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Podoliak N, Salamon P, Lejček L, Kužel P, Novotná V. Undulations of Smectic A Layers in Achiral Liquid Crystals Manifested as Stripe Textures. Phys Rev Lett 2023; 131:228101. [PMID: 38101389 DOI: 10.1103/physrevlett.131.228101] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 04/28/2023] [Accepted: 10/24/2023] [Indexed: 12/17/2023]
Abstract
Self-assembly of organic molecules represents a fascinating playground to create various liquid crystalline nanostructures. In this Letter, we study layer undulations on micrometer scale in smectic A phases for achiral compounds, experimentally demonstrated as regular stripe patterns induced by thermal treatment. Undulations, including their anharmonic properties, are evaluated by means of polarimetric imaging and light diffraction experiments in cells with various thicknesses. The key role in stripe formation is played by high negative values of the thermal expansion coefficient.
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Affiliation(s)
- Natalia Podoliak
- Institute of Physics of the Czech Academy of Sciences, Na Slovance 2, CZ-182 00 Prague 8, Czech Republic
| | - Peter Salamon
- Institute for Solid State Physics and Optics, Wigner Research Centre for Physics, P.O. Box 49, Budapest H-1525, Budapest, Hungary
| | - Lubor Lejček
- Institute of Physics of the Czech Academy of Sciences, Na Slovance 2, CZ-182 00 Prague 8, Czech Republic
| | - Petr Kužel
- Institute of Physics of the Czech Academy of Sciences, Na Slovance 2, CZ-182 00 Prague 8, Czech Republic
| | - Vladimíra Novotná
- Institute of Physics of the Czech Academy of Sciences, Na Slovance 2, CZ-182 00 Prague 8, Czech Republic
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2
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Troha T, Klimovič F, Ostatnický T, Kadlec F, Kužel P, Němec H. Ultrafast Long-Distance Electron-Hole Plasma Expansion in GaAs Mediated by Stimulated Emission and Reabsorption of Photons. Phys Rev Lett 2023; 130:226301. [PMID: 37327420 DOI: 10.1103/physrevlett.130.226301] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 06/23/2022] [Revised: 03/13/2023] [Accepted: 05/03/2023] [Indexed: 06/18/2023]
Abstract
Electron-hole plasma expansion with velocities exceeding c/50 and lasting over 10 ps at 300 K was evidenced by time-resolved terahertz spectroscopy. This regime, in which the carriers are driven over >30 μm is governed by stimulated emission due to low-energy electron-hole pair recombination and reabsorption of the emitted photons outside the plasma volume. At low temperatures a speed of c/10 was observed in the regime where the excitation pulse spectrally overlaps with emitted photons, leading to strong coherent light-matter interaction and optical soliton propagation effects.
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Affiliation(s)
- Tinkara Troha
- Institute of Physics ASCR, Na Slovance 2, 182 00 Prague 8, Czech Republic
| | - Filip Klimovič
- Charles University, Faculty of Mathematics and Physics, Ke Karlovu 3, 121 16 Prague 2, Czech Republic
| | - Tomáš Ostatnický
- Charles University, Faculty of Mathematics and Physics, Ke Karlovu 3, 121 16 Prague 2, Czech Republic
| | - Filip Kadlec
- Institute of Physics ASCR, Na Slovance 2, 182 00 Prague 8, Czech Republic
| | - Petr Kužel
- Institute of Physics ASCR, Na Slovance 2, 182 00 Prague 8, Czech Republic
| | - Hynek Němec
- Institute of Physics ASCR, Na Slovance 2, 182 00 Prague 8, Czech Republic
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Kumar P, Šilhavík M, Parida MR, Němec H, Červenka J, Kužel P. Terahertz charge transport dynamics in 3D graphene networks with localization and band regimes. Nanoscale Adv 2023; 5:2933-2940. [PMID: 37260493 PMCID: PMC10228342 DOI: 10.1039/d2na00844k] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Grants] [Track Full Text] [Figures] [Subscribe] [Scholar Register] [Received: 11/24/2022] [Accepted: 05/01/2023] [Indexed: 06/02/2023]
Abstract
Terahertz steady-state and time-resolved conductivity and permittivity spectra were measured in 3D graphene networks assembled in free-standing covalently cross-linked graphene aerogels. Investigation of a transition between reduced-graphene oxide and graphene controlled by means of high-temperature annealing allowed us to elucidate the role of defects in the charge carrier transport in the materials. The THz spectra reveal increasing conductivity and decreasing permittivity with frequency. This contrasts with the Drude- or Lorentz-like conductivity typically observed in various 2D graphene samples, suggesting a significant contribution of a relaxational mechanism to the conductivity in 3D graphene percolated networks. The charge transport in the graphene aerogels exhibits an interplay between the carrier hopping among localized states and a Drude contribution of conduction-band carriers. Upon photoexcitation, carriers are injected into the conduction band and their dynamics reveals picosecond lifetime and femtosecond dephasing time. Our findings provide important insight into the charge transport in complex graphene structures.
