Ultrafast electron crystallography of the cooperative reaction path in vanadium dioxide

Ultrafast Charge Carrier Dynamics in Vanadium Dioxide, VO2: Nonequilibrium Contributions to the Photoinduced Phase Transitions

Vanadium oxide (VO2) is an exotic phase-change material with diverse applications ranging from thermochromic smart windows to thermal sensors, neuromorphic computing, and tunable metasurfaces. Nonetheless, the mechanism responsible for its metal-insulator phase transition remains a subject of vigorous debate. Here, we investigate the ultrafast dynamics of the photoinduced phase transition in VO2 under low perturbation conditions. By experimentally examining carrier relaxation dynamics at energy levels near the VO2 band gap (0.6-0.92 eV), we note that numerous optical features do not correspond to the first-order phase transition. Previous studies indeed induced such a phase transition, but they relied on fluences at least an order of magnitude higher, leading to temperature increases well above the transition threshold (340 K). Instead, for excitation fluences that correspond to lattice temperatures only in slight excess of the phase transition (absolute temperatures < 500 K), we find that the marked changes in optical properties are dominated by a shift in the electronic density of states/Fermi level. We find that this effect is a lattice-driven process and does not occur until sufficient energy has been transferred from the excited electrons into the phonon subsystem.

Complex Oxides for Brain-Inspired Computing: A Review

The fields of brain-inspired computing, robotics, and, more broadly, artificial intelligence (AI) seek to implement knowledge gleaned from the natural world into human-designed electronics and machines. In this review, the opportunities presented by complex oxides, a class of electronic ceramic materials whose properties can be elegantly tuned by doping, electron interactions, and a variety of external stimuli near room temperature, are discussed. The review begins with a discussion of natural intelligence at the elementary level in the nervous system, followed by collective intelligence and learning at the animal colony level mediated by social interactions. An important aspect highlighted is the vast spatial and temporal scales involved in learning and memory. The focus then turns to collective phenomena, such as metal-to-insulator transitions (MITs), ferroelectricity, and related examples, to highlight recent demonstrations of artificial neurons, synapses, and circuits and their learning. First-principles theoretical treatments of the electronic structure, and in situ synchrotron spectroscopy of operating devices are then discussed. The implementation of the experimental characteristics into neural networks and algorithm design is then revewed. Finally, outstanding materials challenges that require a microscopic understanding of the physical mechanisms, which will be essential for advancing the frontiers of neuromorphic computing, are highlighted.

A Review of Phase-Change Materials and Their Potential for Reconfigurable Intelligent Surfaces

Phase-change materials (PCMs) and metal-insulator transition (MIT) materials have the unique feature of changing their material phase through external excitations such as conductive heating, optical stimulation, or the application of electric or magnetic fields, which, in turn, results in changes to their electrical and optical properties. This feature can find applications in many fields, particularly in reconfigurable electrical and optical structures. Among these applications, the reconfigurable intelligent surface (RIS) has emerged as a promising platform for both wireless RF applications as well as optical ones. This paper reviews the current, state-of-the-art PCMs within the context of RIS, their material properties, their performance metrics, some applications found in the literature, and how they can impact the future of RIS.

Femtosecond-resolved imaging of a single-particle phase transition in energy-filtered ultrafast electron microscopy

Using an energy filter in transmission electron microscopy has enabled elemental mapping at the atomic scale and improved the precision of structural determination by gating inelastic and elastic imaging electrons, respectively. Here, we use an energy filter in ultrafast electron microscopy to enhance the temporal resolution toward the domain of atomic motion. Visualizing transient structures with femtosecond temporal precision was achieved by selecting imaging electrons in a narrow energy distribution from dense chirped photoelectron packets with broad longitudinal momentum distributions and thus typically exhibiting picosecond durations. In this study, the heterogeneous ultrafast phase transitions of vanadium dioxide (VO₂) nanoparticles, a representative strongly correlated system, were filmed and attributed to the emergence of a transient, low-symmetry metallic phase caused by different local strains. Our approach enables electron microscopy to access the time scale of elementary nuclear motion to visualize the onset of the structural dynamics of matter at the nanoscale.

