Tracking the insulator-to-metal phase transition in VO2 with few-femtosecond extreme UV transient absorption spectroscopy

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.

Photoinduced hidden monoclinic metallic phase of VO2 driven by local nucleation

The insulator-to-metal transition in VO2 has garnered extensive attention for its potential applications in ultrafast switches, neuronal network architectures, and storage technologies. However, the photoinduced insulator-to-metal transition remains controversial, especially whether a complete structural transformation from the monoclinic to rutile phase is necessary. Here we employ the real-time time-dependent density functional theory to track the dynamic evolution of atomic and electronic structures in photoexcited VO2, revealing the emergence of a long-lived monoclinic metal phase under low electronic excitation. The emergence of the metal phase in the monoclinic structure originates from the dissociation of the local V-V dimer, driven by the self-trapped and self-amplified dynamics of photoexcited holes, rather than by an electron-electron correction. On the other hand, the monoclinic-to-rutile phase transition does appear at higher electronic excitation. Our findings validate the existence of monoclinic metal phase and provide a comprehensive picture of the insulator-to-metal transition in photoexcited VO2.

Near-Field Nanoimaging of Phases and Carrier Dynamics in Vanadium Dioxide Nanobeams

The stable coexistence of insulating and metallic phases in strained vanadium dioxide (VO2) has garnered significant research interest due to the intriguing phase transition phenomena. However, the temporal behavior of charge carriers in different phases of VO2 remains elusive. Herein, we employ near-field optical nanoscopy to capture nanoscale alternating phase domains in bent VO2 nanobeams. By conducting transient measurements across the different phases, we observed a prolonged carrier recombination lifetime in the metallic phase of VO2, accompanied by an accelerated diffusion process. Our findings reveal nanoscale carrier dynamics in VO2 nanobeams, offering insights that can facilitate further investigations into phase-change materials and their potential applications in sensing and microelectromechanical devices.

Nanoscale optical nonreciprocity with nonlinear metasurfaces

Optical nonreciprocity is manifested as a difference in the transmission of light for the opposite directions of excitation. Nonreciprocal optics is traditionally realized with relatively bulky components such as optical isolators based on the Faraday rotation, hindering the miniaturization and integration of optical systems. Here we demonstrate free-space nonreciprocal transmission through a metasurface comprised of a two-dimensional array of nanoresonators made of silicon hybridized with vanadium dioxide (VO2). This effect arises from the magneto-electric coupling between Mie modes supported by the resonator. Nonreciprocal response of the nanoresonators occurs without the need for external bias; instead, reciprocity is broken by the incident light triggering the VO2 phase transition for only one direction of incidence. Nonreciprocal transmission is broadband covering over 100 nm in the telecommunication range in the vicinity of λ = 1.5 µm. Each nanoresonator unit cell occupies only ~0.1 λ3 in volume, with the metasurface thickness measuring about half-a-micron. Our self-biased nanoresonators exhibit nonreciprocity down to very low levels of intensity on the order of 150 W/cm2 or a µW per nanoresonator. We estimate picosecond-scale transmission fall times and sub-microsecond scale transmission rise. Our demonstration brings low-power, broadband and bias-free optical nonreciprocity to the nanoscale.

Reinvented: An Attosecond Chemist

Attosecond science requires a substantial rethinking of how to make measurements on very short timescales; how to acquire the necessary equipment, technology, and personnel; and how to build a set of laboratories for such experiments. This entails a rejuvenation of the author in many respects, in the laboratory itself, with regard to students and postdocs, and in generating funding for research. It also brings up questions of what it means to do attosecond science, and the discovery of the power of X-ray spectroscopy itself, which complements the short timescales addressed. The lessons learned, expressed in the meanderings of this autobiographical article, may be of benefit to others who try to reinvent themselves.

Ab initio-simulated optical response of hot electrons in gold and ruthenium

Optical femtosecond pump-probe experiments allow to measure the dynamics of ultrafast heating of metals with high accuracy. However, the theoretical analysis of such experiments is often complicated because of the indirect connection of the measured signal and the desired temperature transients. Establishing such a connection requires an accurate model of the optical constants of a metal, depending on both the electron temperature Te and the lattice temperature Tl. In this paper, we present first-principles simulations of the two-temperature scenario with Te ≫ Tl, showing the optical response of hot electrons to laser irradiation in gold and ruthenium. Comparing our simulations with the Kubo-Greenwood approach, we discuss the influence of electron-phonon and electron-electron scattering on the intraband contribution to optical constants. Applying the simulated optical constants to the analysis of ultrafast heating of ruthenium thin films we highlight the importance of the latter scattering channel to understand the measured heating dynamics.

Plasmonic switches based on VO2as the phase change material

In this paper, a comprehensive review of the recent advancements in the design and development of plasmonic switches based on vanadium dioxide (VO2) is presented. Plasmonic switches are employed in applications such as integrated photonics, plasmonic logic circuits and computing networks for light routing and switching, and are based on the switching of the plasmonic properties under the effect of an external stimulus. In the last few decades, plasmonic switches have seen a significant growth because of their ultra-fast switching speed, wide spectral tunability, ultra-compact size, and low losses. In this review, first, the mechanism of the semiconductor to metal phase transition in VO2is discussed and the reasons for employing VO2over other phase change materials for plasmonic switching are described. Subsequently, an exhaustive review and comparison of the current state-of-the-art plasmonic switches based on VO2proposed in the last decade is carried out. As the phase transition in VO2can be activated by application of temperature, voltage or optical light pulses, this review paper has been categorized into thermally-activated, electrically-activated, and optically-activated plasmonic switches based on VO2operating in the visible, near-infrared, infrared and terahertz frequency regions.

