Abstract-Departure from BCS response in photoexcited superconducting NbN films observed by terahertz spectroscopy

M. Šindler, C. Kadlec, P. Kužel, K. Ilin, M. Siegel, and H. Němec

https://journals.aps.org/prb/abstract/10.1103/PhysRevB.97.054507

We investigate time-resolved terahertz conductivity of thin superconducting NbN films with various thicknesses upon their excitation by intense femtosecond laser pulses. The recovery dynamics following a complete destruction of the superconducting state occurs via a growth of superconducting islands in the normal-state environment. This is in contrast with previous observations of the recovery upon strong-field terahertz excitation [R. Matsunaga and R. Shimano, Phys. Rev. Lett. 109, 187002 (2012)]. We observe that the density of electronic states in the superconducting islands deviates from the BCS theory predictions on a subnanosecond time scale, while equilibrium terahertz conductivity spectra confirm the standard BCS behavior in the ground state.

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Abstract-Quantum theory of terahertz conductivity of semiconductor nanostructures

T. Ostatnický, V. Pushkarev, H. Němec, and P. Kužel

https://journals.aps.org/prb/accepted/77073YacS4c1dc6f966b4590831758b60bf70392b

Efficient and controlled charge carrier transport through nanoelements is currently a primordial question in the research of nanoelectronic materials and structures. We develop a quantum-mechanical theory of the conductivity spectra of confined charge carriers responding to an electric field from dc regime up to optical frequencies. The broken translation symmetry induces a broadband drift-diffusion current, which is not taken into account in the analysis based on Kubo formula and relaxation time approximation. We show that this current is required to ensure that the dc conductivity of isolated nanostructures correctly attains zero. It causes a significant reshaping of the conductivity spectra up to terahertz or multi-terahertz spectral ranges, where the electron scattering rate is typically comparable to or larger than the probing frequency.

Abstract-Quantum behavior of terahertz photoconductivity in silicon nanocrystals networks

V. Pushkarev, T. Ostatnický, H. Němec, T. Chlouba, F. Trojánek, P. Malý, M. Zacharias, S. Gutsch, D. Hiller, and P. Kužel 

https://journals.aps.org/prb/abstract/10.1103/PhysRevB.95.125424

Quantum-size effects are essential for understanding the terahertz conductivity of semiconductor nanocrystals, particularly at low temperatures. We derived a quantum mechanical expression for the linear terahertz response of nanocrystals; its introduction into an appropriate effective medium model provides a comprehensive microscopic approach for the analysis of terahertz conductivity spectra as a function of frequency, temperature, and excitation fluence. We performed optical pump–terahertz probe experiments in multilayer Si quantum dot networks with various degrees of percolation at 300 and 20 K and with variable pump fluence (initial carrier density) over nearly three orders of magnitude. Our theoretical approach was successfully applied to quantitatively interpret all the measured data within a single model. A careful data analysis made it possible to assess the distribution of sizes of nanocrystals participating to the photoconduction. We show and justify that such conductivity-weighted distribution may differ from the size distribution obtained by standard analysis of transmission electron microscopy images.

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Abstract-Bulk magnetic terahertz metamaterials based on dielectric microspheres

Bulk magnetic terahertz metamaterials based on dielectric microspheres 

https://www.osapublishing.org/oe/abstract.cfm?uri=oe-24-16-18340

M. Šindler, C. Kadlec, F. Dominec, P. Kužel, C. Elissalde, A. Kassas, J. Lesseur, D. Bernard, P. Mounaix, and H. Němec

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.

© 2016 Optical Society of America

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Abstract-Self-referenced ultra-broadband transient terahertz

F. D’Angelo, H. Němec, S. H. Parekh, P. Kužel, M. Bonn, and D. Turchinovich

https://www.osapublishing.org/oe/abstract.cfm?uri=oe-24-9-10157

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.

© 2016 Optical Society of America

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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 (0.64≤x≤1) 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-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

http://journals.aps.org/prb/accepted/b2077O18M8816f2c954073c1568be39f6a1181571

Silicon nanocrystals prepared by thermal decomposition of silicon-rich 2-5 nm thick SiOx layers (0.64 <= x <= 1) 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.