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Affiliation(s)
- Prabhat Kumar
- FZU - Institute of Physics of the Czech Academy of Sciences Na Slovance 2 Prague 8 18221 Czech Republic
| | - Martin Šilhavík
- FZU - Institute of Physics of the Czech Academy of Sciences Na Slovance 2 Prague 8 18221 Czech Republic
| | - Manas R Parida
- FZU - Institute of Physics of the Czech Academy of Sciences Na Slovance 2 Prague 8 18221 Czech Republic
| | - Hynek Němec
- FZU - Institute of Physics of the Czech Academy of Sciences Na Slovance 2 Prague 8 18221 Czech Republic
| | - Jiří Červenka
- FZU - Institute of Physics of the Czech Academy of Sciences Na Slovance 2 Prague 8 18221 Czech Republic
| | - Petr Kužel
- FZU - Institute of Physics of the Czech Academy of Sciences Na Slovance 2 Prague 8 18221 Czech Republic
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Hovančík D, Repček D, Borodavka F, Kadlec C, Carva K, Doležal P, Kratochvílová M, Kužel P, Kamba S, Sechovský V, Pospíšil J. Terahertz Magnetic and Lattice Excitations in van der Waals Ferromagnet VI 3. J Phys Chem Lett 2022; 13:11095-11104. [PMID: 36417905 DOI: 10.1021/acs.jpclett.2c02944] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 06/16/2023]
Abstract
We use the synergy of infrared, terahertz, and Raman spectroscopies with DFT calculations to shed light on the magnetic and lattice properties of VI3. The structural transition at TS1 = 79 K is accompanied by a large splitting of polar phonon modes. Below TS1, strong ferromagnetic fluctuations are observed. The variations of phonon frequencies at 55 K induced by magnetoelastic coupling enhanced by spin-orbit interaction indicate the proximity of long-range ferromagnetic order. Below TC = 50 K, two Raman modes simultaneously appear and show dramatic softening in the narrow interval around the temperature TS2 of the second structural transition associated with the order-order magnetic phase transition. Below TS2, a magnon in the THz range appears in Raman spectra. The THz magnon observed in VI3 indicates the application potential of 2D van der Waals ferromagnets in ultrafast THz spintronics, which has previously been considered the exclusive domain of antiferromagnets.