Following the Nonthermal Phase Transition in Niobium Dioxide by Time-Resolved Harmonic Spectroscopy

Photoinduced phase transitions in correlated materials promise diverse applications from ultrafast switches to optoelectronics. Resolving those transitions and possible metastable phases temporally are key enablers for these applications, but challenge existing experimental approaches. Extreme nonlinear optics can help probe phase changes, as higher-order nonlinearities have higher sensitivity and temporal resolution to band structure and lattice deformations. Here the ultrafast transition from the semiconducting to the metallic phases in polycrystalline thin-film NbO_{2} is investigated by time-resolved harmonic spectroscopy. The emission strength of all harmonic orders shows a steplike suppression when the excitation fluence exceeds a threshold (∼11-12  mJ/cm^{2}), below the fluence required for the thermal transition-a signature of the nonthermal emergence of a metallic phase within 100±20  fs. This observation is backed by full ab initio simulations as well as a 1D chain model of high-harmonic generation from both phases. Our results demonstrate femtosecond harmonic probing of phase transitions and nonthermal dynamics in solids.

Accelerating Quantum Materials Development with Advances in Transmission Electron Microscopy

Quantum materials are driving a technology revolution in sensing, communication, and computing, while simultaneously testing many core theories of the past century. Materials such as topological insulators, complex oxides, superconductors, quantum dots, color center-hosting semiconductors, and other types of strongly correlated materials can exhibit exotic properties such as edge conductivity, multiferroicity, magnetoresistance, superconductivity, single photon emission, and optical-spin locking. These emergent properties arise and depend strongly on the material's detailed atomic-scale structure, including atomic defects, dopants, and lattice stacking. In this review, we describe how progress in the field of electron microscopy (EM), including in situ and in operando EM, can accelerate advances in quantum materials and quantum excitations. We begin by describing fundamental EM principles and operation modes. We then discuss various EM methods such as (i) EM spectroscopies, including electron energy loss spectroscopy (EELS), cathodoluminescence (CL), and electron energy gain spectroscopy (EEGS); (ii) four-dimensional scanning transmission electron microscopy (4D-STEM); (iii) dynamic and ultrafast EM (UEM); (iv) complementary ultrafast spectroscopies (UED, XFEL); and (v) atomic electron tomography (AET). We describe how these methods could inform structure-function relations in quantum materials down to the picometer scale and femtosecond time resolution, and how they enable precision positioning of atomic defects and high-resolution manipulation of quantum materials. For each method, we also describe existing limitations to solve open quantum mechanical questions, and how they might be addressed to accelerate progress. Among numerous notable results, our review highlights how EM is enabling identification of the 3D structure of quantum defects; measuring reversible and metastable dynamics of quantum excitations; mapping exciton states and single photon emission; measuring nanoscale thermal transport and coupled excitation dynamics; and measuring the internal electric field and charge density distribution of quantum heterointerfaces- all at the quantum materials' intrinsic atomic and near atomic-length scale. We conclude by describing open challenges for the future, including achieving stable sample holders for ultralow temperature (below 10K) atomic-scale spatial resolution, stable spectrometers that enable meV energy resolution, and high-resolution, dynamic mapping of magnetic and spin fields. With atomic manipulation and ultrafast characterization enabled by EM, quantum materials will be poised to integrate into many of the sustainable and energy-efficient technologies needed for the 21st century.

Harnessing the Metal-Insulator Transition of VO2 in Neuromorphic Computing

Future-generation neuromorphic computing seeks to overcome the limitations of von Neumann architectures by colocating logic and memory functions, thereby emulating the function of neurons and synapses in the human brain. Despite remarkable demonstrations of high-fidelity neuronal emulation, the predictive design of neuromorphic circuits starting from knowledge of material transformations remains challenging. VO2 is an attractive candidate since it manifests a near-room-temperature, discontinuous, and hysteretic metal-insulator transition. The transition provides a nonlinear dynamical response to input signals, as needed to construct neuronal circuit elements. Strategies for tuning the transformation characteristics of VO2 based on modification of material properties, interfacial structure, and field couplings, are discussed. Dynamical modulation of transformation characteristics through in situ processing is discussed as a means of imbuing synaptic function. Mechanistic understanding of site-selective modification; external, epitaxial, and chemical strain; defect dynamics; and interfacial field coupling in modifying local atomistic structure, the implications therein for electronic structure, and ultimately, the tuning of transformation characteristics, is emphasized. Opportunities are highlighted for inverse design and for using design principles related to thermodynamics and kinetics of electronic transitions learned from VO2 to inform the design of new Mott materials, as well as to go beyond energy-efficient computation to manifest intelligence.

Correlated insulator collapse due to quantum avalanche via in-gap ladder states

The significant discrepancy observed between the predicted and experimental switching fields in correlated insulators under a DC electric field far-from-equilibrium necessitates a reevaluation of current microscopic understanding. Here we show that an electron avalanche can occur in the bulk limit of such insulators at arbitrarily small electric field by introducing a generic model of electrons coupled to an inelastic medium of phonons. The quantum avalanche arises by the generation of a ladder of in-gap states, created by a multi-phonon emission process. Hot-phonons in the avalanche trigger a premature and partial collapse of the correlated gap. The phonon spectrum dictates the existence of two-stage versus single-stage switching events which we associate with charge-density-wave and Mott resistive phase transitions, respectively. The behavior of electron and phonon temperatures, as well as the temperature dependence of the threshold fields, demonstrates how a crossover between the thermal and quantum switching scenarios emerges within a unified framework of the quantum avalanche.

