Search

Terahertz (THz) radiation with sub-millimeter wavelength falls in the gap between optical and radio frequencies. Conventional THz emitters do not intertwine with spin degrees of freedom. However, it was recently shown that broadband THz radiation can be efficiently created also by exploiting spin-based effects on ultrafast time scales. Here, we demonstrate the generation and control of THz radiation from microstructured spintronic THz emitters based on the inverse spin-Hall effect. Using time-domain THz spectroscopy, we compare the THz spectra of different stripe patterns made of Fe/Pt bilayers with a spectrum obtained from an extended Fe/Pt bilayer film. It is found that the THz spectrum can be altered by a proper choice of the microstructure dimensions. The experimentally observed spectra are interpreted in terms of a simplified multi-slit interference model, which captures the main experimental features. Our results pave the way for an efficient control of THz light emitted from magnetic heterostructures. This is a crucial step forward for the design and realization of directional THz sources. [ABSTRACT FROM AUTHOR]

Terahertz (THz) sciences and technologies have contributed to a rapid development of a wide range of applications and expanded the frontiers in fundamental science. Spintronic terahertz emitters offer conceptual advantages since the spin orientation in the magnetic layer can be easily controlled either by the externally applied magnetic field or by the internal magnetic field distribution determined by the specific shape of the magnetic elements. Here, we report a switchable terahertz source based on micropatterned magnetic heterostructures driven by femtosecond laser pulses. We show that the precise tunability of the polarization state is facilitated by the underlying magnetization texture of the magnetic layer that is dictated by the shape of the microstructure. These results also reveal the underlying physical mechanisms of a nonuniform magnetization state on the generation of ultrafast spin currents in the magnetic heterostructures. Our findings indicate that the emission of the linearly polarized THz waves can be switched on and off by saturating the sample using a biasing magnetic field, opening fascinating perspectives for integrated on-chip THz devices with wide-ranging potential applications. [ABSTRACT FROM AUTHOR]

Ultrafast first-order phase transitions exhibit distinct transition pathways and dynamical properties that are not accessible during quasi-equilibrium transitions. Phenomena arising at the ultrafast timescale are important for understanding the transition mechanisms and in applications using the fast switching of electronic properties or magnetism. These transitions are accompanied by nanoscale structural dynamics that have been challenging to explore by optical or electronic transport probes. Here, X-ray nanodiffraction imaging shows that the nanoscale structural dynamics arising in ultrafast phase transitions differ dramatically from the transitions under slowly varying parameters. The solid-solid phase transitions in a FeRh thin film involve concurrent structural and magnetic changes and can be sensitively probed by monitoring their diffraction signatures following femtosecond optical excitation. Time-dependent nanodiffraction maps with 100-ps temporal and 25-nm spatial resolutions reveal that the preexisting nanoscale variation in phase composition results in spatially inhomogeneous changes of phase fraction after ultrafast optical excitation. The spatial inhomogeneity leads to nanoscale temperature variations and subsequent in-plane heat transport, which are responsible for spatially distinct relaxation pathways on nanometer length scales. The spatial gradients of the phase composition and elastic strain increase upon excitation rather than exhibiting the decrease previously reported in quasi-equilibrium transformations. Long-range elastic interactions thus do not play significant roles in the ultrafast phase transition. These microscopic insights into first-order phase transitions provide routes to manipulate nanoscopic phases in functional materials on ultrafast time scales by engineering initial nanoscale phase distributions. [ABSTRACT FROM AUTHOR]

Hybrid organic–inorganic perovskites such as methylammonium lead iodide have emerged as promising semiconductors for energy‐relevant applications. The interactions between charge carriers and lattice vibrations, giving rise to polarons, have been invoked to explain some of their extraordinary optoelectronic properties. Here, time‐resolved optical spectroscopy is performed, with off‐resonant pumping and electronic probing, to examine several representative lead iodide perovskites. The temporal oscillations of electronic bandgaps induced by coherent lattice vibrations are reported, which is attributed to antiphase octahedral rotations that dominate in the examined 3D and 2D hybrid perovskites. The off‐resonant pumping scheme permits a simplified observation of changes in the bandgap owing to the Ag vibrational mode, which is qualitatively different from vibrational modes of other symmetries and without increased complexity of photogenerated electronic charges. The work demonstrates a strong correlation between the lead–iodide octahedral framework and electronic transitions, and provides further insights into the manipulation of coherent optical phonons and related properties in hybrid perovskites on ultrafast timescales. [ABSTRACT FROM AUTHOR]