VO2-based ultra-reconfigurable intelligent reflective surface for 5G applications

As demand for higher capacity wireless communications increases, new approaches are needed to improve capacity. The lack of configurable radio platforms and power consumed to create new signals are some of the limitations preventing further advancements. To address these limitations, we propose an Ultra-Reconfigurable Intelligent Surface (URIS) platform based on the metal-to-insulator transition property of VO₂. A VO₂ layer is placed on a high-density micro-heater matrix consisting of pixels that can be electronically switched on. With this manner of control, heat can be transferred to selected areas of the VO₂ layer and convert it to highly conductive metallic phase. This technique allows dynamically changing the shape of the reflection surface with high speed. We numerically investigated the heat activated switching and RF reflection characteristics of a reflectarray designed for potential 5G applications operating at 32 GHz. It consists of heating pixels with the size of 40 × 40 μm which can generate metallic VO₂ patches or arbitrary shapes with ~ 100 × 100 μm spatial resolution. Our analyses resulted in large phase range of ~ 300° and approximate losses of −2 dB. The proposed device can serve as a novel platform for ultra-reconfigurable reflectarrays, other IRSs, and various wide spectral range RF applications.

Active optical metasurfaces: comprehensive review on physics, mechanisms, and prospective applications

Optical metasurfaces with subwavelength thickness hold considerable promise for future advances in fundamental optics and novel optical applications due to their unprecedented ability to control the phase, amplitude, and polarization of transmitted, reflected, and diffracted light. Introducing active functionalities to optical metasurfaces is an essential step to the development of next-generation flat optical components and devices. During the last few years, many attempts have been made to develop tunable optical metasurfaces with dynamic control of optical properties (e.g., amplitude, phase, polarization, spatial/spectral/temporal responses) and early-stage device functions (e.g., beam steering, tunable focusing, tunable color filters/absorber, dynamic hologram, etc) based on a variety of novel active materials and tunable mechanisms. These recently-developed active metasurfaces show significant promise for practical applications, but significant challenges still remain. In this review, a comprehensive overview of recently-reported tunable metasurfaces is provided which focuses on the ten major tunable metasurface mechanisms. For each type of mechanism, the performance metrics on the reported tunable metasurface are outlined, and the capabilities/limitations of each mechanism and its potential for various photonic applications are compared and summarized. This review concludes with discussion of several prospective applications, emerging technologies, and research directions based on the use of tunable optical metasurfaces. We anticipate significant new advances when the tunable mechanisms are further developed in the coming years.

Nanoscale-femtosecond dielectric response of Mott insulators captured by two-color near-field ultrafast electron microscopy

Characterizing and controlling the out-of-equilibrium state of nanostructured Mott insulators hold great promises for emerging quantum technologies while providing an exciting playground for investigating fundamental physics of strongly-correlated systems. Here, we use two-color near-field ultrafast electron microscopy to photo-induce the insulator-to-metal transition in a single VO₂ nanowire and probe the ensuing electronic dynamics with combined nanometer-femtosecond resolution (10⁻²¹ m ∙ s). We take advantage of a femtosecond temporal gating of the electron pulse mediated by an infrared laser pulse, and exploit the sensitivity of inelastic electron-light scattering to changes in the material dielectric function. By spatially mapping the near-field dynamics of an individual nanowire of VO₂, we observe that ultrafast photo-doping drives the system into a metallic state on a timescale of ~150 fs without yet perturbing the crystalline lattice. Due to the high versatility and sensitivity of the electron probe, our method would allow capturing the electronic dynamics of a wide range of nanoscale materials with ultimate spatiotemporal resolution.

The fs control of an insulator-to-metal transition down to a few nanometers and its real-time/real space observation remain a challenge. Here, the authors demonstrate a method based on ultrafast electron microscopy to provide a nm/fs resolved view of the electronic dynamics in a single VO₂ nanowire.