Transient dynamics of the phase transition in VO2 revealed by mega-electron-volt ultrafast electron diffraction

Vanadium dioxide (VO2) exhibits an insulator-to-metal transition accompanied by a structural transition near room temperature. This transition can be triggered by an ultrafast laser pulse. Exotic transient states, such as a metallic state without structural transition, were also proposed. These unique characteristics let VO2 have great potential in thermal switchable devices and photonic applications. Although great efforts have been made, the atomic pathway during the photoinduced phase transition is still not clear. Here, we synthesize freestanding quasi-single-crystal VO2 films and examine their photoinduced structural phase transition with mega-electron-volt ultrafast electron diffraction. Leveraging the high signal-to-noise ratio and high temporal resolution, we observe that the disappearance of vanadium dimers and zigzag chains does not coincide with the transformation of crystal symmetry. After photoexcitation, the initial structure is strongly modified within 200 femtoseconds, resulting in a transient monoclinic structure without vanadium dimers and zigzag chains. Then, it continues to evolve to the final tetragonal structure in approximately 5 picoseconds. In addition, only one laser fluence threshold instead of two thresholds suggested in polycrystalline samples is observed in our quasi-single-crystal samples. Our findings provide essential information for a comprehensive understanding of the photoinduced ultrafast phase transition in VO2.

High-Strain-Induced Local Modification of the Electronic Properties of VO2 Thin Films

Vanadium dioxide (VO₂) is a popular candidate for electronic and optical switching applications due to its well-known semiconductor-metal transition. Its study is notoriously challenging due to the interplay of long- and short-range elastic distortions, as well as the symmetry change and the electronic structure changes. The inherent coupling of lattice and electronic degrees of freedom opens the avenue toward mechanical actuation of single domains. In this work, we show that we can manipulate and monitor the reversible semiconductor-to-metal transition of VO₂ while applying a controlled amount of mechanical pressure by a nanosized metallic probe using an atomic force microscope. At a critical pressure, we can reversibly actuate the phase transition with a large modulation of the conductivity. Direct tunneling through the VO₂-metal contact is observed as the main charge carrier injection mechanism before and after the phase transition of VO₂. The tunneling barrier is formed by a very thin but persistently insulating surface layer of the VO₂. The necessary pressure to induce the transition decreases with temperature. In addition, we measured the phase coexistence line in a hitherto unexplored regime. Our study provides valuable information on pressure-induced electronic modifications of the VO₂ properties, as well as on nanoscale metal-oxide contacts, which can help in the future design of oxide electronics.

Tunable Tribovoltaic Effect via Metal-Insulator Transition

Tribovoltaic direct-current (DC) nanogenerator made of dynamic semiconductor heterojunction is emerging as a promising mechanical energy harvesting technology. However, fundamental understanding of the mechano-electronic carrier excitation and transport at dynamic semiconductor interfaces remains to be investigated. Here, we demonstrated for the first time, that tribovoltaic DC effect can be tuned with metal-insulator transition (MIT). In a representative MIT material (vanadium dioxide, VO₂), we found that the short-circuit current (I_SC) can be enhanced by >20 times when the material is transformed from insulating to metallic state upon static or dynamic heating, while the open-circuit voltage (V_OC) turns out to be unaffected. Such phenomenon may be understood by the Hubbard model for Mott insulator: orders' magnitude increase in conductivity is induced when the nearest hopping changes dramatically and overcomes the Coulomb repulsion, while the Coulomb repulsion giving rise to the quasi-particle excitation energy remains relatively stable.

Kinetics of Catalyst-Free and Position-Controlled Low-Pressure Chemical Vapor Deposition Growth of VO2 Nanowire Arrays on Nanoimprinted Si Substrates

In the present article, the position-controlled and catalytic-free synthesis of vanadium dioxide (VO₂) nanowires (NWs) grown by the chemical vapor deposition (CVD) on nanoimprinted silicon substrates in the form of nanopillar arrays was analyzed. The NW growth on silicon nanopillars with different cross-sectional areas was studied, and it has been shown that the NWs' height decreases with an increase in their cross-sectional area. The X-ray diffraction technique, scanning electron microscopy, and X-ray photoelectron spectroscopy showed the high quality of the grown VO₂ NWs. A qualitative description of the growth rate of vertical NWs based on the material balance equation is given. The dependence of the growth rate of vertical and horizontal NWs on the precursor concentration in the gas phase and on the growth time was investigated. It was found that the height of vertical VO₂ NWs along the [100] direction exhibited a linear dependence on time and increased with an increase in the precursor concentration. For horizontal VO₂ NWs, the height along the direction [011] varied little with the growth time and precursor concentration. These results suggest that the high-aspect ratio vertical VO₂ NWs formed due to different growth modes of their crystal faces forming the top of the growing VO₂ crystals and their lateral crystal faces related to the difference between the free energies of these crystal faces and implemented experimental conditions. The results obtained permit a better insight into the growth of high-aspect ratio VO₂ NWs and into the formation of large VO₂ NW arrays with a controlled composition and properties.

