Search

RFeO3 is an ideal candidate for the fabrication of spintronic devices, and its rare earth site doping is an important means to regulate and produce many unique spin behaviors. In this work, we report a high-quality Dy0.5Tb0.5FeO3 single crystal grown by optical floating zone method and its magnetic properties are studied explicitly. A type Ⅱ spin switching effect and suppressed spin reorientation transition are found in the Magnetization vs Temperature curves in Dy0.5Tb0.5FeO3. In the temperature region of spin reorientation transition, the transition of Fe3+ magnetic sublattice configuration becomes very complicated by the influence of competitive interaction with magnetic Dy3+/Tb3+ ions. Interestingly, a field tunable double-hysteresis loop is observed near the antiferromagnetic transition temperature of rare earth ions sublattice, which has never been reported in RFeO3 family compounds. By studying the dependence of double-hysteresis loop at different temperatures, the magnetic phase transition and spin reorientation transition induced by the applied magnetic field is inferred. This novel physical phenomenon helps us understand the mechanism of spin configuration transition better at low temperatures.

Generation of terahertz (THz) wave from air plasma induced by femtosecond laser pulses with a single central frequency (the so-called "single-color") is one of the fundamental interactions between light and matter, and is also the basis of subsequent pumping schemes using two- or multi-color laser fields. Recently, more states of media beyond gas (e.g., atomic cluster and liquid) via photo-ionization have brought new experimental observations of THz radiation, which can no longer be simply attributed to the mainstream model of the transition-Cherenkov radiation (TCR), thus making the whole picture unclear. Here, we revisited the mechanism of this dynamic process in a new view of the traveling-wave antenna (TWA) model. By successfully reproducing the reported far-field THz radiation profiles from various plasma filament arrangements, the wide applicability of the TWA theory has been revealed. On the other hand in the microscopic view, we investigated the plasma oscillation during filamentation aiming at further bridging the plasma filament and the antenna. Accordingly, THz-plasma resonance has been theoretically and experimentally demonstrated as the elementary THz emitter, paving the way towards fully understanding this important single-color plasma based THz source.

The dynamic control of magnetization by short laser pulses has recently attracted interest. The transient magnetization at the metallic magnetic interface has been investigated through second-harmonic generation and the time-resolved magneto-optical effect. However, the ultrafast light-driven magneto-optical nonlinearity in ferromagnetic heterostructures for terahertz (THz) radiation remains unclear. Here, we present THz generation from a metallic heterostructure, Pt/CoFeB/Ta, which is ascribed to an ∼6–8% contribution from the magnetization-induced optical rectification and an ∼94–92% contribution from both spin-to-charge current conversion and ultrafast demagnetization. Our results show that THz-emission spectroscopy is a powerful tool to study the picosecond-time-scale nonlinear magneto-optical effect in ferromagnetic heterostructures.

Understanding of ultrafast spin dynamics is crucial for future ultrafast and energy efficient magnetic memory and storage applications. We study the ultrafast laser-induced magneto-optical response of a CoFeB/MgO/CoFeB magnetic tunneling junction (MTJ), when exciting with a short laser pulse, as a function of magnetic configuration and pump fluence. The ultrafast magnetization of the MTJ drops rapidly in the timescale of 0.33–0.37 ps, which is driven by both electron-spin scattering and spin transport mutually. Subsequently, the energy from the electron and spin reservoirs transfers into the lattice with the timescale of 1.5–2.0 and 5.0–15.0 ps through the electron–phonon and spin–phonon interactions, respectively. Our results suggest that the interfacial spin-orientation of CoFeB/MgO/CoFeB could modulate the interaction constant between spins and phonons. These findings provide insight into the role of MTJ interface in spin dynamics, which will be helpful for opto-spintronic tunnel junction stack designs and applications.