Large non-thermal contribution to picosecond strain pulse generation using the photo-induced phase transition in VO2

Picosecond strain pulses are a versatile tool for investigation of mechanical properties of meso- and nano-scale objects with high temporal and spatial resolutions. Generation of such pulses is traditionally realized via ultrafast laser excitation of a light-to-strain transducer involving thermoelastic, deformation potential, or inverse piezoelectric effects. These approaches unavoidably lead to heat dissipation and a temperature rise, which can modify delicate specimens, like biological tissues, and ultimately destroy the transducer itself limiting the amplitude of generated picosecond strain. Here we propose a non-thermal mechanism for generating picosecond strain pulses via ultrafast photo-induced first-order phase transitions (PIPTs). We perform experiments on vanadium dioxide VO₂ films, which exhibit a first-order PIPT accompanied by a lattice change. We demonstrate that during femtosecond optical excitation of VO₂ the PIPT alone contributes to ultrafast expansion of this material as large as 0.45%, which is not accompanied by heat dissipation, and, for excitation density of 8 mJ cm⁻², exceeds the contribution from thermoelastic effect by a factor of five.

Ultrafast driving of vanadium dioxide can induce a large structural phase transition, which can be used to generate picosecond strain pulses. Here the authors show that the photo-induced phase transition can contribute 0.45% strain without causing undesirable heating.

Macromolecular Nanocrystal Structural Analysis with Electron and X-Rays: A Comparative Review

Crystallography has long been the unrivaled method that can provide the atomistic structural models of macromolecules, using either X-rays or electrons as probes. The methodology has gone through several revolutionary periods, driven by the development of new sources, detectors, and other instrumentation. Novel sources of both X-ray and electrons are constantly emerging. The increase in brightness of these sources, complemented by the advanced detection techniques, has relaxed the traditionally strict need for large, high quality, crystals. Recent reports suggest high-quality diffraction datasets from crystals as small as a few hundreds of nanometers can be routinely obtained. This has resulted in the genesis of a new field of macromolecular nanocrystal crystallography. Here we will make a brief comparative review of this growing field focusing on the use of X-rays and electrons sources.

Sources of error in Debye-Waller-effect measurements relevant to studies of photoinduced structural dynamics

We identify and quantify several practical effects likely to be present in both static and ultrafast electron-scattering experiments that may interfere with the Debye-Waller (DW) effect. Using 120-nm thick, small-grained, polycrystalline aluminum foils as a test system, we illustrate the impact of specimen tilting, in-plane translation, and changes in z height on Debye-Scherrer-ring intensities. We find that tilting by less than one degree can result in statistically-significant changes in diffracted-beam intensities for large specimen regions containing > 10⁵ nanocrystalline grains. We demonstrate that, in addition to effective changes in the field of view with tilting, slight texturing of the film can result in deviations from expected DW-effect behavior. Further, we find that in-plane translations of as little as 20 nm also produce statistically-significant intensity changes, while normalization to total image counts eliminates such effects arising from changes in z height. The results indicate that the use of polycrystalline films in ultrafast electron-scattering experiments can greatly reduce the negative impacts of these effects as compared to single-crystal specimens, though it does not entirely eliminate them. Thus, it is important to account for such effects when studying thin-foil specimens having relatively short reciprocal-lattice rods.

Photoinduced Strain Release and Phase Transition Dynamics of Solid-Supported Ultrathin Vanadium Dioxide

The complex phase transitions of vanadium dioxide (VO₂) have drawn continual attention for more than five decades. Dynamically, ultrafast electron diffraction (UED) with atomic-scale spatiotemporal resolution has been employed to study the reaction pathway in the photoinduced transition of VO₂, using bulk and strain-free specimens. Here, we report the UED results from 10-nm-thick crystalline VO₂ supported on Al₂O₃(0001) and examine the influence of surface stress on the photoinduced structural transformation. An ultrafast release of the compressive strain along the surface-normal direction is observed at early times following the photoexcitation, accompanied by faster motions of vanadium dimers that are more complex than simple dilation or bond tilting. Diffraction simulations indicate that the reaction intermediate involved on picosecond times may not be a single state, which implies non-concerted atomic motions on a multidimensional energy landscape. At longer times, a laser fluence multiple times higher than the thermodynamic enthalpy threshold is required for complete conversion from the initial monoclinic structure to the tetragonal lattice. For certain crystalline domains, the structural transformation is not seen even on nanosecond times following an intense photoexcitation. These results signify a time-dependent energy distribution among various degrees of freedom and reveal the nature of and the impact of strain on the photoinduced transition of VO₂.