Decoupled ultrafast electronic and structural phase transitions in photoexcited monoclinic VO2

Photoexcitation has emerged as an efficient way to trigger phase transitions in strongly correlated materials. There are great controversies about the atomistic mechanisms of structural phase transitions (SPTs) from monoclinic (M₁-) to rutile (R-) VO₂ and its association with electronic insulator-metal transitions (IMTs). Here, we illustrate the underlying atomistic processes and decoupling nature of photoinduced SPT and IMT in nonequilibrium states. The photoinduced SPT proceeds in the order of dilation of V-V pairs and increase of twisting angles after a small delay of ~40 fs. Dynamic simulations with hybrid functionals confirm the existence of isostructural IMT. The photoinduced SPT and IMT exhibit the same thresholds of electronic excitations, indicating similar fluence thresholds in experiments. The IMT is quasi-instantaneously (<10 fs) generated, while the SPT takes place with time a constant of 100 to 300 fs. These findings clarify some key controversies in the literature and provide insights into nonequilibrium phase transitions in correlated materials.

Tunable directional filter for mid-infrared optical transmission switching

Controlling the spectral and angular response of infrared (IR) radiation is a challenging task of paramount importance to various emerging photonic applications. Here, we overcome these problems by proposing and analyzing a new design of a tunable narrowband directional optical transmission filter. The presented thermally controlled multilayer filter leverages the temperature dependent phase change properties of vanadium dioxide (VO₂) to enable efficient and reversible fast optical switching by using a pump-probe laser excitation setup. More specifically, transmission is blocked for high intensity probe lasers due to the VO₂ metallic properties induced at elevated temperatures while at low probe laser intensities high transmission through the filter occurs only for a narrowband IR range confined to near normal incident angles. The proposed multilayer composite dielectric filter is expected to have applications in optical communications, where it can act as dual functional infrared filter and optical switch.

High-harmonic spectroscopy of quantum phase transitions in a high-Tc superconductor

We report on the nonlinear optical signatures of quantum phase transitions in the high-temperature superconductor YBCO, observed through high harmonic generation. While the linear optical response of the material is largely unchanged when cooling across the phase transitions, the nonlinear optical response sensitively imprints two critical points, one at the critical temperature of the cuprate with the exponential growth of the surface harmonic yield in the superconducting phase and another critical point, which marks the transition from strange metal to pseudogap phase. To reveal the underlying microscopic quantum dynamics, a strong-field quasi-Hubbard model was developed, which describes the measured optical response dependent on the formation of Cooper pairs. Further, the theory provides insight into the carrier scattering dynamics and allows us to differentiate between the superconducting, pseudogap, and strange metal phases. The direct connection between nonlinear optical response and microscopic dynamics provides a powerful methodology to study quantum phase transitions in correlated materials. Further implications are light wave control over intricate quantum phases, light-matter hybrids, and application for optical quantum computing.

Recent Development of Ultrafast Optical Characterizations for Quantum Materials

The advent of intense ultrashort optical pulses spanning a frequency range from terahertz to the visible has opened a new era in the experimental investigation and manipulation of quantum materials. The generation of strong optical field in an ultrashort time scale enables the steering of quantum materials nonadiabatically, inducing novel phenomenon or creating new phases which may not have an equilibrium counterpart. Ultrafast time-resolved optical techniques have provided rich information and played an important role in characterization of the nonequilibrium and nonlinear properties of solid systems. Here, some of the recent progress of ultrafast optical techniques and their applications to the detection and manipulation of physical properties in selected quantum materials are reviewed. Specifically, the new development in the detection of the Higgs mode and photoinduced nonequilibrium response in the study of superconductors by time-resolved terahertz spectroscopy are discussed.

Electromagnetically induced modification of gold optical properties

The reflection of light from a metal film, i.e., a mirror, is among the most fundamental and well-understood effects in optics. If the film thickness is greater than the wavelength, reflection is strong and is explained in simple terms by the Fresnel equations. For film thickness much less than the wavelength, reflection is far weaker and more exotic effects become possible. This is especially so if the light illuminating the film is pulsed at the femtosecond time scale. In this work, a phenomenon is proposed where few-femtosecond laser pulses temporarily modify a thin metal film's optical properties via processes that appear linear and classical in nature. By casting a pulsed standing-wave pattern across the metal surface, we consider the possibility that conduction electrons are redistributed to create temporary regions of partly enhanced or reduced density without the excitation of inter-band transitions. The process would constitute a temporary change to the conductivity of the metal, and thus, may be observable as changes to the metal's transmittance and reflectance. In regions where the density is enhanced (reduced), the transmittance is decreased (increased). The concept is termed Electromagnetically Induced Modification (EIM) and is premised on the fact that the pulse length is shorter than the relaxation time of the conduction electrons. An experiment is conducted to test the concept by measuring the change in reflectance and transmittance of gold films with thickness ranging from 20-300 Angstrom. The results show that the film's transmittance decreases only when the standing-wave pattern is present. As the pulse length is increased, or as the film thickness is increased, the changes disappear. The changes show little dependence on the pulse intensity as it is varied by a factor of two. To gain further insight, the Drude theory is used to develop a simplified model for EIM, which qualitatively agrees with the observations. However, neither the experiment nor the model can prove the validity of the EIM concept. As such, an assessment is made for the potential of alternative well-known processes to explain the observations.

