Felix Laboratory

Friday, May 29, 2015

Imperial College Centre for Terahertz Science and Engineering- Strategic Associations


Centre for Terahertz Science and Engineering

Strategic Associations



Abstract-Terahertz carpet cloak based on a ring resonator metasurface


B. Orazbayev, N. Mohammadi Estakhri, M. Beruete, and A. Alù
Phys. Rev. B 91, 195444 – Published 29 May 2015
https://journals.aps.org/prb/abstract/10.1103/PhysRevB.91.195444

In this work we present the concept and design of an ultrathin (λ/22) terahertz (THz) unidirectional carpet cloak based on the local phase compensation approach enabled by gradient metasurfaces. A triangular surface bump with center height of 4.1 mm (1.1λ) and tilt angle of 20° is covered with a metasurface composed of an array of suitably designed closed ring resonators with a transverse gradient of surface impedance. The ring resonators provide a wide range of control for the reflection phase with small absorption losses, enabling efficient phase manipulation along the edge of the bump. Our numerical results demonstrate a good performance of the designed cloak in both near field and far field, and the cloaked object mimics a flat ground plane within a broad range of incidence angles, over 35° angular spectrum centered at 45°. The presented cloak design can be applied in radar and antenna systems as a thin, lightweight, and easy to fabricate solution for radio and THz frequencies.
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Thursday, May 28, 2015

Abstract-Observing microscopic structures of a relativistic object using a time-stretch strategy

http://www.nature.com/srep/2015/150528/srep10330/full/srep10330.html



Emission of light by a single electron moving on a curved trajectory (synchrotron radiation) is one of the most well-known fundamental radiation phenomena. However experimental situations are more complex as they involve many electrons, each being exposed to the radiation of its neighbors. This interaction has dramatic consequences, one of the most spectacular being the spontaneous formation of spatial structures inside electrons bunches. This fundamental effect is actively studied as it represents one of the most fundamental limitations in electron accelerators, and at the same time a source of intense terahertz radiation (Coherent Synchrotron Radiation, or CSR). Here we demonstrate the possibility to directly observe the electron bunch microstructures with subpicosecond resolution, in a storage ring accelerator. The principle is to monitor the terahertz pulses emitted by the structures, using a strategy from photonics, time-stretch, consisting in slowing-down the phenomena before recording. This opens the way to unpreceeded possibilities for analyzing and mastering new generation high power coherent synchrotron sources.

Abstract-Charge transport in silicon nanocrystal superlattices in the terahertz regime


H. Němec, V. Zajac, P. Kužel, P. Malý, S. Gutsch, D. Hiller, and M. Zacharias
Phys. Rev. B 91, 195443 – Published 28 May 2015
https://journals.aps.org/prb/abstract/10.1103/PhysRevB.91.195443
Silicon nanocrystals prepared by thermal decomposition of silicon-rich 2–5-nm-thick SiOx layers (0.64x1) are investigated using time-resolved terahertz spectroscopy. The samples consist of a superlattice of isolated monolayers composed of Si nanocrystals with controlled variable size and filling fraction. Experiments with variable optical pump fluence over almost two orders of magnitude allow us to determine the depolarization fields in the structure. Careful consideration of the local fields along with Monte Carlo calculations of the microscopic conductivity of Si nanocrystals supported by structural characterization of the samples provide detailed information about the electrical connectivity of nanocrystals and about the charge transport among them. Well below the percolation threshold, nanocrystals grow mostly isolated from each other. In thicker or in more Si-enriched layers, nanocrystals merge during their growth and form tens-of-nanometer-sized photoconducting Si structures with a good electrical connection. In addition, in thick SiOx layers, imperfectly connected clusters of Si nanocrystals are observed which develop probably at the end of the growth process and allow only limited charge transport due to energy barriers.


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    Abstract-Terahertz response of patterned epitaxial graphene



    Christian Sorger, Sascha Preu1,2, Johannes Schmidt3, Stephan Winnerl3, Yuliy V Bludov4, Nuno M R Peres4, Mikhail I Vasilevskiy4 and Heiko B Weber1
    http://iopscience.iop.org/1367-2630/17/5/053045
    1 Department of Physics, Friedrich-Alexander-University Erlangen-Nuremberg (FAU), Staudtstraße 7, D-91058 Erlangen, Germany
    2 Department of Electrical Engineering and Information Technology, Technical University Darmstadt, Merckstraße 25, D-64283 Darmstadt, Germany
    3 Institute of Ion Beam Physics and Materials Research, Helmholtz-Zentrum Dresden-Rossendorf, Bautzner Landstraße 400, D-01314 Dresden, Germany
    4 Department of Physics and Center of Physics, University of Minho, Campus de Gualtar, P-4710-057 Braga, Portugal 


    We study the interaction between polarized terahertz (THz) radiation and micro-structured large-area graphene in transmission geometry. In order to efficiently couple the radiation into the two-dimensional material, a lateral periodic patterning of a closed graphene sheet by intercalation doping into stripes is chosen. We observe unequal transmittance of the radiation polarized parallel and perpendicular to the stripes. The relative contrast, partly enhanced by Fabry–Perot oscillations reaches 20%. The effect even increases up to 50% when removing graphene stripes in analogy to a wire grid polarizer. The polarization dependence is analyzed in a large frequency range from <80 GHz to 3 THz, including the plasmon–polariton resonance. The results are in excellent agreement with theoretical calculations based on the electronic energy spectrum of graphene and the electrodynamics of the patterned structure.