Coherent ultrafast photoemission from a single quantized state of a one-dimensional emitter.

Femtosecond laser-driven photoemission source provides an unprecedented femtosecond-resolved electron probe not only for atomic-scale ultrafast characterization but also for free-electron radiation sources. However, for conventional metallic electron source, intense lasers may induce a considerable broadening of emitting energy level, which results in large energy spread (>600 milli-electron volts) and thus limits the spatiotemporal resolution of electron probe. Here, we demonstrate the coherent ultrafast photoemission from a single quantized energy level of a carbon nanotube. Its one-dimensional body can provide a sharp quantized electronic excited state, while its zero-dimensional tip can provide a quantized energy level act as a narrow photoemission channel. Coherent resonant tunneling electron emission is evidenced by a negative differential resistance effect and a field-driven Stark splitting effect. The estimated energy spread is ~57 milli-electron volts, which suggests that the proposed carbon nanotube electron source may promote electron probe simultaneously with subangstrom spatial resolution and femtosecond temporal resolution.

Giant acceleration of polaron transport by ultrafast laser-induced coherent phonons.

Polaron formation is ubiquitous in polarized materials, but severely hampers carrier transport for which effective controlling methods are urgently needed. Here, we show that laser-controlled coherent phonon excitation enables orders of magnitude enhancement of carrier mobility via accelerating polaron transport in a prototypical material, lithium peroxide (Li2O2). The selective excitation of specific phonon modes, whose vibrational pattern directly overlap with the polaronic lattice deformation, can remarkably reduce the energy barrier for polaron hopping. The strong nonadiabatic couplings between the electronic and ionic subsystem play a key role in triggering the migration of polaron, via promoting phonon-phonon scattering in q space within sub-picoseconds. These results extend our understanding of polaron transport dynamics to the nonequilibrium regime and allow for optoelectronic devices with ultrahigh on-off ratio and ultrafast responsibility, competitive with those of state-of-the-art devices fabricated based on free electron transport.

Coherent Phonon Assisted Ultrafast Order-Parameter Reversal and Hidden Metallic State in Ta_{2}NiSe_{5}.

The nonequilibrium dynamics during photoinduced insulator-to-metal transition (IMT) in the excitonic insulator (EI) candidate Ta_{2}NiSe_{5} have been investigated, which reproduce the timescale and spectral features of the ultrafast switch and reveal intricate many-body interactions involving multidegrees of freedom. The key role of lattice order parameter (OP) reversal, occurring on a timescale comparable to that of purely electronic processes (<100 fs), is identified. This reversal is enabled by the anharmonic interactions between EI-OP-coupled phonons and the conventional coherent phonons, leading to a modified potential energy landscape and a high-frequency mode up-conversion. The phonon excitation depends on the dynamics of photocarriers distributed around the Fermi level, and thus intertwines with the excitonic quenching and the complete gap collapse. These findings provide a comprehensive understanding of exciton-phonon dynamics in correlated quantum materials.

Super-resolved time-frequency measurements of coupled phonon dynamics in a 2D quantum material.

Methods to probe and understand the dynamic response of materials following impulsive excitation are important for many fields, from materials and energy sciences to chemical and neuroscience. To design more efficient nano, energy, and quantum devices, new methods are needed to uncover the dominant excitations and reaction pathways. In this work, we implement a newly-developed superlet transform-a super-resolution time-frequency analytical method-to analyze and extract phonon dynamics in a laser-excited two-dimensional (2D) quantum material. This quasi-2D system, 1T-TaSe2, supports both equilibrium and metastable light-induced charge density wave (CDW) phases mediated by strongly coupled phonons. We compare the effectiveness of the superlet transform to standard time-frequency techniques. We find that the superlet transform is superior in both time and frequency resolution, and use it to observe and validate novel physics. In particular, we show fluence-dependent changes in the coupled dynamics of three phonon modes that are similar in frequency, including the CDW amplitude mode, that clearly demonstrate a change in the dominant charge-phonon couplings. More interestingly, the frequencies of the three phonon modes, including the strongly-coupled CDW amplitude mode, remain time- and fluence-independent, which is unusual compared to previously investigated materials. Our study opens a new avenue for capturing the coherent evolution and couplings of strongly-coupled materials and quantum systems.

Traversing Double-Well Potential Energy Surfaces: Photoinduced Concurrent Intralayer and Interlayer Structural Transitions in XTe2 (X = Mo, W).

