Perturbative regime of terahertz high-harmonics generation in topological insulators

In this Letter, terahertz high harmonic generation processes in topological insulators of the bismuth and antimony chalcogenides family are investigated. Field conversion efficiencies are determined and clean cubic and quintic power-law scaling is observed for third and fifth harmonics, up to driving terahertz fields of 140 kV/cm. This is in contrast to all previous experiments on terahertz harmonics generation in Dirac materials where a non-perturbative regime has been observed already at few 10s kV/cm driving fields. Our nonlinear THz spectroscopy experiments are complemented by THz pump - optical probe measurements showing distinctly different relaxation dynamics of the carriers in the topologically-protected Dirac states at the surfaces and the bulk. The THz-induced dynamics of surface states reveal ultrafast relaxation that prevents accumulation effects, and results in a clear perturbative regime of THz harmonics generation that is different to graphene or Dirac semimetals with their slower relaxation times in the few ps regime

Terahertz signatures of ultrafast Dirac fermion relaxation at the surface of topological insulators

Topologically protected surface states present rich physics and promising spintronic, optoelectronic, and photonic applications that require a proper understanding of their ultrafast carrier dynamics. Here, we investigate these dynamics in topological insulators (TIs) of the bismuth and antimony chalcogenide family, where we isolate the response of Dirac fermions at the surface from the response of bulk carriers by combining photoexcitation with below-bandgap terahertz (THz) photons and TI samples with varying Fermi level, including one sample with the Fermi level located within the bandgap. We identify distinctly faster relaxation of charge carriers in the topologically protected Dirac surface states (few hundred femtoseconds), compared to bulk carriers (few picoseconds). In agreement with such fast cooling dynamics, we observe THz harmonic generation without any saturation effects for increasing incident fields, unlike graphene which exhibits strong saturation. This opens up promising avenues for increased THz nonlinear conversion efficiencies, and high-bandwidth optoelectronic and spintronic information and communication applications.Parts of this research were carried out at ELBE at the Helmholtz-Zentrum Dresden-Rossendorf e.V., a member of the Helmholtz Association. The films are grown in IRE RAS within the framework of the state task. This work was supported by the RFBR grants Nos. 18-29-20101, 19-02-00598. N.A., S.K., and I.I. acknowledge support from the European Union’s Horizon 2020 research and innovation program under grant agreement No. 737038 (TRANSPIRE). T.V.A.G.O. and L.M.E. acknowledge the support by the Würzburg-Dresden Cluster of Excellence on Complexity and Topology in Quantum Matter (ct.qmat). K.-J.T. acknowledges funding from the European Union’s Horizon 2020 research and innovation program under Grant Agreement No. 804349 (ERC StG CUHL) and financial support through the MAINZ Visiting Professorship. ICN2 was supported by the Severo Ochoa program from Spanish MINECO Grant No. SEV-2017-0706

Impulsive Fermi magnon-phonon resonance in antiferromagnetic CoF2CoF_{2}

Understanding spin-lattice interactions in antiferromagnets is one of the most fundamental issues at the core of the recently emerging and booming fields of antiferromagnetic spintronics and magnonics. Recently, coherent nonlinear spin-lattice coupling was discovered in an antiferromagnet which opened the possibility to control the nonlinear coupling strength and thus showing a novel pathway to coherently control magnon-phonon dynamics. Here, utilizing intense narrow band terahertz (THz) pulses and tunable magnetic fields up to 7 T, we experimentally realize the conditions of the Fermi magnon-phonon resonance in antiferromagnetic $CoF_{2}$. These conditions imply that both the spin and the lattice anharmonicities harvest energy transfer between the subsystems, if the magnon eigenfrequency $f_{m}$ is twice lower than the frequency of the phonon $2f_{m}=f_{ph}$. Performing THz pump-infrared probe spectroscopy in conjunction with simulations, we explore the coupled magnon-phonon dynamics in the vicinity of the Fermi-resonance and reveal the corresponding fingerprints of an impulsive THz-induced response. This study focuses on the role of nonlinearity in spin-lattice interactions, providing insights into the control of coherent magnon-phonon energy exchange