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Affiliation(s)
- Dávid Hovančík
- Faculty of Mathematics and Physics, Department of Condensed Matter Physics, Charles University, Ke Karlovu 5, 121 16Prague 2, Czech Republic
| | - Dalibor Repček
- Institute of Physics, Czech Academy of Sciences, Na Slovance 2, 182 21Prague 8, Czech Republic
- Faculty of Nuclear Sciences and Physical Engineering, Czech Technical University in Prague, Břehová 7, 115 19Prague 1, Czech Republic
| | - Fedir Borodavka
- Institute of Physics, Czech Academy of Sciences, Na Slovance 2, 182 21Prague 8, Czech Republic
| | - Christelle Kadlec
- Institute of Physics, Czech Academy of Sciences, Na Slovance 2, 182 21Prague 8, Czech Republic
| | - Karel Carva
- Faculty of Mathematics and Physics, Department of Condensed Matter Physics, Charles University, Ke Karlovu 5, 121 16Prague 2, Czech Republic
| | - Petr Doležal
- Faculty of Mathematics and Physics, Department of Condensed Matter Physics, Charles University, Ke Karlovu 5, 121 16Prague 2, Czech Republic
| | - Marie Kratochvílová
- Faculty of Mathematics and Physics, Department of Condensed Matter Physics, Charles University, Ke Karlovu 5, 121 16Prague 2, Czech Republic
| | - Petr Kužel
- Institute of Physics, Czech Academy of Sciences, Na Slovance 2, 182 21Prague 8, Czech Republic
| | - Stanislav Kamba
- Institute of Physics, Czech Academy of Sciences, Na Slovance 2, 182 21Prague 8, Czech Republic
| | - Vladimír Sechovský
- Faculty of Mathematics and Physics, Department of Condensed Matter Physics, Charles University, Ke Karlovu 5, 121 16Prague 2, Czech Republic
| | - Jiří Pospíšil
- Faculty of Mathematics and Physics, Department of Condensed Matter Physics, Charles University, Ke Karlovu 5, 121 16Prague 2, Czech Republic
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Troha T, Ostatnický T, Kužel P. Improving security in terahertz wireless links using beam symmetry of vortex and Gaussian beams. Opt Express 2021; 29:30461-30472. [PMID: 34614769 DOI: 10.1364/oe.433606] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [What about the content of this article? (0)] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 06/07/2021] [Accepted: 08/27/2021] [Indexed: 06/13/2023]
Abstract
We present an effective way to improve the security of a point-to-point terahertz wireless link on a physical layer supported by numerical calculations in the frame of Fourier optics. The improvement is based on original countermeasures which exploit three independent degrees of freedom of the carrier wave: its intensity and azimuthal and radial symmetry. When the transmission line is intercepted, the light beam is subject to changes in either of the three degrees of freedom. We propose a strategy to measure these changes and they are quantified by a single eavesdropping parameter that is shown to be correlated to the secrecy capacity of the transmission. Consequently, its excessive value serves as an indication of the beam interception. We consider the carrier wave in the form of Gaussian and vortex beams. Comparison between the two reveals that vortex beam ensures a even higher level of security.
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Olejník K, Seifert T, Kašpar Z, Novák V, Wadley P, Campion RP, Baumgartner M, Gambardella P, Němec P, Wunderlich J, Sinova J, Kužel P, Müller M, Kampfrath T, Jungwirth T. Terahertz electrical writing speed in an antiferromagnetic memory. Sci Adv 2018; 4:eaar3566. [PMID: 29740601 PMCID: PMC5938222 DOI: 10.1126/sciadv.aar3566] [Citation(s) in RCA: 57] [Impact Index Per Article: 9.5] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 10/30/2017] [Accepted: 02/08/2018] [Indexed: 05/08/2023]
Abstract
The speed of writing of state-of-the-art ferromagnetic memories is physically limited by an intrinsic gigahertz threshold. Recently, realization of memory devices based on antiferromagnets, in which spin directions periodically alternate from one atomic lattice site to the next has moved research in an alternative direction. We experimentally demonstrate at room temperature that the speed of reversible electrical writing in a memory device can be scaled up to terahertz using an antiferromagnet. A current-induced spin-torque mechanism is responsible for the switching in our memory devices throughout the 12-order-of-magnitude range of writing speeds from hertz to terahertz. Our work opens the path toward the development of memory-logic technology reaching the elusive terahertz band.