Design and implementation of an optimal laser pulse front tilting scheme for ultrafast electron diffraction in reflection geometry with high temporal resolution

Ultrafast electron diffraction is a powerful technique to investigate out-of-equilibrium atomic dynamics in solids with high temporal resolution. When diffraction is performed in reflection geometry, the main limitation is the mismatch in group velocity between the overlapping pump light and the electron probe pulses, which affects the overall temporal resolution of the experiment. A solution already available in the literature involved pulse front tilt of the pump beam at the sample, providing a sub-picosecond time resolution. However, in the reported optical scheme, the tilted pulse is characterized by a temporal chirp of about 1 ps at 1 mm away from the centre of the beam, which limits the investigation of surface dynamics in large crystals. In this paper, we propose an optimal tilting scheme designed for a radio-frequency-compressed ultrafast electron diffraction setup working in reflection geometry with 30 keV electron pulses containing up to 10⁵ electrons/pulse. To characterize our scheme, we performed optical cross-correlation measurements, obtaining an average temporal width of the tilted pulse lower than 250 fs. The calibration of the electron-laser temporal overlap was obtained by monitoring the spatial profile of the electron beam when interacting with the plasma optically induced at the apex of a copper needle (plasma lensing effect). Finally, we report the first time-resolved results obtained on graphite, where the electron-phonon coupling dynamics is observed, showing an overall temporal resolution in the sub-500 fs regime. The successful implementation of this configuration opens the way to directly probe structural dynamics of low-dimensional systems in the sub-picosecond regime, with pulsed electrons.

Shaped cathodes for the production of ultra-short multi-electron pulses

An electrostatic electron source design capable of producing sub-20 femtoseconds (rms) multi-electron pulses is presented. The photoelectron gun concept builds upon geometrical electric field enhancement at the cathode surface. Particle tracer simulations indicate the generation of extremely short bunches even beyond 40 cm of propagation. Comparisons with compact electron sources commonly used for femtosecond electron diffraction are made.

Beating Darwin-Bragg losses in lab-based ultrafast x-ray experiments

The use of low temperature thermal detectors for avoiding Darwin-Bragg losses in lab-based ultrafast experiments has begun. An outline of the background of this new development is offered, showing the relevant history and initiative taken by this work.

Ultrafast electron microscopy integrated with a direct electron detection camera

In the past decade, we have witnessed the rapid growth of the field of ultrafast electron microscopy (UEM), which provides intuitive means to watch atomic and molecular motions of matter. Yet, because of the limited current of the pulsed electron beam resulting from space-charge effects, observations have been mainly made to periodic motions of the crystalline structure of hundreds of nanometers or higher by stroboscopic imaging at high repetition rates. Here, we develop an advanced UEM with robust capabilities for circumventing the present limitations by integrating a direct electron detection camera for the first time which allows for imaging at low repetition rates. This approach is expected to promote UEM to a more powerful platform to visualize molecular and collective motions and dissect fundamental physical, chemical, and materials phenomena in space and time.

Defect-mediated phonon dynamics in TaS2 and WSe2

We report correlative crystallographic and morphological studies of defect-dependent phonon dynamics in single flakes of 1T-TaS₂ and 2H-WSe₂ using selected-area diffraction and bright-field imaging in an ultrafast electron microscope. In both materials, we observe in-plane speed-of-sound acoustic-phonon wave trains, the dynamics of which (i.e., emergence, propagation, and interference) are strongly dependent upon discrete interfacial features (e.g., vacuum/crystal and crystal/crystal interfaces). In TaS₂, we observe cross-propagating in-plane acoustic-phonon wave trains of differing frequencies that undergo coherent interference approximately 200 ps after initial emergence from distinct interfacial regions. With ultrafast bright-field imaging, the properties of the interfering wave trains are observed to correspond to the beat frequency of the individual oscillations, while intensity oscillations of Bragg spots generated from selected areas within the region of interest match well with the real-space dynamics. In WSe₂, distinct acoustic-phonon dynamics are observed emanating and propagating away from structurally dissimilar morphological discontinuities (vacuum/crystal interface and crystal terrace), and results of ultrafast selected-area diffraction reveal thickness-dependent phonon frequencies. The overall observed dynamics are well-described using finite element analysis and time-dependent linear-elastic continuum mechanics.