Abstract-Systematic study of terahertz response of SrTiO3 based heterostructures: Influence of strain, temperature, and electric field

Phys. Rev. B 89, 214116 – Published 30 June 2014

V. Skoromets, C. Kadlec, J. Drahokoupil, J. Schubert, J. Hlinka, and P. Kužel

https://journals.aps.org/prb/abstract/10.1103/PhysRevB.89.214116

Epitaxial heterostructures consisting of a variable number of SrTiO3/DyScO3 bilayers deposited on DyScO3substrates were investigated using time-domain terahertz spectroscopy down to helium temperatures. Interdigitated electrodes deposited on top of the structures allowed probing of the terahertz response upon an applied electric field. The phase transition into a ferroelectric state is observed in SrTiO3 films in all samples close to room temperature (between 250 and 310 K) due to in-plane epitaxial strain induced by the substrate and intercalated layers of DyScO3. Evolution of the dielectric spectra with temperature and external electric field is described by a general model which involves a damped harmonic oscillator (soft mode) coupled to a Debye relaxation (central mode). Both modes are connected with the soft mode eigenvector, as recently shown by molecular dynamics simulations, and they reflect a strong anharmonicity of the soft-mode potential. At high temperatures the soft-mode frequency variation drives all the changes observed in the spectra with temperature and applied field. At low temperatures, deep in the ferroelectric phase, the soft mode significantly hardens and loses its importance for the terahertz dynamics; the central mode becomes stronger and it almost completely determines the shape of the measured spectra. The observed variation of phase transition temperature and of the dielectric response among the structures is ascribed to a partial epitaxial strain relaxation confirmed also by x-ray diffraction.

DOI: http://dx.doi.org/10.1103/PhysRevB.89.214116

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Abstract-Ultrafast carrier response of Br+-irradiated In0.53Ga0.47As excited at telecommunication wavelengths

L. Fekete¹, H. Němec¹, Z. Mics¹, F. Kadlec¹, P. Kužel^1,a), V. Novák², J. Lorinčík^3,4, M. Martin⁵, J. Mangeney⁵, J. C. Delagnes⁶ and P. Mounaix⁶
^{http://scitation.aip.org/content/aip/journal/jap/111/9/10.1063/1.4709441}

1 Institute of Physics, ASCR, Na Slovance 2, 182 21 Prague 8, Czech Republic
2 Institute of Physics ASCR, Cukrovarnická 10, 162 53 Prague 6, Czech Republic
3 Institute of Photonics and Electronics ASCR, Chaberská 57, 18251 Prague 8, Czech Republic
4 Department of Physics, Faculty of Science, J.E. Purkinje University, Ceske mladeze 8, 40096, Usti nad Labem, Czech Republic
5 Institut d’Electronique Fondamentale, Université Paris XI, UMR CNRS 8622, 91405 Orsay Cedex, France
6 Laboratoire Ondes et Matière d’Aquitaine, Université Bordeaux I, UMR CNRS 5798, 351 Cours de la Libération, 33405 Talence Cedex, France
View the Scitation page for Institute of Physics AS CR (IOP).
View the Scitation page for JE Purkyn? University.
View the Scitation page for University of Bordeaux.
View the Scitation page for University of Paris-Sud.
a) E-mail: kuzelp@fzu.cz.
J. Appl. Phys. 111, 093721 (2012); http://dx.doi.org/10.1063/1.4709441

We present results of infrared pump—terahertz probe experiments applied to a set of In_0.53Ga_0.47As films irradiated with heavy ions (Br⁺) at doses from 10⁹ to 10¹²cm⁻².Photoexcitation at 1400 nm (0.89 eV) allowed us to characterize the response close to telecommunications’ wavelengths whilst avoiding the intervalley carrier scattering observed when a shorter wavelength excitation is used. The excitation fluence was varied in our experiments in order to characterize the dynamics in detail: the lifetimes and mobilities of both electrons and holes were retrieved, and the trap filling and carrier diffusion were clearly observed. The In_0.53Ga_0.47As film irradiated by the dose of 10¹²cm⁻² exhibits simultaneously ultrashort electronlifetime (∼300 fs) and very high electron mobility (2800 cm²V⁻¹s⁻¹). These findings are particularly important for the design of terahertz emitters controlled by lasers operating at standard telecommunication wavelengths.