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Affiliation(s)
- Kamil Olejník
- Institute of Physics, Czech Academy of Sciences, Cukrovarnická 10, 162 00 Praha 6, Czech Republic
- Corresponding author.
| | - Tom Seifert
- Fritz Haber Institute of the Max Planck Society, Faradayweg 4-6, 14195 Berlin, Germany
| | - Zdeněk Kašpar
- Institute of Physics, Czech Academy of Sciences, Cukrovarnická 10, 162 00 Praha 6, Czech Republic
- Faculty of Mathematics and Physics, Charles University, Ke Karlovu 3, 121 16 Prague 2, Czech Republic
| | - Vít Novák
- Institute of Physics, Czech Academy of Sciences, Cukrovarnická 10, 162 00 Praha 6, Czech Republic
| | - Peter Wadley
- School of Physics and Astronomy, University of Nottingham, Nottingham NG7 2RD, UK
| | - Richard P. Campion
- School of Physics and Astronomy, University of Nottingham, Nottingham NG7 2RD, UK
| | - Manuel Baumgartner
- Department of Materials, ETH Zürich, Hönggerbergring 64, CH-8093 Zürich, Switzerland
| | - Pietro Gambardella
- Department of Materials, ETH Zürich, Hönggerbergring 64, CH-8093 Zürich, Switzerland
| | - Petr Němec
- Faculty of Mathematics and Physics, Charles University, Ke Karlovu 3, 121 16 Prague 2, Czech Republic
| | - Joerg Wunderlich
- Institute of Physics, Czech Academy of Sciences, Cukrovarnická 10, 162 00 Praha 6, Czech Republic
- Hitachi Cambridge Laboratory, J. J. Thomson Avenue, Cambridge CB3 0HE, UK
| | - Jairo Sinova
- Institute of Physics, Czech Academy of Sciences, Cukrovarnická 10, 162 00 Praha 6, Czech Republic
- Institut für Physik, Johannes Gutenberg Universität Mainz, 55128 Mainz, Germany
| | - Petr Kužel
- Institute of Physics, Czech Academy of Sciences, Na Slovance 2, 182 21 Praha 8, Czech Republic
| | - Melanie Müller
- Fritz Haber Institute of the Max Planck Society, Faradayweg 4-6, 14195 Berlin, Germany
| | - Tobias Kampfrath
- Fritz Haber Institute of the Max Planck Society, Faradayweg 4-6, 14195 Berlin, Germany
- Department of Physics, Freie Universität Berlin, Arnimallee 14, 14195 Berlin, Germany
| | - Tomas Jungwirth
- Institute of Physics, Czech Academy of Sciences, Cukrovarnická 10, 162 00 Praha 6, Czech Republic
- School of Physics and Astronomy, University of Nottingham, Nottingham NG7 2RD, UK
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7
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Šindler M, Kadlec C, Dominec F, Kužel P, Elissalde C, Kassas A, Lesseur J, Bernard D, Mounaix P, Němec H. Bulk magnetic terahertz metamaterials based on dielectric microspheres. Opt Express 2016; 24:18340-18345. [PMID: 27505797 DOI: 10.1364/oe.24.018340] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [What about the content of this article? (0)] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/06/2023]
Abstract
Rigid metamaterials were prepared by embedding TiO2 microspheres into polyethylene. These structures exhibit a series of Mie resonances where the lowest-frequency one is associated with a strong dispersion in the effective magnetic permeability. Using time-domain terahertz spectroscopy, we experimentally demonstrated the magnetic nature of the observed resonance. The presented approach shows a way for low-cost massive fabrication of mechanically stable terahertz metamaterials based on dielectric microresonators.
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D'Angelo F, Němec H, Parekh SH, Kužel P, Bonn M, Turchinovich D. Self-referenced ultra-broadband transient terahertz spectroscopy using air-photonics. Opt Express 2016; 24:10157-10171. [PMID: 27137624 DOI: 10.1364/oe.24.010157] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [What about the content of this article? (0)] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/05/2023]
Abstract
Terahertz (THz) air-photonics employs nonlinear interactions of ultrashort laser pulses in air to generate and detect THz pulses. As air is virtually non-dispersive, the optical-THz phase matching condition is automatically met, thus permitting the generation and detection of ultra-broadband THz pulses covering the entire THz spectral range without any gaps. Air-photonics naturally offers unique opportunities for ultra-broadband transient THz spectroscopy, yet many critical challenges inherent to this technique must first be resolved. Here, we present explicit guidelines for ultra-broadband transient THz spectroscopy with air-photonics, including a novel method for self-referenced signal acquisition minimizing the phase error, and the numerically-accurate approach to the transient reflectance data analysis.