The microscopic arrangement of atoms and molecules is the determining factor in how materials behave and perform; i.e., the structure determines the property, a traditional paradigm in materials science. Photoexcitation-driven manipulation of the crystal structure and associated electronic properties in quantum materials provides opportunities for the exploration of exotic physics and practical applications; however, a generalized mechanism for such symmetry engineering is absent. Here, by ultrafast electron diffraction, structure factor calculation, and TDDFT-MD simulations, we report the photoinduced concurrent intralayer and interlayer structural transitions in the Td and 1T' phases of XTe2 (X = Mo, W). We discuss the modification of multiple quantum electronic states associated with the intralayer and interlayer structural transitions, such as the topological band inversion and the higher-order topological state. The twin structures and the stacking faults in XTe2 are also identified by ultrafast structural responses. The comprehensive study of the ultrafast structural response in XTe2 suggests the traversal of all double-well potential energy surfaces (DWPES) by laser excitation, which is expected to be an intrinsic mechanism in the field of photoexcitation-driven global/local symmetry engineering and also a critical ingredient inducing the exotic properties in the non-equilibrium state in a large number of material systems.

Calibrating Out-of-Equilibrium Electron-Phonon Couplings in Photoexcited MoS2.

Nonequilibrium electron-phonon coupling (EPC) serves as a dominant interaction in a multitude of transient processes, including photoinduced phase transitions, coherent phonon generation, and possible light-induced superconductivity. Here we use monolayer MoS2 as a prototype to investigate the variation in electron-phonon couplings under laser excitation, on the basis of real-time time-dependent density functional theory simulations. Phonon softening, anisotropic modification of the deformation potential, and enhancement of EPC are observed, which are attributed to the reduced electronic screening and modulated potential energy surfaces by photoexcitation. Furthermore, by tracking the transient deformation potential and nonthermal electronic population, we can monitor the ultrafast time evolution of the energy exchange rate between electrons and phonons upon laser excitation. This work provides an effective strategy to investigate the nonequilibrium EPC and constructs a scaffold for understanding nonequilibrium states beyond the multitemperature models.

Optical Control of Multistage Phase Transition via Phonon Coupling in MoTe_{2}.

The temporal characters of laser-driven phase transition from 2H to 1T^{'} has been investigated in the prototype MoTe_{2} monolayer. This process is found to be induced by fundamental electron-phonon interactions, with an unexpected phonon excitation and coupling pathway closely related to the nonequilibrium relaxation of photoexcited electrons. The order-to-order phase transformation is dissected into three substages, involving energy and momentum scattering processes from optical (A_{1}^{'} and E^{'}) to acoustic phonon modes [LA(M)] in subpicosecond timescale. An intermediate metallic state along the nonadiabatic transition pathway is also identified. These results have profound implications on nonequilibrium phase engineering strategies.

Creation of a novel inverted charge density wave state.

Charge density wave (CDW) order is an emergent quantum phase that is characterized by periodic lattice distortion and charge density modulation, often present near superconducting transitions. Here, we uncover a novel inverted CDW state by using a femtosecond laser to coherently reverse the star-of-David lattice distortion in 1T-TaSe2. We track the signature of this novel CDW state using time- and angle-resolved photoemission spectroscopy and the time-dependent density functional theory to validate that it is associated with a unique lattice and charge arrangement never before realized. The dynamic electronic structure further reveals its novel properties that are characterized by an increased density of states near the Fermi level, high metallicity, and altered electron-phonon couplings. Our results demonstrate how ultrafast lasers can be used to create unique states in materials by manipulating charge-lattice orders and couplings.

Indirect to Direct Charge Transfer Transition in Plasmon-Enabled CO2 Photoreduction.

Understanding hot carrier dynamics between plasmonic nanomaterials and its adsorbate is of great importance for plasmon-enhanced photoelectronic processes such as photocatalysis, optical sensing and spectroscopic analysis. However, it is often challenging to identify specific dominant mechanisms for a given process because of the complex pathways and ultrafast interactive dynamics of the photoelectrons. Here, using CO2 reduction as an example, the underlying mechanisms of plasmon-driven catalysis at the single-molecule level using time-dependent density functional theory calculations is clearly probed. The CO2 molecule adsorbed on two typical nanoclusters, Ag20 and Ag147 , is photoreduced by optically excited plasmon, accompanied by the excitation of asymmetric stretching and bending modes of CO2 . A nonlinear relationship has been identified between laser intensity and reaction rate, demonstrating a synergic interplay and transition from indirect hot-electron transfer to direct charge transfer, enacted by strong localized surface plasmons. These findings offer new insights for CO2 photoreduction and for the design of effective pathways toward highly efficient plasmon-mediated photocatalysis.

Manipulating Weyl quasiparticles by orbital-selective photoexcitation in WTe2.