In this paper, a chiral metamaterial (MM) structure composed of bilayer twisted split-ring resonator (SRR) arrays is proposed and demonstrated to realize polarization-controlled, dual-directional, and multifunctional polarization conversion for terahertz (THz) waves. Simulated results show that the converter can selectively achieve linear-to-linear, or linear to left-handed circular, or linear to right-handed circular’s polarization conversion according to the polarization state and propagating direction of the incident THz wave. Stokes parameters, ellipticity, and polarized rotation angle are introduced to determine the polarization states of the output waves. The circular polarization transmission coefficients and surface current distributions at the resonance frequency points are simulated to explain and demonstrate the physical origin of polarization conversion. The proposed multifunctional converter can find potential applications in THz sensing, imaging and communication areas.

A material’s magnetic state and its dynamics are of great fundamental research interest and are also at the core of a wide plethora of modern technologies. However, reliable access to magnetization dynamics in materials and devices on the technologically relevant ultrafast timescale, and under realistic device-operation conditions, remains a challenge. Here, we demonstrate a method of ultrafast terahertz (THz) magnetometry, which gives direct access to the (sub-)picosecond magnetization dynamics even in encapsulated materials or devices in a contact-free fashion, in a fully calibrated manner, and under ambient conditions. As a showcase for this powerful method, we measure the ultrafast magnetization dynamics in a laser-excited encapsulated iron film. Our measurements reveal and disentangle distinct contributions originating from (i) incoherent hot-magnon-driven magnetization quenching and (ii) coherent acoustically-driven modulation of the exchange interaction in iron, paving the way to technologies utilizing ultrafast heat-free control of magnetism. High sensitivity and relative ease of experimental arrangement highlight the promise of ultrafast THz magnetometry for both fundamental studies and the technological applications of magnetism., Nature Communications, 11 (1), ISSN:2041-1723

The atomically thin layered transition metal dichalcogenide (TMDCs) PtSe2 is a new emerging two-dimension (2D) material, which has a transition from indirect-gap semiconductor to semimetal with increasing thickness. Defects in 2D TMDCs are very ubiquitous and play a crucial role in understanding many electronics and optoelectronics in TMDCs. In this article, PtSe2 films with different thickness are obtained by selenizing the variously thick Pt films. Extrapolation of the onset absorption from the visible-infrared spectrum demonstrates that the selenization of 1 and 3 nm Pt films shows semiconductor-like character, while those of thick Pt films (with thickness of 10 and 15 nm) show metallic behavior. The transient absorption (TA) spectroscopy reveals that all films show immediately photobleaching signals after photoexcitation at 800 nm, and the subsequent relaxation process shows strongly thickness dependence. The thinnest film shows biexponential relaxation with typical time constants of 1.28 and 101.2 ps. In contrast, the photobleaching signals are developed into photoinduced absorption signals in thicker films, and the absorption magnitude increases with the thickness of the films. The optical pump and terahertz (THz) probe spectroscopy reveals that all samples show positive photoconductivity after photoexcitation of 800 nm, the subsequent recovery is completed within 2.0 ps, and the recovery time constant decreases with the increase of the films' thickness. Our TA and THz spectroscopy results reveal that the defect states of the films play dominated role in the relaxation of photocarrier after photoexcitation, and edgesite states are inferred to make dominated contributions to the defect states in the selenization of platinum films.

Spin transport is crucial for future spintronic devices operating at bandwidths up to the terahertz range. In F|N thin-film stacks made of a ferromagnetic/ferrimagnetic layer F and a normal-metal layer N, spin transport is mediated by (1) spin-polarized conduction electrons and/or (2) torque between electron spins. To identify a crossover from (1) to (2), we study laser-driven spin currents in F|Pt stacks where F consists of model materials with different degrees of electrical conductivity. For the magnetic insulators yttrium iron garnet, gadolinium iron garnet (GIG) and γ−Fe2O3, identical dynamics is observed. It arises from the terahertz interfacial spin Seebeck effect (SSE), is fully determined by the relaxation of the electrons in the metal layer, and provides a rough estimate of the spin-mixing conductance of the GIG/Pt and γ−Fe2O3/Pt interfaces. Remarkably, in the half-metallic ferrimagnet Fe3O4 (magnetite), our measurements reveal two spin-current components with opposite direction. The slower, positive component exhibits SSE dynamics and is assigned to torque-type magnon excitation of the A- and B-spin sublattices of Fe3O4. The faster, negative component arises from the pyrospintronic effect and can consistently be assigned to ultrafast demagnetization of minority-spin hopping electrons. This observation supports the magneto-electronic model of Fe3O4. In general, our results provide a route to the contact-free separation of torque- and conduction-electron-mediated spin currents., The authors acknowledge funding by the German Research Foundation through the collaborative research centers SFB TRR 227 “Ultrafast spin dynamics” (Project ID 328545488, projects A05, B02, and B03), SFB TRR 173 “Spin + X” (Project No. 358671374, projects A01 and B02), the European Union H2020 program through the project CoG TERAMAG(GrantNo.681917), the Spanish Ministry of Science and Innovation (Project No. PID2020-112914RB-I00), the National Natural Science Foundation of China (Grants No. 61988102 and No. 61975110), and the Czech Science Foundation through Project GA CR/Grant No. 21-28876J. P.J.-C. acknowledges the Spanish MECD for support through the FPU program (References No. FPU014/02546 and No. EST17/00382.)

