Possible Eliashberg-Type Superconductivity Enhancement Effects in a Two-Band Superconductor MgB_{2} Driven by Narrow-Band THz Pulses.

We study THz-driven condensate dynamics in epitaxial thin films of MgB_{2}, a prototype two-band superconductor (SC) with weak interband coupling. The temperature and excitation density dependent dynamics follow the behavior predicted by the phenomenological bottleneck model for the single-gap SC, implying adiabatic coupling between the two condensates on the ps timescale. The amplitude of the THz-driven suppression of condensate density reveals an unexpected decrease in pair-breaking efficiency with increasing temperature-unlike in the case of optical excitation.

Field-resolved THz-pump laser-probe measurements with CEP-unstable THz light sources.

Radiation sources with a stable carrier-envelope phase (CEP) are highly demanded tools for field-resolved studies of light-matter interaction, providing access both to the amplitude and phase information of dynamical processes. At the same time, many coherent light sources, including those with outstanding power and spectral characteristics lack CEP stability, and so far could not be used for this type of research. In this work, we present a method enabling linear and non-linear phase-resolved terahertz (THz) -pump laser-probe experiments with CEP-unstable THz sources.

Flexomagnetism and vertically graded Néel temperature of antiferromagnetic CrO thin films.

Antiferromagnetic insulators are a prospective materials platform for magnonics, spin superfluidity, THz spintronics, and non-volatile data storage. A magnetomechanical coupling in antiferromagnets offers vast advantages in the control and manipulation of the primary order parameter yet remains largely unexplored. Here, we discover a new member in the family of flexoeffects in thin films of CrO.

Coherent coupling of metamaterial resonators with dipole transitions of boron acceptors in Si.

We investigate the coherent coupling of metamaterial resonators with hydrogen-like boron acceptors in Si at cryogenic temperatures. When the resonance frequency of the metamaterial, chosen to be in the range 7-9 THz, superimposes the transition frequency from the ground state of the acceptor to an excited state, Rabi splitting as large as 0.4 THz is observed.

Mid- and far-infrared localized surface plasmon resonances in chalcogen-hyperdoped silicon.

Plasmonic sensing in the infrared region employs the direct interaction of the vibrational fingerprints of molecules with the plasmonic resonances, creating surface-enhanced sensing platforms that are superior to traditional spectroscopy. However, the standard noble metals used for plasmonic resonances suffer from high radiative losses as well as fabrication challenges, such as tuning the spectral resonance positions into mid- to far-infrared regions, and the compatibility issue with the existing complementary metal-oxide-semiconductor (CMOS) manufacturing platform. Here, we demonstrate the occurrence of mid-infrared localized surface plasmon resonances (LSPR) in thin Si films hyperdoped with the known deep-level impurity tellurium.

High electron mobility in strained GaAs nanowires.

Transistor concepts based on semiconductor nanowires promise high performance, lower energy consumption and better integrability in various platforms in nanoscale dimensions. Concerning the intrinsic transport properties of electrons in nanowires, relatively high mobility values that approach those in bulk crystals have been obtained only in core/shell heterostructures, where electrons are spatially confined inside the core. Here, it is demonstrated that the strain in lattice-mismatched core/shell nanowires can affect the effective mass of electrons in a way that boosts their mobility to distinct levels.

Stretchable Thin Film Mechanical-Strain-Gated Switches and Logic Gate Functions Based on a Soft Tunneling Barrier.

Mechanical-strain-gated switches are cornerstone components of material-embedded circuits that perform logic operations without using conventional electronics. This technology requires a single material system to exhibit three distinct functionalities: strain-invariant conductivity and an increase or decrease of conductivity upon mechanical deformation. Herein, mechanical-strain-gated electric switches based on a thin-film architecture that features an insulator-to-conductor transition when mechanically stretched are demonstrated.

High-field THz pulses from a GaAs photoconductive emitter for non-linear THz studies.

We report the emission of high-field terahertz pulses from a GaAs large-area photoconductive emitter pumped with a Ti:Sapphire amplifier laser system at 800 nm wavelength and 1 kHz repetition rate. The maximum estimated terahertz electric field at the focus is ≳ 230 kV/cm. We also demonstrate the capability of the terahertz field to cause a non-linear effect, which usually requires high-field terahertz pulses generated through optical rectification or an air plasma.