Multifunctional and Transformative Metaphotonics with Emerging Materials

Future technologies underpinning multifunctional physical and chemical systems and compact biological sensors will rely on densely packed transformative and tunable circuitry employing nanophotonics. For many years, plasmonics was considered as the only available platform for subwavelength optics, but the recently emerged field of resonant metaphotonics may provide a versatile practical platform for nanoscale science by employing resonances in high-index dielectric nanoparticles and metasurfaces. Here, we discuss the recently emerged field of metaphotonics and describe its connection to material science and chemistry. For tunabilty, metaphotonics employs a variety of the recently highlighted materials such as polymers, perovskites, transition metal dichalcogenides, and phase change materials. This allows to achieve diverse functionalities of metasystems and metasurfaces for efficient spatial and temporal control of light by employing multipolar resonances and the physics of bound states in the continuum. We anticipate expanding applications of these concepts in nanolasers, tunable metadevices, metachemistry, as well as a design of a new generation of chemical and biological ultracompact sensing devices.

Attosecond spectroscopy for the investigation of ultrafast dynamics in atomic, molecular and solid-state physics

Since the first demonstration of the generation of attosecond pulses (1 as = 10^-18s) in the extreme-ultraviolet spectral region, several measurement techniques have been introduced, at the beginning for the temporal characterization of the pulses, and immediately after for the investigation of electronic and nuclear ultrafast dynamics in atoms, molecules and solids with unprecedented temporal resolution. The attosecond spectroscopic tools established in the last two decades, together with the development of sophisticated theoretical methods for the interpretation of the experimental outcomes, allowed to unravel and investigate physical processes never observed before, such as the delay in photoemission from atoms and solids, the motion of electrons in molecules after prompt ionization which precede any notable nuclear motion, the temporal evolution of the tunneling process in dielectrics, and many others. This review focused on applications of attosecond techniques to the investigation of ultrafast processes in atoms, molecules and solids. Thanks to the introduction and ongoing developments of new spectroscopic techniques, the attosecond science is rapidly moving towards the investigation, understanding and control of coupled electron-nuclear dynamics in increasingly complex systems, with ever more accurate and complete investigation techniques. Here we will review the most common techniques presenting the latest results in atoms, molecules and solids.

Tuning Phase Transition and Thermochromic Properties of Vanadium Dioxide Thin Films via Cobalt Doping

Vanadium dioxide (VO₂) featuring a distinct thermally triggered phase transition is regarded as the most attractive thermochromic material for smart window applications. However, the high transition temperature (∼67 °C) and moderate luminous transmittance (<50%) of the pristine VO₂ circumvent room temperature applications. In this work, epitaxial cobalt-doped VO₂ thin films were fabricated to tailor the electric and optical properties on a c-plane sapphire substrate. At the highest doping concentration of 10%, the transition temperature of VO₂ is reduced to 44 °C, accompanied by a high luminous transmittance of 79% for single-element Co-doped VO₂. The roles of cobalt doping and detailed band variation are fully explained experimentally and by modeling (DFT calculation), respectively. Furthermore, the dramatically increased carrier concentration in cobalt-doped VO₂ underscores the promising future of cobalt-doped VO₂ unveiled by temperature-dependent Hall effect measurement.

Extreme ultraviolet-excited time-resolved luminescence spectroscopy using an ultrafast table-top high-harmonic generation source

We present a table-top extreme ultraviolet (XUV) beamline for measuring time- and frequency-resolved XUV-excited optical luminescence (XEOL) with additional femtosecond-resolution XUV transient absorption spectroscopy functionality. XUV pulses are generated via high-harmonic generation using a near-infrared pulse in a noble gas medium and focused to excite luminescence from a solid sample. The luminescence is collimated and guided into a streak camera where its spectral components are temporally resolved with picosecond temporal resolution. We time-resolve XUV-excited luminescence and compare the results to luminescence decays excited at longer wavelengths for three different materials: (i) sodium salicylate, an often used XUV scintillator; (ii) fluorescent labeling molecule 4-carbazole benzoic (CB) acid; and (iii) a zirconium metal oxo-cluster labeled with CB, which is a photoresist candidate for extreme-ultraviolet lithography. Our results establish time-resolved XEOL as a new technique to measure transient XUV-driven phenomena in solid-state samples and identify decay mechanisms of molecules following XUV and soft-x-ray excitation.

Design of an On-Chip Plasmonic Modulator Based on Hybrid Orthogonal Junctions Using Vanadium Dioxide

We present the design of a plasmonic modulator based on hybrid orthogonal silver junctions using vanadium dioxide as the modulating material on a silicon-on-insulator. The modulator has an ultra-compact footprint of 1.8 μm × 1 μm with a 100 nm × 100 nm modulating section based on the hybrid orthogonal geometry. The modulator takes advantage of the large change in the refractive index of vanadium dioxide during its phase transition to achieve a high modulation depth of 46.89 dB/μm. The simulated device has potential applications in the development of next generation high frequency photonic modulators for optical communications which require nanometer scale footprints, large modulation depth and small insertion losses.