Terahertz-slicing -- an all-optical synchronization for 4th generation light sources

A conceptually new approach to synchronizing accelerator-based light sources and external laser systems is presented. The concept is based on utilizing a sufficiently intense accelerator-based single-cycle terahertz pulse to slice a thereby intrinsically synchronized femtosecond-level part of a longer picosecond laser pulse in an electro-optic crystal. A precise synchronization of the order of 10 fs is demonstrated, allowing for real-time lock-in amplifier signal demodulation. We demonstrate successful operation of the concept with three benchmark experiments using a 4th generation accelerator-based terahertz light source, i.e. (i) far-field terahertz time-domain spectroscopy, (ii) terahertz high harmonic generation spectroscopy, and (iii) terahertz scattering-type scanning near-field optical microscopy

Efficient optical-to-terahertz conversion in large-area InGaAs photo-Dember emitters with increased indium content

Optical-to-terahertz (THz) conversion of 800 nm femtosecond laser pulses in large-area bias-free InGaAs emitters based on photo-Dember (PD) and lateral photo-Dember (LPD) effects is experimentally investigated. We use metamorphic buffers to grow submicro-meter thick InxGa1-xAs layers with indium mole fractions x=0.37, 0.53, and 0.70 on a GaAs substrate. A strong enhancement of THz output energy with an increase of indium content is observed. On the surface of the sample providing the strongest emission (x=0.7), we have fabricated a 1.5 cm2 area of asymmetri-cally shaped metallic grating for LPD emission. This LPD emitter allows achieving high conversion efficiency of 0.24 . 10-3 and a broad generation band-width of up to 6 THz. We also demon-strate that there is no significant differ-rence in the conversion efficiency when operating at 1 and 200 kHz repetition rates. Our results show that large area LPD emitters give a convenient, competitive way to generate intense high-repetition-rate THz pulses

Возможности позитронной эмиссионной томографии, совмещенной с компьютерной томографией, с 18F-ФДГ в диагностике почечно-клеточного рака

Positron emission tomography combined with computed tomography (PET/CT) is a method, capable of evaluation of not only structural, but also metabolic alterations in organs and tissues. 18F-fluorodeoxyglucose (18F-PDG) PET/CT is widely used in diagnostics of various malignant tumors, but its role in renal cell carcinoma (RCC) remains obscure.Despite its limited effectiveness in primary and differential diagnosis of RCC, PET/CT can be used to determine the grade of malignancy and the histological type of tumor. According to various authors, 18F-FDG standardized uptake value (SUV) of a renal neoplasm can serve as an independent prognostic factor of overall survival.18F-FDG PET/CT is widely used to detect local recurrence and metastatic lesions in various neoplastic processes. Numerous studies confirm the high efficiency of PET/CT in detection of metastases and recurrence discernment for RCC.The ability to assess the metabolic activity of neoplastic foci using PET/CT may allow for evaluation of RCC therapy effectiveness, which is confirmed by clinical studies of various targeted therapy agents, such as tyrosine kinase inhibitors, mammalian target of rapamycin inhibitors, and monoclonal antibodies.Thus, there is growing evidence that 18F-FDG PET/CT is a useful tool in diagnostics of RCC.Позитронная эмиссионная томография, совмещенная с компьютерной томографией (ПЭТ/КТ), является методом оценки не только структурных, но и метаболических изменений в органах и тканях. ПЭТ/КТ с 18F-фтордезоксиглюкозой (18F-ФДГ) широко применяется в диагностике различных злокачественных опухолей, однако роль данного метода при почечно-клеточном раке (ПКР) остается неясной.Несмотря на ограниченную эффективность в первичной и дифференциальной диагностике ПКР, ПЭТ/КТ может быть использована для определения степени злокачественности и гистологического типа опухоли. Кроме этого, по данным различных авторов, показатель стандартизированной величины поглощения (standardized uptake value, SUV) 18F-ФДГ неопластического очага при ПКР может служить независимым прогностическим фактором общей выживаемости.ПЭТ/КТ с 18F-ФДГ широко применяется для выявления местного рецидива и метастатической диссеминации при различных неопластических процессах. Результаты многочисленных исследований подтверждают высокую эффективность ПЭТ/КТ в оценке распространенности и диагностике рецидива ПКР.Возможность определения метаболической активности неопластических очагов с помощью ПЭТ/КТ позволяет использовать этот метод для оценки эффективности терапии ПКР, что подтверждается данными клинических исследований различных таргетных препаратов, таких как ингибиторы тирозинкиназы, ингибиторы мишени рапамицина млекопитающих и моноклональные антитела.Таким образом, все больше данных свидетельствует в пользу того, что ПЭТ/КТ с 18F-ФДГ является полезным инструментом в диагностике ПКР.