Limitation of THz conversion efficiency in DSTMS pumped by intense femtosecond pulses.

Terahertz (THz) generation via optical rectification (OR) of near-infrared femtosecond pulses in DSTMS is systematically studied using a quasi-3D theoretical model, which takes into account cascaded OR, three-photon absorption (3PA) of the near-infrared radiation, and material dispersion/absorption properties. The simulation results and the comparison with experimental data for pump pulses with the center wavelength of 1.4 µm indicate that the 3PA process is one of the main limiting factors for THz generation in DSTMS at high pump fluences.

A Two-Dimensional Polyimide-Graphene Heterostructure with Ultra-fast Interlayer Charge Transfer.

Two-dimensional polymers (2DPs) are a class of atomically/molecularly thin crystalline organic 2D materials. They are intriguing candidates for the development of unprecedented organic-inorganic 2D van der Waals heterostructures (vdWHs) with exotic physicochemical properties. In this work, we demonstrate the on-water surface synthesis of large-area (cm ), monolayer 2D polyimide (2DPI) with 3.

Nonlinear Charge Transport in InGaAs Nanowires at Terahertz Frequencies.

We probe the electron transport properties in the shell of GaAs/InGaAs core/shell nanowires at high electric fields using optical pump/THz probe spectroscopy with broadband THz pulses and peak electric fields up to 0.6 MV/cm. The plasmon resonance of the photoexcited charge carriers exhibits a systematic redshift and a suppression of its spectral weight for THz driving fields exceeding 0.

Terahertz absorption-saturation and emission from electron-doped germanium quantum wells.

We study radiative relaxation at terahertz frequencies in n-type Ge/SiGe quantum wells, optically pumped with a terahertz free electron laser. Two wells coupled through a tunneling barrier are designed to operate as a three-level laser system with non-equilibrium population generated by optical pumping around the 1→3 intersubband transition at 10 THz. The non-equilibrium subband population dynamics are studied by absorption-saturation measurements and compared to a numerical model.

Up to 70 THz bandwidth from an implanted Ge photoconductive antenna excited by a femtosecond Er:fibre laser.

Phase-stable electromagnetic pulses in the THz frequency range offer several unique capabilities in time-resolved spectroscopy. However, the diversity of their application is limited by the covered spectral bandwidth. In particular, the upper frequency limit of photoconductive emitters - the most widespread technique in THz spectroscopy - reaches only up to 7 THz in the regular transmission mode due to absorption by infrared-active optical phonons.

Inhibition of Neutrophil Elastase and Cathepsin G As a New Approach to the Treatment of Psoriasis: From Fundamental Biology to Development of New Target-Specific Drugs.

Psoriasis therapy remains an extremely relevant area of modern drug design, due to necessity of adverse reaction reduction, inherent for actual methods of therapy. It was established that two serine proteases-neutrophil elastase 1 (HNE1) and cathepsin G (CatG)-are the key agents in psoriasis development. The collected molecular data for the presented targets form the basis for the molecular modeling strategy for the search for and identification of new target-specific inhibitors.

Electron dynamics in In Ga As shells around GaAs nanowires probed by terahertz spectroscopy.

We present the electrical properties of GaAs/In Ga As core/shell nanowires (NWs) measured by ultrafast optical pump-terahertz probe spectroscopy. This contactless technique was used to measure the photoconductivity of NWs with shell compositions of x = 0.20, 0.

Nonlinear plasmonic response of doped nanowires observed by infrared nanospectroscopy.

We report a strong shift of the plasma resonance in highly-doped GaAs/InGaAs core/shell nanowires (NWs) for intense infrared excitation observed by scattering-type scanning near-field infrared microscopy. The studied NWs show a sharp plasma resonance at a photon energy of about 125 meV in the case of continuous wave excitation by a CO laser. Probing the same NWs with the pulsed free-electron laser with peak electric field strengths up to several 10 kV cm reveals a power-dependent redshift to about 95 meV and broadening of the plasmonic resonance.