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Nádvorník L, Němec P, Janda T, Olejník K, Novák V, Skoromets V, Němec H, Kužel P, Trojánek F, Jungwirth T, Wunderlich J. Long-range and high-speed electronic spin-transport at a GaAs/AlGaAs semiconductor interface. Sci Rep 2016; 6:22901. [PMID: 26980667 PMCID: PMC4793250 DOI: 10.1038/srep22901] [Citation(s) in RCA: 13] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/10/2015] [Accepted: 02/24/2016] [Indexed: 11/09/2022] Open
Abstract
Spin-valves or spin-transistors in magnetic memories and logic elements are examples of structures whose functionality depends crucially on the length and time-scales at which spin-information is transferred through the device. In our work we employ spatially resolved optical pump-and-probe technique to investigate these fundamental spin-transport parameters in a model semiconductor system. We demonstrate that in an undoped GaAs/AlGaAs layer, spins are detected at distances reaching more than ten microns at times as short as nanoseconds. We have achieved this unprecedented combination of long-range and high-speed electronic spin-transport by simultaneously suppressing mechanisms that limit the spin life-time and the mobility of carriers. By exploring a series of structures we demonstrate that the GaAs/AlGaAs interface can provide superior spin-transport characteristics whether deposited directly on the substrate or embedded in complex semiconductor heterostructures. We confirm our conclusions by complementing the optical experiments with dc and terahertz photo-conductivity measurements.
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Affiliation(s)
- L Nádvorník
- Institute of Physics ASCR, v.v.i., Cukrovarnická 10, 16253 Praha 6, Czech Republic.,Faculty of Mathematics and Physics, Charles University, Ke Karlovu 3, 12116 Praha 2, Czech Republic
| | - P Němec
- Faculty of Mathematics and Physics, Charles University, Ke Karlovu 3, 12116 Praha 2, Czech Republic
| | - T Janda
- Institute of Physics ASCR, v.v.i., Cukrovarnická 10, 16253 Praha 6, Czech Republic.,Faculty of Mathematics and Physics, Charles University, Ke Karlovu 3, 12116 Praha 2, Czech Republic
| | - K Olejník
- Institute of Physics ASCR, v.v.i., Cukrovarnická 10, 16253 Praha 6, Czech Republic
| | - V Novák
- Institute of Physics ASCR, v.v.i., Cukrovarnická 10, 16253 Praha 6, Czech Republic
| | - V Skoromets
- Institute of Physics ASCR, v.v.i., Na Slovance 2, 18221 Praha 8, Czech Republic
| | - H Němec
- Institute of Physics ASCR, v.v.i., Na Slovance 2, 18221 Praha 8, Czech Republic
| | - P Kužel
- Institute of Physics ASCR, v.v.i., Na Slovance 2, 18221 Praha 8, Czech Republic
| | - F Trojánek
- Faculty of Mathematics and Physics, Charles University, Ke Karlovu 3, 12116 Praha 2, Czech Republic
| | - T Jungwirth
- Institute of Physics ASCR, v.v.i., Cukrovarnická 10, 16253 Praha 6, Czech Republic.,School of Physics and Astronomy, University of Nottingham, Nottingham NG7 2RD, UK
| | - J Wunderlich
- Institute of Physics ASCR, v.v.i., Cukrovarnická 10, 16253 Praha 6, Czech Republic.,Hitachi Cambridge Laboratory, J. J. Thomson Avenue, CB3 0HE Cambridge, UK
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Dominec F, Kadlec C, Němec H, Kužel P, Kadlec F. Transition between metamaterial and photonic-crystal behavior in arrays of dielectric rods. Opt Express 2014; 22:30492-30503. [PMID: 25606995 DOI: 10.1364/oe.22.030492] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [What about the content of this article? (0)] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/04/2023]
Abstract
Using finite-difference time-domain simulations, we study the interactions of electromagnetic radiation with a square array of dielectric rods parallel to the electric vector. We observe the electric and magnetic Mie resonances which induce intervals of negative effective permittivity and permeability and which contribute to the formation of the photonic band gaps. Owing to the interplay of these phenomena, a narrow spectral range with a negative refractive index can occur. However, this requires the filling fraction of the dielectric to fall into a well defined interval of values and its permittivity to exceed a minimum of about 50. We discuss these phenomena from the perspective of both photonic crystal and metamaterial concepts.