Terahertz-wave decoding of femtosecond extreme-ultraviolet light pulses

In recent years, femtosecond extreme-ultraviolet (XUV) and x-ray pulses from free-electron lasers have developed into important probes to monitor processes and dynamics in matter on femtosecond-time and angstrom-length scales. With the rapid progress of versatile ultrafast x-ray spectroscopy techniques and more sophisticated data analysis tools, accurate single-pulse information on the arrival time, duration, and shape of the probing x-ray and XUV pulses becomes essential. Here, we demonstrate that XUV pulses can be converted into terahertz electromagnetic pulses using a spintronic terahertz emitter. We observe that the duration, arrival time, and energy of each individual XUV pulse is encoded in the waveform of the associated terahertz pulses, and thus can be readily deduced from single-shot terahertz time-domain detection

Theoretical and experimental studies of relativistic oversized Ka-band surface-wave oscillator based on 2D periodical corrugated structure

Based on a quasioptical approach and direct particle-in-cell simulations, we study dynamics of oversized relativistic surface-wave oscillators (SWOs) of the Cherenkov type with 2D periodical corrugated structures of cylindrical geometry. Such corrugation allows significant rarefication of the spectrum of modes with different azimuthal indices. As a result, selective excitation of a mode with a given azimuthal index is possible. Azimuthal index of the generated mode depends on the voltage rise time. For short (nanosecond scale) rise time, generation of an azimuthally symmetric mode can be realized. For longer (hundreds nanoseconds to microseconds) rise time, the modes with high azimuthal indexes would be excited. These conclusions are supported by the experiments where Ka-band SWOs with 2D corrugated structures were realized based on the 300  keV/100  A/4  μs thermionic accelerator SATURN. For an oversize factor of 16, stable narrow-band generation with output power of 1.5–2 MW was obtained at the frequency of 32.5 GHz corresponding to the mode with an azimuthal index of m=3. The project of Ka-band subgigawatt power SWOs operating at the azimuthally symmetric mode based on 500  keV/4  kA/20  ns high current explosive-emission accelerator SINUS-6 is under development

Synchronization of radiation in an oversized coaxial Ka-band backward wave oscillator using two-dimensional Bragg structure

A coaxial Ka-band backward wave oscillator with a two-dimensional Bragg structure located at the output of the interaction space has been studied. This structure has a double-period corrugation and provides azimuthal electromagnetic energy fluxes, which act on the synchronized radiation of an oversized tubular electron beam. Proof-of-principle experiments were conducted based on the Saturn thermionic accelerator (300  keV/200  A/2  μs). In accordance with simulations, narrow-band generation was obtained at a frequency of 30 GHz and a power level of 1.5–2 MW. As a result, the possibility of using a two-dimensional distributed feedback mechanism in oscillators of the Cherenkov type has been demonstrated