Quantitative hyperspectral coherent diffractive imaging spectroscopy of a solid-state phase transition in vanadium dioxide

Solid-state systems can host a variety of thermodynamic phases that can be controlled with magnetic fields, strain, or laser excitation. Many phases that are believed to exhibit exotic properties only exist on the nanoscale, coexisting with other phases that make them challenging to study, as measurements require both nanometer spatial resolution and spectroscopic information, which are not easily accessible with traditional x-ray spectromicroscopy techniques. Here, we use coherent diffractive imaging spectroscopy (CDIS) to acquire quantitative hyperspectral images of the prototypical quantum material vanadium oxide across the vanadium L 2,3 and oxygen K x-ray absorption edges with nanometer-scale resolution. We extract the full complex refractive indices of the monoclinic insulating and rutile conducting phases of VO2 from a single sample and find no evidence for correlation-driven phase transitions. CDIS will enable quantitative full-field x-ray spectromicroscopy for studying phase separation in time-resolved experiments and other extreme sample environments where other methods cannot operate.

Phase-Changing in Graphite Assisted by Interface Charge Injection

Among many phase-changing materials, graphite is probably the most studied and interesting: the rhombohedral (3R) and hexagonal (2H) phases exhibit dramatically different electronic properties. However, up to now the only way to promote 3R to 2H phase transition is through exposure to elevated temperatures (above 1000 °C); thus, it is not feasible for modern technology. In this work, we demonstrate that 3R to 2H phase transition can be promoted by changing the charged state of 3D graphite, which promotes the repulsion between the layers and significantly reduces the energy barrier between the 3R and 2H phases. In particular, we show that charge transfer from lithium nitride (α-Li₃N) to graphite can lower the transition temperature down to 350 °C. The proposed interlayer slipping model potentially offers the control over topological states at the interfaces between different phases, making this system even more attractive for future electronic applications.

Selective MOCVD synthesis of VO2 crystals on nanosharp Si structures

High-quality single VO₂ nanocrystals and ordered arrays of VO₂ nanorings were selectively synthesized by chemical vapor deposition (CVD) respectively on the tip apices and on the sidewall scallops.

Photoinduced Broad-band Tunable Terahertz Absorber Based on a VO2 Thin Film

The demand for terahertz (THz) communication and detection fuels continuous research for high performance of THz absorption materials. In addition to varying the materials and their structure passively, an alternative approach is to modulate a THz wave actively by tuning an external stimulus. Correlated oxides are ideal materials for this because the effects of a small external control parameter can be amplified by inner electronic correlations. Here, by utilizing an unpatterned strongly correlated electron oxide VO₂ thin film, a photoinduced broad-band tunable THz absorber is realized first. The absorption, transmission, reflection, and phase of THz waves can all be actively controlled by an external pump laser above room temperature. By varying the laser fluence, the average broad-band absorption can be tuned from 18.9 to 74.7% and the average transmission can be tuned from 9.2 to 69.2%. Meanwhile, a broad-band antireflection is obtained at 5.6 mJ/cm², and a π-phase shift of a reflected THz wave is achieved when the fluence increases greater than 5.7 mJ/cm². Apart from other modulators, the photoexcitation-assisted dual-phase competition is identified as the origin of this active THz multifunctional modulation. Our work suggests that advantages of controllable phase separation in strongly correlated electron systems could provide viable routes in the creation of active optical components for THz waves.

Effects of charge fluctuation and charge regulation on the phase transitions in stoichiometric VO2

Detailed electrical and photoemission studies were carried out to probe the chemical nature of the insulating ground state of VO₂, whose properties have been an issue for accurate prediction by common theoretical probes. The effects of a systematic modulation of oxygen over-stoichiometry of VO₂ from 1.86 to 2.44 on the band structure and insulator-metal transitions are presented for the first time. Results offer a different perspective on the temperature- and doping-induced IMT process. They suggest that charge fluctuation in the metallic phase of intrinsic VO₂ results in the formation of e^- and h⁺ pairs that lead to delocalized polaronic V³⁺ and V⁵⁺ cation states. The metal-to-insulator transition is linked to the cooperative effects of changes in the V-O bond length, localization of V³⁺ electrons at V⁵⁺ sites, which results in the formation of V⁴⁺-V⁴⁺ dimers, and removal of [Formula: see text] screening electrons. It is shown that the nature of phase transitions is linked to the lattice V³⁺/V⁵⁺ concentrations of stoichiometric VO₂ and that electronic transitions are regulated by the interplay between charge fluctuation, charge redistribution, and structural transition.

Electron-electron correlations and structural, spectral and polarization properties of tetragonal BaTiO3

To analyze the role of electron-electron correlation effects in structural (local-geometry), spectral and polarization properties of tetragonal BaTiO3we apply DFT +Uapproach. We demonstrate that the system properties drastically change when the value of the local Coulomb repulsionUcrosses the critical valueUc≈ 7 eV. In particular, the correlation effects cause a change of the ratio of the in-plane and inter-plane Ti-O bond lengths, which results in a flip of the order of the Tid-bands and change of the polarizability of the system. Since the consensus value ofUin BaTiO3is unknown, we discuss how the obtained results may be revealed in experimental data, especially in the optical response and ultrafast charge dynamics, where effectiveUis dynamically tuned.

Optical modulation in hybrid antiresonant hollow-core fiber infiltrated with vanadium dioxide phase change nanocrystals

We present a study of optical modulation by the effect of temperature-induced insulator-to-metal phase transition of vanadium dioxide (VO₂) nanocrystals deposited in an antiresonance hollow-core fiber (AR-HCF). We fabricate such a VO₂-coated fiber by embedding alkylsilane functionalized VO₂ nanocrystals into the air holes of an AR-HCF. With this fiber, we achieve an optical loss modulation of ∼60% at a temperature above ∼53^∘C over an ultrabroad spectral range that encompasses the S+C+L band.