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Mitrofanov O, Dominec F, Kužel P, Reno JL, Brener I, Chung UC, Elissalde C, Maglione M, Mounaix P. Near-field probing of Mie resonances in single TiO2 microspheres at terahertz frequencies. Opt Express 2014; 22:23034-23042. [PMID: 25321774 DOI: 10.1364/oe.22.023034] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [What about the content of this article? (0)] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/04/2023]
Abstract
We show experimentally that poly-crystalline TiO2 spheres, 20-30 μm in diameter, exhibit a magnetic dipole Mie resonance in the terahertz (THz) frequency band (1.0-1.6 THz) with a narrow line-width (<40 GHz). We detect and investigate the magnetic dipole and electric dipole resonances in single high-permittivity TiO2 microspheres, using a near-field probe with a sub-wavelength (~λ/50) size aperture and THz time-domain spectroscopy technique. The Mie resonance signatures are observed in the electric field amplitude and phase spectra, as well as in the electric field distribution near the microspheres. The narrow line-width and the sub-wavelength size (λ/10) make the TiO2 microspheres excellent candidates for realizing low-loss THz metamaterials.
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Němec H, Kratochvílová I, Kužel P, Šebera J, Kochalska A, Nožár J, Nešpůrek S. Charge carrier mobility in poly[methyl(phenyl)silylene] studied by time-resolved terahertz spectroscopy and molecular modelling. Phys Chem Chem Phys 2010; 13:2850-6. [PMID: 21305068 DOI: 10.1039/c0cp00774a] [Citation(s) in RCA: 10] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
Abstract
Time-resolved terahertz spectroscopy and combination of quantum chemistry modeling and molecular dynamics simulations were used for the determination of charge carrier mobility in poly[methyl(phenyl)silylene]. Using time-resolved THz spectroscopy we established the on-chain charge carrier drift mobility in PMPSi as 0.02 cm(2) V(-1) s(-1). This value is low due to the formation of polarons: the hole is self-trapped in a potential formed by local chain distortion and the transient THz conductivity spectra show signatures of its oscillations within this potential well. This view is supported by the agreement between experimental and calculated values of the on-chain charge carrier mobility.
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Affiliation(s)
- Hynek Němec
- Institute of Physics AS CR, Na Slovance 2, 182 21 Prague 8, Czech Republic.
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13
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Jungwirth T, Horodyská P, Tesařová N, Němec P, Subrt J, Malý P, Kužel P, Kadlec C, Mašek J, Němec I, Orlita M, Novák V, Olejník K, Sobáň Z, Vašek P, Svoboda P, Sinova J. Systematic study of Mn-doping trends in optical properties of (Ga,Mn)As. Phys Rev Lett 2010; 105:227201. [PMID: 21231417 DOI: 10.1103/physrevlett.105.227201] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 07/28/2010] [Revised: 10/08/2010] [Indexed: 05/30/2023]
Abstract
We report on a systematic study of optical properties of (Ga,Mn)As epilayers spanning the wide range of accessible Mn(Ga) dopings. The material synthesis was optimized for each nominal Mn doping in order to obtain films which are as close as possible to uniform uncompensated (Ga,Mn)As mixed crystals. We observe a broad maximum in the mid-infrared absorption spectra whose position exhibits a prevailing blueshift for increasing Mn doping. In the visible range, a peak in the magnetic circular dichroism also shifts with increasing Mn doping. The results are consistent with the description of ferromagnetic (Ga,Mn)As based on the microscopic valence band theory. They also imply that opposite trends seen previously in the optical data on a limited number of samples are not generic and cannot serve as an experimental basis for postulating the impurity band model of ferromagnetic (Ga,Mn)As.
