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Tuesday, May 24, 2016

Abstract-Composition control of plasmon-phonon interaction using topological quantum-phase transition in photoexcited (Bi1-xInx)2Se3

ACS Photonics, Just Accepted Manuscript
DOI: 10.1021/acsphotonics.6b00021
Publication Date (Web): May 24, 2016
Copyright © 2016 American Chemical Society

Plasmonics is a technology aiming at light modulation via collective charge oscillations. Topological insulators, where Dirac-like metallic surfaces coexist with normal insulating bulk, have recently attracted great attentions in plasmonics due to their topology-originated outstanding properties. Here, we introduce a new methodology for controlling the interaction of plasmon with phonon in topological insulators, which is a key for utilizing the unique spectral profiles for photonic applications. By using both static and ultrafast terahertz spectroscopy, we show that the interaction can be tuned by controlling the chemical composition of (Bi1-xInx)2Se3micro-ribbon arrays. The topological quantum-phase transition induced by varying the composition drives a dramatic change in the strength of the plasmon-phonon interaction. This was possible due to the availability of manipulating the spatial overlap between topological surface plasmonic states and underlying bulk phonon. Especially, we control the laser-induced ultrafast evolution of the transient spectral peaks arising from the plasmon-phonon interaction by varying the spatial overlap across the topological phase transition. This study may provide a new platform for realizing topological insulator-based ultrafast plasmonic devices.

Ultra-fast method to create terahertz radiation advances materials science,5,10,16&typ=artikel&lang=en

Uppsala physicists have in an international collaboration developed a new method for creating laser pulses which are shorter, have much higher intensity and cover the THz frequency range better than current sources. The study is published today in the authoritative journal Nature Photonics and is of great importance to materials research.
“Many interesting, dynamic phenomena of interest to materials science occur within the so-called terahertz spectral range but it has been difficult so far to generate such short pulses,” says Pablo Maldonado, one of the researchers behind the study.
The THz range has become increasingly important to science and engineering since so many dynamic processes such as molecular vibrations or magnetic spin waves usually vibrate with THz-frequencies. Therefore, there are many important areas of application for THz radiation such as medical diagnostics, security scanning at airports, molecular sensors or even wireless communications. However, it has been difficult to realise THz sources which cover the entire frequency domain and supply ultra-short pulses of sufficient intensity.
In collaboration with researchers from Germany, France and the USA, Uppsala University researchers Pablo Maldonado and Peter Oppeneer have developed a new THz laser emitter which has better properties than every such device so far made. It builds upon principles of ultra-fast spin transport developed by the Uppsala physicists.
Ultra-fast superdiffusion spin currents are generated by laser excitation in a nanometre thin metallic magnetic layer and move through the adjacent layer in less than a picosecond (10-12 seconds). There they induce the extremely short-lived charge currents which emit intensive THz radiation with a pulse width shorter than 0.5 picoseconds. In order to find the best THz emitter, the researchers from Mainz and Greifswald (Germany) synthesised more than 70 different thin metallic layer systems, which were measured in Berlin. The best emitter was found to consist of three different metal layers which together are less than six nanometres thick.
“It was pleasing that our theory of ultra-fast spin currents could be used in this way and that we can not only explain how spin currents are generated but also how they can be applied to create brilliant THz laser pulses,” says Pablo Maldonado.
Article reference:
Efficient metallic spintronic emitters of ultrabroadband terahertz radiation 
T. Seifert1, S. Jaiswal2,3, U. Martens4, J. Hannegan5, L. Braun1, P. Maldonado6, F. Freimuth7,
A. Kronenberg2, J. Henrizi2, In. Radu8, E. Beaurepaire9, Y. Mokrousov7, P.M. Oppeneer6, M. Jourdan2, G. Jakob2, D. Turchinovich10, L.M. Hayden5, M. Wolf1, M. Münzenberg4, M. Kläui2, T. Kampfrath1

Abstract-Efficient metallic spintronic emitters of ultrabroadband terahertz radiation