Optical control of structural and electronic properties of Weyl semimetals allows development of switchable and dissipationless topological devices at the ultrafast scale. An unexpected orbital-selective photoexcitation in type-II Weyl material WTe2 is reported under linearly polarized light (LPL), inducing striking transitions among several topologically-distinct phases mediated by effective electron-phonon couplings. The symmetry features of atomic orbitals comprising the Weyl bands result in asymmetric electronic transitions near the Weyl points, and in turn a switchable interlayer shear motion with respect to linear light polarization, when a near-infrared laser pulse is applied. Consequently, not only annihilation of Weyl quasiparticle pairs, but also increasing separation of Weyl points can be achieved, complementing existing experimental observations. In this work, we provide a new perspective on manipulating the Weyl node singularity and coherent control of electron and lattice quantum dynamics simultaneously.

Phase Transition Photodetection in Charge Density Wave Tantalum Disulfide.

The charge density wave (CDW) phase is a macroscopic quantum state with periodic charge density modulation accompanied by periodic lattice distortion in low-dimensional metals. External fields, such as an electric field and optical excitation, can trigger the transitions among different CDW states, leaving an under-explored mechanism and attracting great interest toward optoelectronic applications. Here, we explore a photoinduced phase transition in 1T-TaS2 under an electrical field. By analyzing the phase transition probability, we obtained a linear dependence of the phase transition barrier on the electric field and laser energy density. Additionally, the threshold laser energy for the phase transition decreases linearly with an increasing applied electrical field. Finally, picojoule photodetection was realized in the visible and near-infrared ranges near the CDW transition edge. Our work will promote the understanding of the CDW phase transition mechanism as well as open pathways for optoelectronic applications.

Ultrafast charge ordering by self-amplified exciton-phonon dynamics in TiSe2.

The origin of charge density waves (CDWs) in TiSe[Formula: see text] has long been debated, mainly due to the difficulties in identifying the timescales of the excitonic pairing and electron-phonon coupling (EPC). Without a time-resolved and microscopic mechanism, one has to assume simultaneous appearance of CDW and periodic lattice distortions (PLD). Here, we accomplish a complete separation of ultrafast exciton and PLD dynamics and unravel their interplay in our real-time time-dependent density functional theory simulations. We find that laser pulses knock off the exciton order and induce a homogeneous bonding-antibonding transition in the initial 20 fs, then the weakened electronic order triggers ionic movements antiparallel to the original PLD. The EPC comes into play after the initial 20 fs, and the two processes mutually amplify each other leading to a complete inversion of CDW ordering. The self-amplified dynamics reproduces the evolution of band structures in agreement with photoemission experiments. Hence we resolve the key processes in the initial dynamics of CDWs that help elucidate the underlying mechanism.

Extreme nonlinear strong-field photoemission from carbon nanotubes.

Strong-field photoemission produces attosecond (10-18 s) electron pulses that are synchronized to the waveform of the incident light. This nonlinear photoemission lies at the heart of current attosecond technologies. Here we report a new nonlinear photoemission behaviour-the nonlinearity in strong-field regime sharply increases (approaching 40th power-law scaling), making use of sub-nanometric carbon nanotubes and 800 nm pulses. As a result, the carrier-envelope phase sensitive photoemission current shows a greatly improved modulation depth of up to 100% (with a total modulation current up to 2 nA). The calculations reveal that the behaviour is an interplay of valence band optical-field emission with charge interaction, and the nonlinear dynamics can be tunable by changing the bandgap of carbon nanotubes. The extreme nonlinear photoemission offers a new means of producing extreme temporal-spatial resolved electron pulses, and provides a new design philosophy for attosecond electronics and photonics.

Photoexcitation Induced Quantum Dynamics of Charge Density Wave and Emergence of a Collective Mode in 1T-TaS2.

Photoexcitation is a powerful means in distinguishing different interactions and manipulating the states of matter, especially in charge density wave (CDW) materials. The CDW state of 1T-TaS2 has been widely studied experimentally mainly because of its intriguing laser-induced ultrafast responses of electronic and lattice subsystems. However, the microscopic atomic dynamics and underlying electronic mechanism upon photoexcitation remain unclear. Here, we demonstrate photoexcitation induced ultrafast dynamics of CDW in 1T-TaS2 using time-dependent density functional theory molecular dynamics. We discover a novel collective oscillation mode between the CDW state and a transient state induced by photodoping, which is significantly different from thermally induced phonon mode and attributed to the modification of the potential energy surface from laser excitation. In addition, our finding validates nonthermal melting of CDW induced at low light intensities, supporting that conventional hot electron model is inadequate to explain photoinduced dynamics. Our results provide a deep insight into the coherent electron and lattice quantum dynamics during the formation and excitation of CDW in 1T-TaS2.