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Affiliation(s)
- T Jungwirth
- Institute of Physics ASCR, v.v.i., Cukrovarnicka´ 10, 162 53 Praha 6, Czech Republic
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14
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Ostapchuk T, Petzelt J, Hlinka J, Bovtun V, Kužel P, Ponomareva I, Lisenkov S, Bellaiche L, Tkach A, Vilarinho P. Broad-band dielectric spectroscopy and ferroelectric soft-mode response in the Ba(0.6)Sr(0.4)TiO(3) solid solution. J Phys Condens Matter 2009; 21:474215. [PMID: 21832494 DOI: 10.1088/0953-8984/21/47/474215] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/31/2023]
Abstract
Ceramic Ba(0.6)Sr(0.4)TiO(3) (BST-0.6) samples were studied in the broad spectral range of 10(6)-10(14) Hz by using several dielectric techniques in between 20 and 800 K. The dominant dielectric dispersion mechanism in the paraelectric phase was shown to be of strongly anharmonic soft-phonon origin. The whole soft-mode response in the vicinity of the ferroelectric transition was shown to consist of two coupled overdamped THz excitations, which show classical features of a coupled soft and central mode, known from many ferroelectric crystals with a dynamics near the displacive and order-disorder crossover. Similar behaviour has been recently revealed and theoretically simulated in pure BaTiO(3) (see Ponomareva et al 2008 Phys. Rev. B 77 012102 and Hlinka et al 2008 Phys. Rev. Lett. 101 167402). Also for the BST system, this feature was confirmed by the theory based on molecular dynamics simulations with an effective first-principles Hamiltonian. In all the ferroelectric phases, additional relaxation dispersion appeared in the GHz range, assigned to ferroelectric domain-wall dynamics. The microwave losses were analysed from the point of view of applications. The paraelectric losses above 1 GHz are comparable with those in single crystals and appear to be of intrinsic multi-phonon origin. The ceramic BST system is therefore well suited for applications in the whole microwave range.
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Affiliation(s)
- T Ostapchuk
- Institute of Physics, Na Slovance 2, 18221 Prague 8, Czech Republic
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15
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Kadlec C, Kadlec F, Němec H, Kužel P, Schubert J, Panaitov G. High tunability of the soft mode in strained SrTiO(3)/DyScO(3) multilayers. J Phys Condens Matter 2009; 21:115902. [PMID: 21693932 DOI: 10.1088/0953-8984/21/11/115902] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/30/2023]
Abstract
SrTiO(3)/DyScO(3) epitaxial multilayers with variable number and thickness (10-100 nm) of bilayers deposited on DyScO(3) substrates were investigated by means of time-domain terahertz spectroscopy at room temperature. A tensile strain develops in the SrTiO(3) films and shifts the eigenfrequency of the ferroelectric soft mode down by ∼25-45 cm(-1) with respect to the value found for single crystals. In all films the soft mode strongly hardens upon the electrical bias and a linear coupling to a silent excitation of relaxation type at 10 cm(-1) is observed. We show that the change in the THz and sub-THz response of the layers with an increasing field is determined solely by the soft mode eigenfrequency and we propose a phenomenological model describing the origin of the tunability and the peculiar properties of the ferroelectric soft mode in the terahertz spectral range.
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Affiliation(s)
- C Kadlec
- Institute of Physics, Academy of Sciences of the Czech Republic, Na Slovance 2, 182 21 Prague 8, Czech Republic
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Kadlec F, Berta M, Kužel P, Lopot F, Polakovič V. Assessing skin hydration status in haemodialysis patients using terahertz spectroscopy: a pilot/feasibility study. Phys Med Biol 2008; 53:7063-71. [DOI: 10.1088/0031-9155/53/24/004] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [What about the content of this article? (0)] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/12/2022]
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Němec H, Kadlec F, Kužel P. Methodology of an optical pump-terahertz probe experiment: An analytical frequency-domain approach. J Chem Phys 2002. [DOI: 10.1063/1.1512648] [Citation(s) in RCA: 56] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [What about the content of this article? (0)] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/14/2022] Open
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