The ultrafast photocurrents in a typical 3D topological insulator (TI) Sb2Te3 were probed with THz emission spectroscopy by tuning the polarization of excitation pulses and the sample’s azimuthal angle. The different ultrafast photocurrents driven by the linear photogalvanic effect, circular photogalvanic effect, and thermoelectric effect were distinguished from THz emission signal. Results potentially provide insights into the ultrafast all optical modulation of THz emission with non-external bias field, which could play a vital role in future TI-based high-speed THz optoelectronic and optospintronic devices.

We demonstrate that the tunneling magneto-resistance (TMR) can be used to modulate terahertz (THz) wave propagation through a magnetic tunnel junctions (MTJ). Operating at the THz frequency range, a maximal modulation depth of 60% is reached for the parallel state of the MTJ with the thickness of 77.45 nm, by using a small magnetic field of 30 mT. The THz conductivity spectrum of the MTJ is governed by the spin-dependent electron tunneling. This findings open that the MTJ device will have potential applications in THz magnetic modulators, filtering and sensing.

By employing ultrafast time-resolved optical pump terahertz (THz) probe spectroscopy, we report a comparative THz response on CVD grown topological insulator (TI) Sb 2 Te 3 under different temperatures. Both photo induced carrier dynamics attributed to the surface and bulk electrons are observed. The electron system undergoes an ultrafast excitation and relaxation and releases through intra-band and inter-band scattering.

We present a method to enhance the terahertz (THz) wave radiation from a femtosecond laser-induced plasma filament by controlling the local gas density within the filament. We develop a theoretical model for THz generation from a laser-induced air plasma filament and the subsequent propagation process, to account for a varying local gas density. By adjusting the local gas density along the filament, the transient current distribution along the filament and the resulting coherent superposition of terahertz waves can be controlled. The location of the gas jet nozzle and the relative phase between multicolor light fields both affect the transient current distribution and thus the strength of the generated THz field. Compared with the conventional terahertz generation by a two-color filament in a homogeneous gas, a three-color filament can realize an increase by 6.12 times in the generated THz pulse energy, with optimized local gas density modulation. Our results suggest that the THz amplification via local gas density modulation can be further improved with well-designed multicolor pulses.

As a newly emergent type-II Dirac semimetal, Platinum Telluride (PtTe2) stands out from other 2D noble-transition-metal dichalcogenides for the unique structure and novel physical properties, such as high carrier mobility, strong electron-phonon coupling and tunable bandgap, which make the PtTe2 a good candidate for applications in optoelectronics, valleytronics and far infrared detectors. Although the transport properties of PtTe2 have been studied extensively, the dynamics of the nonequilibrium carriers remain nearly uninvestigated. Herein we employ optical pump-terahertz (THz) probe spectroscopy (OPTP) to systematically study the photocarrier dynamics of PtTe2 thin films with varying pump fluence, temperature, and film thickness. Upon photoexcitation the THz photoconductivity (PC) of 5 nm PtTe2 film shows abrupt increase initially, while the THz PC changes into negative value in a subpicosecond time scale, followed by a prolonged recovery process that lasted hundreds of picoseconds (ps). This unusual THz PC response observed in the 5 nm PtTe2 film was found to be absent in a 2 nm PtTe2 film. We assign the unexpected negative THz PC as the small polaron formation due to the strong electron-Eg-mode phonon coupling, which is further substantiated by pump fluence- and temperature-dependent measurements as well as the Raman spectroscopy. Moreover, our investigations give a subpicosecond time scale of sequential carrier cooling and polaron formation. The present study provides deep insights into the underlying dynamics evolution mechanisms of photocarrier in type-II Dirac semimetal upon photoexcitation, which is fundamental importance for designing PtTe2-based optoelectronic devices., 18 pages, 4 figures