The spontaneously formed striped polarization nanodomain configuration of a PbTiO3/SrTiO3 superlattice transforms to a uniform polarization state under the above-bandgap illumination with a time dependence varying with the intensity of optical illumination and a well-defined threshold intensity. The recovery after the end of illumination occurs over a temperature-dependent period of tens of seconds at room temperature and shorter times at elevated temperatures. A model in which the screening of the depolarization field depends on the population of trapped electrons correctly predicts the observed temperature and optical intensity dependence. [ABSTRACT FROM AUTHOR]

Intense hard x-ray pulses from a free-electron laser induce irreversible structural damage in a perovskite oxide epitaxial heterostructure when pulse fluences exceed a threshold value. The intensity of x-ray diffraction from a 25-nm thick epitaxial BiFeO3 layer on a SrTiO3 (STO) substrate measured using a series of pulses decreases abruptly with a per-pulse fluence of 2.7 × 106 photons μm−2 at a photon energy of 9.7 keV but remains constant for 1.3 × 106 photons μm−2 or less. The damage resulted in the destruction of the BiFeO3 thin film within the focal spot area and the formation of a deep cavity penetrating into the STO substrate via the removal of tens of nanometers of material per pulse. The damage threshold occurs at a fluence that is insufficient to heat the absorption volume to the melting point. The morphology of the ablated sample is consistent with fracture rather than melting. Together, these results indicate that the damage occurs via a nonthermal process consistent with ultrafast ionization of the absorption volume. [ABSTRACT FROM AUTHOR]

A multimodal imaging instrument has been developed that integrates scanning near‐field optical microscopy with nanofocused synchrotron X‐ray diffraction imaging. The instrument allows for the simultaneous nanoscale characterization of electronic/near‐field optical properties of materials together with their crystallographic structure, facilitating the investigation of local structure–property relationships. The design, implementation and operating procedures of this instrument are reported. The scientific capabilities are demonstrated in a proof‐of‐principle study of the insulator–metal phase transition in samarium sulfide (SmS) single crystals induced by applying mechanical pressure via a scanning tip. The multimodal imaging of an in situ tip‐written region shows that the near‐field optical reflectivity can be correlated with the heterogeneously transformed structure of the near‐surface region of the crystal. [ABSTRACT FROM AUTHOR]

X-ray microscopy has been an indispensable tool to image nanoscale properties for materials research. One of its recent advances is extending microscopic studies to the time domain to visualize the dynamics of nanoscale phenomena. Large-scale X-ray facilities have been the powerhouse of time-resolved X-ray microscopy. Their upgrades, including a significant reduction of the X-ray emittance at storage rings (SRs) and fully coherent ultrashort X-ray pulses at free-electron lasers (FELs), will lead to new developments in instrumentation and will open new scientific opportunities for X-ray imaging of nanoscale dynamics with the simultaneous attainment of unprecedentedly high spatial and temporal resolutions. This review presents recent progress in and the outlook for time-resolved X-ray microscopy in the context of ultrafast nanoscale imaging and its applications to condensed matter physics and materials science. [ABSTRACT FROM AUTHOR]

Understanding and controlling the domain evolution under external stimuli in multiferroic thin films is critical to realizing nanoelectronic devices, including for non-volatile memory, data storage, sensors, and optoelectronics. In this article, we studied the shear-strain effect on the domain evolution with temperature in highly strained BiFeO3 thin films on rhombohedral LaAlO3 substrates using a high-resolution synchrotron X-ray diffraction three dimensional-reciprocal space mapping (3D-RSM) technique. The results revealed significant biaxial, anisotropic, evolution behaviors of the mixed-phase (MC + R′/T′ phases) BiFeO3 ferroelectric domains along the in-plane [100] and [010] axes. These biaxial, anisotropic, evolution behaviors were attributed to the shear-strain-modulated transition pathways of the mixed-phase ferroelectric domains. This viewpoint was further verified in the BiFeO3/LaSrAlO4 (001) system in which no anisotropic evolution behaviors of the mixed-phase domains were found. This work sheds light on the quantitative analysis of domain evolution in multi-domain systems and demonstrates that the shear-strain effect could act as an effective tool to manipulate the domain behavior and control novel functionalities of ferroelectric thin films. [ABSTRACT FROM AUTHOR]