Non-thermal resistive switching in Mott insulator nanowires

Resistive switching can be achieved in a Mott insulator by applying current/voltage, which triggers an insulator-metal transition (IMT). This phenomenon is key for understanding IMT physics and developing novel memory elements and brain-inspired technology. Despite this, the roles of electric field and Joule heating in the switching process remain controversial. Using nanowires of two archetypal Mott insulators—VO₂ and V₂O₃ we unequivocally show that a purely non-thermal electrical IMT can occur in both materials. The mechanism behind this effect is identified as field-assisted carrier generation leading to a doping driven IMT. This effect can be controlled by similar means in both VO₂ and V₂O₃, suggesting that the proposed mechanism is generally applicable to Mott insulators. The energy consumption associated with the non-thermal IMT is extremely low, rivaling that of state-of-the-art electronics and biological neurons. These findings pave the way towards highly energy-efficient applications of Mott insulators.

Despite intensive research on the electrically driven insulator-to-metal transition, this phenomenon is not well understood. Using quasi 1D nanowires of two Mott insulators, the authors reveal the central role of defects in enabling a non-thermal doping driven insulator-to metal transition.

Optical Limiting Based on Huygens' Metasurfaces

Optical limiting is desirable or necessary in a variety of applications that employ high-power light sources or sensitive photodetectors. However, the most prevalent methods compromise between on-state transmission and turndown ratio or rely on narrow transmission windows. We demonstrate that a metasurface-based architecture incorporating phase-change materials enables both high and broadband on-state transmission (-4.8 dB) while also providing a large turndown ratio (25.2 dB). Additionally, this design can be extended for broadband multiwavelength limiting due to the high off-resonance transmittance and readily scalable resonant wavelength. Furthermore, our choice of active material allows for protection in ultrafast laser environments due to the speed of the phase transition. These benefits offer a strong alternative to state-of-the-art optical limiters in technologies ranging from sensor protection to protective eyewear.

Electron correlations and memory effects in ultrafast electron and hole dynamics in VO2

By applying an approach based on time-dependent density functional theory and dynamical mean-field theory (TDDFT+DMFT) we examine the role of electron correlations in the ultrafast breakdown of the insulating M₁ phase in bulk VO₂. We consider the case of a spatially homogeneous ultrafast (femtosecond) laser pulse perturbation and present the dynamics of the melting of the insulating state, in particular the time-dependence of the excited charge density. The time-dependence of the chemical potential of the excited electron and hole subsystems shows that even for such short times the dynamics of the system is significantly affected by memory effects-the time-resolved electron-electron interactions. The results pave the way for obtaining a microscopic understanding of the ultrafast dynamics of strongly-correlated materials.

Global Control of Stacking-Order Phase Transition by Doping and Electric Field in Few-Layer Graphene

The layer stacking order has profound effects on the physical properties of two-dimensional van der Waals heterostructures. For example, graphene multilayers can have distinct electronic band structures and exhibit completely different behaviors depending on the stacking order. Fascinating physical phenomena, such as correlated insulators, superconductors, and ferromagnetism, can also emerge with a periodic variation of the layer stacking order, which is known as the moiré superlattice in van der Waals materials. In this work, we realize the global phase transition between different graphene layer stacking orders and elucidate its microscopic origin. We experimentally determine the energy difference between different stacking orders with the accuracy of μeV/atom. We reveal that both the carrier doping and the electric field can drive the layer-stacking phase transition through different mechanisms: carrier doping can change the energy difference because of a non-negligible work function difference between different stacking orders; the electric field, on the other hand, induces a band-gap opening in ABC-stacked graphene and hence changes the energy difference. Our findings provide a fundamental understanding of the electrically driven stacking-order phase transition in few-layer graphene and demonstrate a reversible and noninvasive method to globally control the stacking order.

Characterisation of the temperature-dependent M1 to R phase transition in W-doped VO2 nanorod aggregates by Rietveld refinement and theoretical modelling

Synchrotron XRD Rietveld refinement is combined with first-principles calculations to probe the effect of W doping on the IMT mechanism in VO₂ nanorods, providing insights into the connection between atomic-scale phenomena and macro-scale properties.

Extreme ultraviolet time- and angle-resolved photoemission setup with 21.5 meV resolution using high-order harmonic generation from a turn-key Yb:KGW amplifier

Characterizing and controlling electronic properties of quantum materials require direct measurements of nonequilibrium electronic band structures over large regions of momentum space. Here, we demonstrate an experimental apparatus for time- and angle-resolved photoemission spectroscopy using high-order harmonic probe pulses generated by a robust, moderately high power (20 W) Yb:KGW amplifier with a tunable repetition rate between 50 and 150 kHz. By driving high-order harmonic generation (HHG) with the second harmonic of the fundamental 1025 nm laser pulses, we show that single-harmonic probe pulses at 21.8 eV photon energy can be effectively isolated without the use of a monochromator. The on-target photon flux can reach 5 × 10¹⁰ photons/s at 50 kHz, and the time resolution is measured to be 320 fs. The relatively long pulse duration of the Yb-driven HHG source allows us to reach an excellent energy resolution of 21.5 meV, which is achieved by suppressing the space-charge broadening using a low photon flux of 1.5 × 10⁸ photons/s at a higher repetition rate of 150 kHz. The capabilities of the setup are demonstrated through measurements in the topological semimetal ZrSiS and the topological insulator Sb_2-xGd_xTe₃.