Lenses accessing multiple focal spots with polarization rotatable functionality or focal spot with extended focal depth is naturally challenging. We propose and experimentally demonstrate an approach to realizing terahertz multi-foci metalens with polarization-rotated focal points. Terahertz polarization-dependent imaging is realized by the multi-foci metalens. Furthermore, we propose and approach to design terahertz metalens with simultaneous extended focal depth and polarization insensitivity. As a proof-of-concept, longitudinal high-tolerance imaging is demonstrated by the metalens.

The ultrafast electron transport is central to the requirement of all-electrically controlled spintronics in metallic collinear antiferromagnetic (AFM) ${\mathrm{Mn}}_{2}\mathrm{Au}$ thin films within the terahertz (THz) range. Here we use the THz time-domain spectroscopy to measure the conductivity spectra of ${\mathrm{Mn}}_{2}\mathrm{Au}$ thin films with different thicknesses and temperatures. We demonstrate an onset of carrier localization in ${\mathrm{Mn}}_{2}\mathrm{Au}$ with decreasing the film thickness from 15 to 5 nm. By using optical pump-THz probe spectroscopy, we found the surface trapping states in the thinnest ${\mathrm{Mn}}_{2}\mathrm{Au}$ film limit its photoconductivity significantly. The DC conductivity and scattering time become larger and longer with decreasing temperature for all samples, which suggests that ${\mathrm{Mn}}_{2}\mathrm{Au}$ behaves in a typical metallic fashion. The THz optical and conductivity properties of ${\mathrm{Mn}}_{2}\mathrm{Au}$ films will advance our understanding of ultrafast electronic transport processes in AFM ${\mathrm{Mn}}_{2}\mathrm{Au}$ thin films.

Magnetic tunnel junctions (MTJs) are a key technology in modern spintronics because they are the basis of read-heads of modern hard disk drives, nonvolatile magnetic random access memories, and sensor applications. In this paper, we demonstrate that tunneling magnetoresistance can influence terahertz (THz) wave propagation through a MTJ. In particular, various magnetic configurations between parallel state and antiparallel state of the magnetizations of the ferromagnetic layers in the MTJ have the effect of changing the conductivity, making a functional modulation of the propagating THz electromagnetic fields. Operating in the THz frequency range, a maximal modulation depth of 60% is reached for the parallel state of the MTJ with a thickness of 77.45 nm, using a magnetic field as low as 30 mT. The THz conductivity spectrum of the MTJ is governed by spin-dependent electron tunneling. It is anticipated that the MTJ device and its tunability scheme will have many potential applications in THz magnetic modulators, filtering, and sensing.

In laser-excited ferromagnetic heterostructures, both ultrafast local magnetization dynamics and spin-transport processes can lead to a THz emission. Here, we demonstrate that the THz emission spectroscopy is a powerful tool to investigate ultrafast magnetization dynamics in laser-excited magnetic systems. The polarity of emitted THz can be used to distinguish which process, local or non-local, dominates the emission of THz in a ferromagnetic heterostructure. The measured THz radiation can be used for rigorous reconstruction of ultrafast magnetization process in the laser-excited magnetic material.