Coexistence of Different Charge-Transfer Mechanisms in the Hot-Carrier Dynamics of Hybrid Plasmonic Nanomaterials.

Hot-carrier dynamics at the interfaces of semiconductors and nanoclusters is of significant importance for photovoltaic and photocatalytic applications. Plasmon-driven charge separation processes are considered to be only dependent on the type of donor-acceptor interactions, that is, the conventional hot-electron-transfer mechanism for van der Waals interactions and the plasmon-induced interfacial charge-transfer transition mechanism for chemical bonds. Here, we demonstrate that the two mechanisms can coexist in a nanoparticle-semiconductor hybrid nanomaterial, both leading to faster transfer than carrier relaxation. The origin of the two mechanisms is attributed to the spatial polarization of the excited hot carriers, where the longitudinal state couples to semiconductors more strongly than the transverse state. Our findings provide a new insight into the photoinduced carrier dynamics, which is relevant for many applications in solar energy conversion, including efficient water splitting, photocatalysis, and photovoltaics.

Momentum-resolved TDDFT algorithm in atomic basis for real time tracking of electronic excitation.

Ultrafast electronic dynamics in solids lies at the core of modern condensed matter and materials physics. To build up a practical ab initio method for studying solids under photoexcitation, we develop a momentum-resolved real-time time dependent density functional theory (rt-TDDFT) algorithm using numerical atomic basis, together with the implementation of both the length and vector gauge of the electromagnetic field. When applied to simulate elementary excitations in two-dimensional materials such as graphene, different excitation modes, only distinguishable in momentum space, are observed. The momentum-resolved rt-TDDFT is important and computationally efficient for the study of ultrafast dynamics in extended systems.

Plasmon-induced nonlinear response of silver atomic chains.

Nonlinear response of a linear silver atomic chain upon ultrafast laser excitation has been studied in real time using the time-dependent density functional theory. We observe the presence of nonlinear responses up to the fifth order in tunneling current, which is ascribed to the excitation of high-energy electrons generated by Landau damping of plasmons. The nonlinear effect is enhanced after adsorption of polar molecules such as water due to the enhanced damping rates during plasmon decay. Increasing the length of atomic chains also increases the nonlinear response, favoring higher-order plasmon excitation. These findings offer new insights towards a complete understanding and ultimate control of plasmon-induced nonlinear phenomena to atomic precision.

Plasmon-driven sub-picosecond breathing of metal nanoparticles.

We present the first real-time atomistic simulation on the quantum dynamics of icosahedral silver nanoparticles under strong laser pulses, using time dependent density functional theory (TDDFT) molecular dynamics. We identify the emergence of sub-picosecond breathing-like radial oscillations starting immediately after laser pulse excitation, with increasing amplitude as the field intensity increases. The ultrafast dynamic response of nanoparticles to laser excitation points to a new mechanism other than equilibrium electron-phonon scattering previously assumed, which takes a much longer timescale. A sharp weakening of all bonds during laser excitation is observed, thanks to plasmon damping into excited electrons in anti-bonding states. This sudden weakening of bonds leads to a uniform expansion of the nanoparticles and launches coherent breathing oscillations.

Facile and straightforward synthesis of superparamagnetic reduced graphene oxide-Fe3O4 hybrid composite by a solvothermal reaction.

A superparamagnetic reduced graphene oxide-Fe(3)O(4) hybrid composite (rGO-Fe(3)O(4)) was prepared via a facile and straightforward method through the solvothermal reaction of iron (III) acetylacetonate (Fe(acac)(3)) and graphene oxide (GO) in ethylenediamine (EDA) and water. By this method, chemical reduction of GO as well as the formation of Fe(3)O(4) nanoparticles (NPs) can be achieved in one step. The Fe(3)O(4) NPs are firmly deposited on the surfaces of rGO, avoiding their reassembly to graphite. The rGO sheets prevent the agglomeration of Fe(3)O(4) NPs and enable a uniform dispersion of these metal oxide particles. The size distribution and coverage density of Fe(3)O(4) NPs deposited on rGO can be controlled by varying the initial mass ratio of GO and iron precursor, Fe(acac)(3). With an initial mass ratio of GO and Fe(acac)(3) of 5:5, the surfaces of rGO sheets are densely covered by spherical Fe(3)O(4) NPs with an average size of 19.9 nm. The magnetic-functionalized rGO hybrid exhibits a good magnetic property and the specific saturation magnetization (M(s)) is 13.2 emu g(-1). The adsorption test of methylene blue from aqueous solution demonstrates the potential application of this rGO-Fe(3)O(4) hybrid composite in removing organic dyes from polluted water.