Bioinspired Toolkit Based on Intermolecular Encoder toward Evolutionary 4D Chiral Plasmonic Materials

Over the last two decades, nanophotonics, including plasmonics and metamaterials, have promised compelling opportunities for exotic control over light-matter interactions. The strong chiral light-matter interaction is a representative example. Three-dimensional (3D) chirality has existed naturally only in organic molecules and bio-organisms, but a negligible chiroptic effect was attained with these naturally occurring materials because of their small absorption cross sections. However, inspired by biological chirality, nanophotonic chiral materials have greatly expanded the design space of accessible chiroptic effects (e.g., pushing the chiral light-matter interaction to an exceptional regime, such as a broad-band circular polarizer, negative refractive index, and sensitive chiral sensing). Nevertheless, it is still a challenge to achieve precisely defined and dynamically reconfigurable chiral morphologies that further increase the chiroptic effect. Biological systems continue to inspire approaches to the design and synthesis of precisely defined 3D nanostructures. In particular, a living organism can program the evolutionary pathway of highly complexed 3D chiral morphology precisely from the molecular scale to the macroscopic scale while simultaneously enabling dynamic reconfiguration of their chirality. What if we could harness the power of biological selectivity and evolutionary capability in synthesizing chiral plasmonic materials? We envisioned that platform technology mimicking biological principles would enable control of 3D chiral structures for effective plasmonic interactions with polarized light and further impart the concept of time-dependent evolution (3D + 1D = 4D) to bring about responsive and dynamic changes in chiral plasmonics. In this Account, we review our efforts to develop the biomolecule-based synthesis of 3D chiral plasmonic materials and share the vision that as in biological systems, chirality can be programmed at the molecular level and hierarchically transferred at multiple scales to develop macroscopic chirality. Accompanied by a biomimetic time-dependent chirality of singular plasmonic nanometals, we also summarize recent achievements in the chemistry and nanophotonics communities pursuing 4D plasmonics that are closely related to our research. The biomimetic and bioinspired approaches discussed in this Account will provide new synthetic insights into implementing chiral nanomaterials and extend the range of accessible nanophotonic design. We hope that the molecular encoding approach will be useful to achieve dynamic light-matter interactions at unprecedented dimensions, time scales, and chirality.

Transient absorption spectroscopy using high harmonic generation: a review of ultrafast X-ray dynamics in molecules and solids

Attosecond science opened the door to observing nuclear and electronic dynamics in real time and has begun to expand beyond its traditional grounds. Among several spectroscopic techniques, X-ray transient absorption spectroscopy has become key in understanding matter on ultrafast time scales. In this review, we illustrate the capabilities of this unique tool through a number of iconic experiments. We outline how coherent broadband X-ray radiation, emitted in high-harmonic generation, can be used to follow dynamics in increasingly complex systems. Experiments performed in both molecules and solids are discussed at length, on time scales ranging from attoseconds to picoseconds, and in perturbative or strong-field excitation regimes. This article is part of the theme issue 'Measurement of ultrafast electronic and structural dynamics with X-rays'.

How optical excitation controls the structure and properties of vanadium dioxide

We combine ultrafast electron diffraction and time-resolved terahertz spectroscopy measurements to link structure and electronic transport properties during the photoinduced insulator-metal transitions in vanadium dioxide. We determine the structure of the metastable monoclinic metal phase, which exhibits antiferroelectric charge order arising from a thermally activated, orbital-selective phase transition in the electron system. The relative contribution of the photoinduced monoclinic and rutile metals to the time-dependent and pump-fluence-dependent multiphase character of the film is established, as is the respective impact of these two distinct phase transitions on the observed changes in terahertz conductivity. Our results represent an important example of how light can control the properties of strongly correlated materials and demonstrate that multimodal experiments are essential when seeking a detailed connection between ultrafast changes in optical-electronic properties and lattice structure.

Attosecond transient absorption instrumentation for thin film materials: Phase transitions, heat dissipation, signal stabilization, timing correction, and rapid sample rotation

We present an extreme ultraviolet (XUV) transient absorption apparatus tailored to attosecond and femtosecond measurements on bulk solid-state thin-film samples, specifically when the sample dynamics are sensitive to heating effects. The setup combines methodology for stabilizing sub-femtosecond time-resolution measurements over 48 h and techniques for mitigating heat buildup in temperature-dependent samples. Single-point beam stabilization in pump and probe arms and periodic time-zero reference measurements are described for accurate timing and stabilization. A hollow-shaft motor configuration for rapid sample rotation, raster scanning capability, and additional diagnostics are described for heat mitigation. Heat transfer simulations performed using a finite element analysis allow comparison of sample rotation and traditional raster scanning techniques for 100 Hz pulsed laser measurements on vanadium dioxide, a material that undergoes an insulator-to-metal transition at a modest temperature of 340 K. Experimental results are presented confirming that the vanadium dioxide (VO₂) sample cannot cool below its phase transition temperature between laser pulses without rapid rotation, in agreement with the simulations. The findings indicate the stringent conditions required to perform rigorous broadband XUV time-resolved absorption measurements on bulk solid-state samples, particularly those with temperature sensitivity, and elucidate a clear methodology to perform them.