TmFeO

Exotic quantum vacuum phenomena are predicted in cavity quantum electrodynamics systems with ultrastrong light-matter interactions. Their ground states are predicted to be vacuum squeezed states with suppressed quantum fluctuations owing to antiresonant terms in the Hamiltonian. However, such predictions have not been realized because antiresonant interactions are typically negligible compared to resonant interactions in light-matter systems. Here we report an unusual, ultrastrongly coupled matter-matter system of magnons that is analytically described by a unique Hamiltonian in which the relative importance of resonant and antiresonant interactions can be easily tuned and the latter can be made vastly dominant. We found a regime where vacuum Bloch-Siegert shifts, the hallmark of antiresonant interactions, greatly exceed analogous frequency shifts from resonant interactions. Further, we theoretically explored the system’s ground state and calculated up to 5.9 dB of quantum fluctuation suppression. These observations demonstrate that magnonic systems provide an ideal platform for exploring exotic quantum vacuum phenomena predicted in ultrastrongly coupled light-matter systems., Ultrastrong light-matter interactions with dominant antiresonant terms are expected to give rise to interesting phenomena such as quantum fluctuation suppression. Here, the authors propose a system of ultrastrongly coupled magnon modes in a rare earth orthoferrite as a platform for exploring such phenomena.

We investigate ultrafast magnetization dynamics in encapsulated iron films, benefitting from their laser-induced terahertz emission. Combined with first-principles theoretical modeling, the experimental results provide quantitative insights into the observed magnetization dynamics.

We studied magnon-magnon ultrastrong coupling in YFeO3 using terahertz magnetospectroscopy in magnetic fields up to 30 T, which led to an extreme breakdown of the rotating-wave approximation where the counter-rotating term dominates the co-rotating term.

Cooperative quantum magnetism One of the earliest and most intensively studied problems in quantum optics is the interaction of a two-level system (an atom) with a single photon. This simple system provides a rich platform for exploring exotic light-matter interactions and the emergence of more complex phenomena such as superradiance, which is a cooperative effect that emerges when the density of atoms is increased and coupling between them is enhanced. Going beyond the light-matter system, Li et al. observed analogous cooperative effects for coupled magnetic systems. The results suggest that ideas in quantum optics could be carried over and used to control and predict exotic phases in condensed matter systems. Science , this issue p. 794

Magnon-polaritons are shown to play a dominant role in the propagation of terahertz (THz) waves through TmFeO3 orthoferrite, if the frequencies of the waves are in the vicinity of the quasi-antiferromagnetic spin resonance mode. Both time-domain THz transmission and emission spectroscopies reveal clear beatings between two modes with frequencies slightly above and slightly below this resonance, respectively. Rigorous modeling of the interaction between the spins of TmFeO3 and the THz light shows that the frequencies correspond to the upper and lower magnon-polariton branches. Our findings reveal the previously ignored importance of propagation effects and polaritons in such heavily debated areas as THz magnonics and THz spectroscopy of electromagnons. It also shows that future progress in these areas calls for an interdisciplinary approach at the interface between magnetism and photonics.

The interplay among ferroelectric, magnetic and elastic degrees of freedom in multiferroics is the key issues in condensed matters, which has been widely investigated by various methods. Here, using ultrafast two-color pump-probe spectroscopy, the picosecond electron-phonon and spin-lattice coupling process in Dysprosium doped-BiFeO3 (BDFO) films on SrTiO3 (STO) substrate have been investigated systematically. The Dy-doping induced structural transition and magnetic enhancement in BDFO is observed by ultrafast electron-phonon and spin-lattice interaction, respectively. The elastic anomalies in BDFO films are revealed by the photo-induced coherent acoustic phonon. With increasing the Dy doping ratio, the frequencies of the acoustic phonon in the films are modulated, and the phonon transmission coefficient between films and substrate is found to approach unity gradually. The ultrafast observation of the tunability of the ferroelectric, magnetic and the elastic properties in the morphotropic phase boundary of rare-earth doped BFO films provides new insights into the integration of BFO in next-generation high frequency electro-magnetic and electroacoustic devices.