Doubly Dressed Bosons: Exciton Polaritons in a Strong Terahertz Field.

We demonstrate the existence of a novel quasiparticle, an exciton in a semiconductor doubly dressed with two photons of different wavelengths: a near infrared cavity photon and terahertz (THz) photon, with the THz coupling strength approaching the ultrastrong coupling regime. This quasiparticle is composed of three different bosons, being a mixture of a matter-light quasiparticle. Our observations are confirmed by a detailed theoretical analysis, treating quantum mechanically all three bosonic fields.

Four-Wave Mixing in Landau-Quantized Graphene.

For Landau-quantized graphene, featuring an energy spectrum consisting of nonequidistant Landau levels, theory predicts a giant resonantly enhanced optical nonlinearity. We verify the nonlinearity in a time-integrated degenerate four-wave mixing (FWM) experiment in the mid-infrared spectral range, involving the Landau levels LL, LL and LL. A rapid dephasing of the optically induced microscopic polarization on a time scale shorter than the pulse duration (∼4 ps) is observed, while a complementary pump-probe experiment under the same experimental conditions reveals a much longer lifetime of the induced population.

Slow Noncollinear Coulomb Scattering in the Vicinity of the Dirac Point in Graphene.

The Coulomb scattering dynamics in graphene in energetic proximity to the Dirac point is investigated by polarization resolved pump-probe spectroscopy and microscopic theory. Collinear Coulomb scattering rapidly thermalizes the carrier distribution in k directions pointing radially away from the Dirac point. Our study reveals, however, that, in almost intrinsic graphene, full thermalization in all directions relying on noncollinear scattering is much slower.

Pressure-dependent structural and electronic properties of quasi-one-dimensional (TMTTF)2PF6.

We have performed detailed x-ray investigations of the quasi-one-dimensional organic conductor (TMTTF)(2)PF(6) at room temperature and hydrostatic pressures up to 27 kbar. Based on the pressure-dependent crystal structure, the electronic band structure was calculated by density functional theory (DFT). Our systematic study provides important information on the coupling among the organic molecules but also to the anions.

Nonperturbative interband response of a bulk InSb semiconductor driven off resonantly by terahertz electromagnetic few-cycle pulses.

Intense multiterahertz pulses are used to study the coherent nonlinear response of bulk InSb by means of field-resolved four-wave mixing spectroscopy. At amplitudes above 5  MV/cm the signals show a clear temporal substructure which is unexpected in perturbative nonlinear optics. Simulations based on a model of a two-level quantum system demonstrate that in spite of the strongly off-resonant character of the excitation the high-field few-cycle pulses drive the interband resonances into a nonperturbative regime of Rabi flopping.

Ultrafast transient generation of spin-density-wave order in the normal state of BaFe2As2 driven by coherent lattice vibrations.

The interplay among charge, spin and lattice degrees of freedom in solids gives rise to intriguing macroscopic quantum phenomena such as colossal magnetoresistance, multiferroicity and high-temperature superconductivity. Strong coupling or competition between various orders in these systems presents the key to manipulate their functional properties by means of external perturbations such as electric and magnetic fields or pressure. Ultrashort and intense optical pulses have emerged as an interesting tool to investigate elementary dynamics and control material properties by melting an existing order.

Femtosecond response of quasiparticles and phonons in superconducting YBa(2)Cu(3)O(7-δ) studied by wideband terahertz spectroscopy.

We measure the anisotropic midinfrared response of electrons and phonons in bulk YBa(2)Cu(3)O(7-δ) after femtosecond photoexcitation. A line shape analysis of specific lattice modes reveals their transient occupation and coupling to the superconducting condensate. The apex oxygen vibration is strongly excited within 150 fs, demonstrating that the lattice absorbs a major portion of the pump energy before the quasiparticles are thermalized.

Infrared spectroscopy of KTa(1-x)Nb(x)O(3) crystals.

Fourier-transform infrared and time-domain THz dielectric spectroscopy measurements on KTa(1-x)Nb(x)O(3) crystals for x = 0.018 and 0.022 were performed.