  • T. Seifert,
  • S. Jaiswal,
  • U. Martens,
  • J. Hannegan,
  • L. Braun,
  • P. Maldonado,
  • F. Freimuth,
  • A. Kronenberg,
  • J. Henrizi,
  • I. Radu,
  • E. Beaurepaire,
  • Y. Mokrousov,
  • P. M. Oppeneer,
  • M. Jourdan,
  • G. Jakob,
  • D. Turchinovich,
  • L. M. Hayden,
  • M. Wolf,
  • M. Münzenberg,
  • M. Kläui

  • T. Kampfrath 
  • (some author names deleted from Labels at the bottom due to blogger word limit)


    Terahertz electromagnetic radiation is extremely useful for numerous applications, including imaging and spectroscopy. It is thus highly desirable to have an efficient table-top emitter covering the 1–30 THz window that is driven by a low-cost, low-power femtosecond laser oscillator. So far, all solid-state emitters solely exploit physics related to the electron charge and deliver emission spectra with substantial gaps. Here, we take advantage of the electron spin to realize a conceptually new terahertz source that relies on three tailored fundamental spintronic and photonic phenomena in magnetic metal multilayers: ultrafast photoinduced spin currents, the inverse spin-Hall effect and a broadband Fabry–Pérot resonance. Guided by an analytical model, this spintronic route offers unique possibilities for systematic optimization. We find that a 5.8-nm-thick W/CoFeB/Pt trilayer generates ultrashort pulses fully covering the 1–30 THz range. Our novel source outperforms laser-oscillator-driven emitters such as ZnTe(110) crystals in terms of bandwidth, terahertz field amplitude, flexibility, scalability and cost.

    Monday, May 23, 2016

    Abstract-Two-dimensional terahertz magnetic resonance spectroscopy of collective spin waves

    Nonlinear manipulation of nuclear and electron spins is the basis for all advanced methods in magnetic resonance including multidimensional nuclear magnetic and electron spin resonance spectroscopies, magnetic resonance imaging, and in recent years, quantum control over individual spins. The methodology is facilitated by the ease with which the regime of strong coupling can be reached between radiofrequency or microwave magnetic fields and nuclear or electron spins respectively, typified by sequences of magnetic pulses that control the magnetic moment directions. The capabilities meet a bottleneck, however, for far-infrared magnetic resonances characteristic of correlated electron materials, molecular magnets, and proteins that contain high-spin transition metal ions. Here we report the development of two-dimensional terahertz magnetic resonance spectroscopy and its use for direct observation of the nonlinear responses of collective spin waves (magnons). The spectra show magnon spin echoes and 2-quantum signals that reveal pairwise correlations between magnons at the Brillouin zone center. They also show resonance-enhanced second-harmonic and difference-frequency signals. Our methods are readily generalizable to multidimensional magnetic resonance spectroscopy and nonlinear coherent control of terahertz-frequency spin systems in molecular complexes, biomolecules, and materials.

    Abstract-Tunable ultrasensitive terahertz sensor based on complementary graphene metamaterials

    RSC Adv., 2016, Accepted Manuscript

    DOI: 10.1039/C5RA21974D
    Received 11 Mar 2016, Accepted 12 May 2016
    First published online 23 May 2016!divAbstract

    In this paper, we propose an ultrasensitive terahertz sensor based on the complementary graphene metamaterial composed of wire-slot and split-ring resonator slot array structure. The destructive interference between two resonators gives rise to a reflection peak enabling ultrasensitive sensing, and sensitivity of 177.7GHz/RIU and FOM of 59.3 can be obtained for the proposed sensor. More importantly, this sensor can not only enhance the absorption of biomelecules and sensing performance, but also dynamically tune the sensing range by shifting the Fermi energy. In addition, the influences of the lateral displacement on the sensing performance are also investigated to improve the sensitivity of sensor. Therefore, this method opens up opportunities for efficiently sensing several organic, explosive, and biomolecules.