The low-order harmonic generation induced by a strong laser field produces a bright, ultrashort, supercontinuum radiation ranging from the terahertz to ultraviolet band. By controlling the phase-delay and ellipticity of the bi-chromatic laser fields, the third harmonic generation is experimentally and theoretically investigated for elucidating the mechanism of the low-order harmonics. The third harmonic generation is found to be strongly suppressed in the counter-rotating bi-chromatic laser field due to the selection rule for harmonic emissions. The continuum-continuum transition in the strong field approximation is extended to explain the third harmonic generation as a function of the phase delay and ellipticity of the bi-chromatic laser fields. Compared with the semi-classical photocurrent model, the continuum-continuum transition on the basis of quantum-mechanical treatment achieves better agreement with the experimental observations. Our work indicates that the overlapping in continuum states via different quantum paths of a single electron plays a role in low-order harmonics generation under elliptical bi-chromatic laser fields.

The exciton effect in two-dimensional (2D) transition metal dichalcogenides (TMDs) plays a dominating role in describing the optical and optoelectronic properties. However, the interplay between the excitons and free carriers has not yet been understood upon photoexcitation in 2D TMDs. Here, we first present a study of the dynamical interplay of excitons and unbound electron–hole pairs using time-resolved terahertz (THz) spectroscopy (TRTS) in a few-layer WS2 laminate. Our experimental results demonstrate that the Auger recombination is observed in the relaxation process of the mobile charge carriers rather than that of excitons upon photoexcitation. The transient complex THz photoconductivity spectroscopy of WS2 is well described by the Drude–Lorentz model of free carriers modulated by the exciton polarization field. Our results provide a comprehensive understanding of nonequilibrium carrier kinetics (both excitons and free carriers) in WS2 laminate, and should be applicable to other 2D systems.

Graphene oxide (GO) is an attractive option for large scale production of graphene. On the other hand, the graphene obtained by the reduction of GO has inevitable structural defects, and the vacant lattice sites will significantly restrict its conductivity. It has been demonstrated that thermal annealing in hydrogen is an efficient method to reduce defects and heal the lattice in GO samples. However, it is still not clear how the defects and/or disordering influence the photoelectric conversion efficiency and the carrier relaxation pathway in GO. Herein, time-domain terahertz (THz) spectroscopy is employed to characterize the properties of the multilayer GO films which were annealed in hydrogen at various temperatures. Upon photoexcitation, a transient increase of the conductivity was observed for the reduced graphene oxide (RGO) samples. The ultrafast carrier relaxation process can be well assigned to the carrier–carrier scattering and carrier–phonon coupling. Our results demonstrated that the RGO films ...

The speed of optical modulation on a terahertz (THz) pulse is mainly dominated by the optical response of the photocarrier. In order to achieve ultrafast THz modulation, the effective method is to reduce the lifetime of the photocarrier by introducing defects that can trap the photocarriers efficiently. In this paper, we reported the ultrafast optical modulation of THz switching in a 10 nm CdTe nanostructure film. After photoexcitation at 800/400 nm, the THz response of the film is extremely fast with a lifetime of ${\sim}{1.3}\;{\rm ps}$∼1.3ps. Further, the ultrafast transient THz transmission shows almost temperature independence down to 100 K. On the other hand, the transient absorption spectroscopy reveals that the lifetime of photocarriers in CdTe nanostructure film lasts as long as several ns. The 1.3 ps THz photoconductivity response is due to the substantial decrease of photocarrier mobility in a CdTe nanostructure, which comes from the increase of the photocarrier scattering between the photocarrier and the surface states of CdTe nanostructural film. Our experimental results provide a new method to design optically driven ultrafast THz response devices, such as THz switch and THz modulator.

THz time-domain spectroscopy (THz-TDS) is used to study the THz-optical properties of a single crystal bismuth ferrite (BiFeO 3 ). The anisotropy of BiFeO 3 is strongly dependent on the temperature. A giant birefringence is observed up to around 3.6.

THz emission spectroscopy is a powerful tool to access ultrafast magnetization dynamics in laser-excited magnetic systems. Here, we demonstrate femtosecond terahertz magnetometry, a method that can be used to rigorously reconstruct the (sub)picosecond-timescale magnetization dynamics in photoexcited magnetic sample from the detected THz emission. Considering the propagation of the THz field from the surface of the sample to the THz electro-optic sampling unit, we establish the transfer function, which allows us to obtain the ultrafast magnetization dynamics in the laser-excited sample in a calibrated manner.