quench. Lightwave tuning of persistent photocurrents can be extended for quantum control of unconventional superconductors and topological matter, with implications on quantum gate and sensing functionalities.

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-Competing Orders and Ultrafast Energy Transfer at the Quantum Limit in a Nb3Sn Superconductor Probed by Terahertz Electrodynamics

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

https://arxiv.org/abs/1808.06731

We report the low-energy electrodynamics of a moderately clean A15 superconductor (SC) following ultrafast excitation to understand and manipulate terahertz (THz) quasi--particle (QP) transport by tuning pump photoexcitation of from competing orders. Using 35-fs optical pulses, we observe a non-thermal enhancement in the low frequency conductivity, opposite to that observed for THz pump, which persists up to an additional critical temperature, above the SC one, from an electronic order in the Martensitic normal state. In the SC state, the fluence dependence of pair breaking kinetics together with an analytic model provides evidence for a `one photon-to-one pair' non-resonant energy transfer during the laser pulse. Such initial transfer of photon energy ℏω to QPs at the {\em quantum} limit, set by 2ΔSC/ℏω=0.33%, 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 in A15 SCs.

Abstract-Terahertz-light quantum tuning of a metastable emergent phase hidden by superconductivity

X.Yang, C. Vaswani, C. Sundahl, M. Mootz, P. Gagel, L. Luo, J. H. Kang, P. P. Orth, I. E. Perakis, C. B. Eom, J. Wang,

https://www.nature.com/articles/s41563-018-0096-3

Sudden’ quantum quench and prethermalization have become a cross-cutting theme for discovering emergent states of matter. Yet this remains challenging in electron matter, especially superconductors. The grand question of what is hidden underneath superconductivity (SC) appears universal, but poorly understood. Here we reveal a long-lived gapless quantum phase of prethermalized quasiparticles (QPs) after a single-cycle terahertz (THz) quench of a Nb3Sn SC gap. Its conductivity spectra is characterized by a sharp coherent peak and a vanishing scattering rate that decreases almost linearly towards zero frequency, which is most pronounced around the full depletion of the condensate and absent for a high-frequency pump. Above a critical pump threshold, such a QP phase with coherent transport and memory persists as an unusual prethermalization plateau, without relaxation to normal and SC thermal states for an order of magnitude longer than the QP recombination and thermalization times. Switching to this metastable ‘quantum QP fluid’ signals non-thermal quench of coupled SC and charge-density-wave (CDW)-like orders and hints quantum control beneath the SC.

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-A Tunable Eight-Wavelength Terahertz Modulator Based on Photonic Crystals

K. Ji, H. Chen, W. Zhou, Y. Zhuang, J. Wang

https://link.springer.com/article/10.1007%2Fs10812-017-0551-y


We propose a tunable eight-wavelength terahertz modulator based on a structure of triple triangular lattice photonic crystals by using photonic crystals in the terahertz regime. The triple triangular lattice was formed by nesting circular, square, and triangular dielectric cylinders. Three square point defects were introduced into the perfect photonic crystal to produce eight defect modes. GaAs was used as the point defects to realize tunability. We used a structure with a reflecting barrier to achieve modulation at high transmission rate. The insertion loss and extinction ratio were 0.122 and 38.54 dB, respectively. The modulation rate was 0.788 dB. The performance of the eightwavelength terahertz modulator showed great potential for use in future terahertz communication systems.

Abstract-Ultrabroadband Terahertz Absorption by Uniaxial Anisotropic Nanowire Metamaterials

Shen, Y. Pang, Y. ; Wang, J. ; Ma, H. ; Pei, Z. ; Qu, S.
http://ieeexplore.ieee.org/xpl/abstractAuthors.jsp?reload=true&arnumber=7169530&filter%3DAND%28p_IS_Number%3A7274213%29

Three-dimensional structure terahertz metamaterial absorbers with the properties of ultrabroadband and polarization-insensitive absorption were proposed. Different from the recent designs of the multilayer horizontal configurations, our design consists of the uniaxial anisotropic gold nanowire arrays that are filled in the frustum pyramid dielectric medium. The principle for the ultrabroadband absorption originates from the overlapping of the different but very closely positioned electromagnetic resonances. Each resonance frequency follows the equation of the odd multiple of the quarter wavelengths, which can be flexibly controlled by varying the height of the corresponding gold nanowires. In addition to the great prospect for the potential applications in terahertz, our design could also be easily extended to the other frequency regimes for a host of applications such as electromagnetic stealth, infrared detection, imaging, and solar cell.

Manipulating Light with a Single Layer of Carbon

My Note: This is somewhat old, but I include it because it's from the source for the most popular blog post found on this blog: "Viewpoint: Stimulated Near-Infrared Light Emission in Graphene".

"Femtosecond Population Inversion and Stimulated Emission of Dense Dirac Fermions in Graphene"

 http://www.ameslab.gov/research/highlights/manipulating-light-single-layer-carbon

 T. Li, L. Luo, M. Hupalo, J. Zhang, M.C. Tringides, J. Schmalian, and J. Wang 

Researchers have shown that it may be possible to make lasers using single-layer sheets of carbon atoms — the novel material known as graphene. Lasers are made from materials that can absorb ordinary light and then emit photons that have matching waves to provide high intensity.To generate laser power, a material must first undergo a population inversion where an excess of electrons is excited. They must then produce optical gain when one photon is emitted spontaneously causing the excited state electrons to undergo a cascade reaction, each one emitting an additional photon coherent with the first, so a large intensity builds up. Graphene exhibits both of these properties. Very short light pulses, only a few femtoseconds (10^-15 seconds) in duration, were used to stimulate the graphene. Almost instantaneously broad population inversions were observed; and the ultrabroad band gain is established at about 10 femtoseconds, producing a much wider tuning range of light (from terahertz to ultraviolet) than in conventional lasing materials. This is remarkable for photonics materials. Comparison of the experiments with newly-developed theoretical approaches neatly explains the findings. This work opens up a wide range of possible uses of graphene in previously-unexplored areas, particularly ultra-fast telecommunications and laser technology.With graphene a little light may go a long way

Abstract-Reversible modulation and ultrafast dynamics of terahertz resonances in strongly photoexcited metamaterials

http://prb.aps.org/abstract/PRB/v86/i12/e125110

I. Chatzakis¹, L. Luo¹, J. Wang^1,*, N.-H. Shen^1,†, T. Koschny¹, J. Zhou^2,‡, and C. M. Soukoulis^1,3
1Ames Laboratory and Department of Physics and Astronomy, Iowa State University, Ames, Iowa 50011, USA
²Center for Integrated Nanotechnologies, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA
³Institute of Electronic Structure and Laser, FORTH, 71110 Heraklion, Crete, Greece

Received 24 December 2011; revised 30 July 2012; published 7 September 2012

We demonstrate an ultrafast reversible modulation of resonant terahertz (THz) response in strongly photoexcited metamaterials. The transient spectral-temporal response of the dipole transition ∼1.6 THz exhibits a distinct nonmonotonic variation as a function of pump fluence. The transition energy shift, strength, spectral width, and density-dependent ultrafast relaxation manifest a remarkable reemergence of the transmission dip after initial quenching. Our simulations, incorporating the first-order diffraction from the photoinduced transient grating, reproduce the salient features, providing a new avenue for designing nonlinear and frequency-agile THz modulators.

©2012 American Physical Society

URL:

http://link.aps.org/doi/10.1103/PhysRevB.86.125110

DOI:

10.1103/PhysRevB.86.125110

PACS:

78.67.Pt, 42.25.Bs, 78.20.-e, 78.47.-p