Due to the strong dependence of electronic properties on the local bonding environment, a full characterization of the structural dynamics in ultrafast experiments is critical. Here, we report the dynamics and structural refinement at nanosecond time scales of a perovskite thin film by combining optical excitation with time-resolved X-ray diffraction. This is achieved by monitoring the temporal response of both integer and half-integer diffraction peaks of LaVO3 in response to an above-band-gap 800 nm pump pulse. We find that the lattice expands by 0.1% out of plane, and the relaxation is characterized by a biexponential decay with 2 and 12 ns time scales. We analyze the relative intensity change in half-integer peaks and show that the distortions to the substructure are small: the oxygen octahedral rotation angles decrease by ∼0.3° and La displacements decrease by ∼0.2 pm, which directly corresponds to an ∼0.8° increase in the V-O-V bond-angles, an in-plane V-O bond length reduction of ∼0.3 pm, and an unchanged out-of-plane bond length. This demonstration of tracking the atomic positions in a pump-probe experiment provides experimentally accessible values for structural and electronic tunability in this class of materials and will stimulate future experiments. [ABSTRACT FROM AUTHOR]

The coupling of ordered electronic phases with lattice, spin, and orbital degrees of freedom are of central interest in strongly correlated systems. Their interplay has been intensively studied from femtosecond to picosecond time scales, while their dynamics beyond nanoseconds are usually assumed to follow lattice cooling. Here, we report an unusual slowing down of the recovery of an electronic phase across a first-order phase transition. Following optical excitation, the recovery time of both transient optical reflectivity and X-ray diffraction intensity from the charge-ordered superstructure in a La1/3Sr2/3FeO3 thin film increases by orders of magnitude as the sample temperature approaches the phase transition temperature. In this regime, the recovery time becomes much longer than the lattice cooling time. The combined experimental and theoretical investigation shows that the slowing down of electronic recovery corresponds to the pseudo-critical dynamics that originates from magnetic interactions close to a weakly first-order phase transition. [ABSTRACT FROM AUTHOR]

Pump-probe studies at synchrotrons using X-ray and laser pulses require accurate determination of the time delay between pulses. This becomes especially important when observing ultrafast responses with lifetimes approaching or even less than the X-ray pulse duration (∼100 ps). The standard approach of inspecting the time response of a detector sensitive to both types of pulses can have limitations due to dissimilar pulse profiles and other experimental factors. Here, a simple alternative is presented, where the frequency response of the detector is monitored versus time delay. Measurements readily demonstrate a time resolution of ∼1 ps. Improved precision is possible by simply extending the data acquisition time. [ABSTRACT FROM AUTHOR]

Optoelectronic detection using photoconductive coplanar stripline devices has been applied to measuring the time profile of x-ray synchrotron pulses, a proof of concept demonstration that may lead to improved time-resolved x-ray studies. Laser sampling of current vs time delay between 12 keV x-ray and 800 nm laser pulses reveal the ∼50 ps x-ray pulse width convoluted with the ∼200 ps lifetime of the conduction band carriers. For GaAs implanted with 8 MeV protons, a time profile closer to the x-ray pulse width is observed. The protons create defects over the entire depth sampled by the x-rays, trapping the x-ray excited conduction electrons and minimizing lifetime broadening of the electrical excitation. [ABSTRACT FROM AUTHOR]

We report time-resolved studies of hydrogen bonding in liquid H2O, in response to direct excitation of the O–H stretch mode at 3 μm, probed viasoft X-ray absorption spectroscopy at the oxygen K-edge. This approach employs a newly developed nanofluidic cell for transient soft X-ray spectroscopy in the liquid phase. Distinct changes in the near-edge spectral region (XANES) are observed, and are indicative of a transient temperature rise of 10 K following transient laser excitation and rapid thermalization of vibrational energy. The rapid heating occurs at constant volume and the associated increase in internal pressure, estimated to be 8 MPa, is manifested by distinct spectral changes that differ from those induced by temperature alone. We conclude that the near-edge spectral shape of the oxygen K-edge is a sensitive probe of internal pressure, opening new possibilities for testing the validity of water models and providing new insight into the nature of hydrogen bonding in water. [ABSTRACT FROM AUTHOR]