Ultrafast Transition Research Articles

Ultrafast transient absorption spectra and kinetics of human blue cone visual pigment at room temperature

The ultrafast photochemical reaction mechanism, transient spectra, and transition kinetics of the human blue cone visual pigment have been recorded at room temperature. Ultrafast time-resolved absorption spectroscopy revealed the progressive formation and decay of several metastable photo-intermediates, corresponding to the Batho to Meta-II photo-intermediates previously observed with bovine rhodopsin and human green cone opsin, on the picosecond to millisecond timescales following pulsed excitation. The experimental data reveal several interesting similarities and differences between the photobleaching sequences of bovine rhodopsin, human green cone opsin, and human blue cone opsin. While Meta-II formation kinetics are comparable between bovine rhodopsin and blue cone opsin, the transition kinetics of earlier photo-intermediates and qualitative characteristics of the Meta-I to Meta-II transition are more similar for blue cone opsin and green cone opsin. Additionally, the blue cone photo-intermediate spectra exhibit a high degree of overlap with uniquely small spectral shifts. The observed variation in Meta-II formation kinetics between rod and cone visual pigments is explained based on key structural differences.

Interplay of coulomb and exciton-phonon coupling controls singlet fission dynamics in two pentacene polymorphs.

Pentacene is an important model organic semiconductor in both the singlet exciton fission (SF) and organic electronics communities. We have investigated the effect of changing crystal structure on the SF process, generating multiple triplet excitons from an initial singlet exciton, and subsequent triplet recombination. Unlike for similar organic semiconductors that have strong SF sensitive to polymorphism, we find almost no quantitative difference between the kinetics of triplet pair (TT) formation in the two dominant polymorphs of pentacene. Both pairwise dimer coupling and momentum-space crystal models predict much faster TT formation from the bright singlet excited state of the Bulk vs ThinFilm polymorph, contrasting with the experiment. GW and Bethe-Salpeter equation calculations, including exciton-phonon coupling, reveal that ultrafast phonon-driven transitions in the ThinFilm polymorph compensate the intrinsically slower purely Coulomb-mediated TT formation channel, rationalizing the similarity in observed rates. Taking into account the influence of subtle structural distinctions on both the direct and phonon-mediated SF pathways reveals a predictive capability to these methods, expected to be applicable to a wide variety of molecular crystals.

Ultrafast (≈10 GHz) mid-IR modulator based on ultrafast electrical switching of the light–matter coupling

We demonstrate a free-space amplitude modulator for mid-infrared radiation (λ ≈ 9.6 μm) that operates at room temperature up to at least 20 GHz (above the −3 dB cutoff frequency measured at 8.2 GHz). The device relies on the ultrafast transition between weak- and strong-coupling regimes induced by the variation of the applied bias voltage. Such transition induces a modulation of the device reflectivity. It is made of a semiconductor heterostructure enclosed in a judiciously designed array of metal–metal optical resonators, that—all-together—behave as an electrically tunable surface. At negative bias, it operates in the weak light–matter coupling regime. Upon application of an appropriate positive bias, the quantum wells populate with electrons, and the device transitions to the strong-coupling regime. The modulator transmission remains linear with input radio frequency power in the 0–9 dBm range. The increase in optical powers up to 25 mW exhibit a weak beginning of saturation a little bit below.

Optical pulse-induced ultrafast antiferrodistortive transition in SrTiO3

The ultrafast dynamics of the antiferrodistortive phase transition in perovskite SrTiO3 is monitored via time-domain Brillouin scattering. Using femtosecond optical pulses, we initiate a thermally driven tetragonal-to-cubic structural transformation and detect the crystal phase through changes in the frequency of Brillouin oscillations (BO) induced by propagating acoustic phonons. Coupling the measured BO frequency with a spatiotemporal heat diffusion model, we demonstrate that, for a sample kept in the tetragonal phase, deposition of sufficient thermal energy induces a rapid transformation of the heat-affected region to the cubic phase. The initial phase change is followed by a slower reverse cubic-to-tetragonal phase transformation occurring on a timescale of hundreds of picoseconds. We attribute this ultrafast phase transformation in the perovskite to a structural resemblance between atomic displacements of the R-point soft optic mode of the cubic phase and the tetragonal phase, both characterized by anti-phase rotation of oxygen octahedra. The structural relaxation time exhibits a strong temperature dependence consistent with the prediction of the equation of motion describing collective oxygen octahedra rotation based on the energy landscape of the phenomenological Landau theory of phase transitions. Evidence of such a fast structural transition in perovskites can open up new avenues in information processing and energy storage sectors.

Expanded Stability of Layered SnSe-PbSe Alloys and Evidence of Displacive Phase Transformation from Rocksalt in Heteroepitaxial Thin Films.

Bulk PbSnSe has a two-phase region, or miscibility gap, as the crystal changes from a van der Waals-bonded orthorhombic 2D layered structure in SnSe-rich compositions to the related 3D-bonded rocksalt structure in PbSe-rich compositions. This structural transition drives a large contrast in the electrical, optical, and thermal properties. We realize low temperature direct growth of epitaxial PbSnSe thin films on GaAs via molecular beam epitaxy using an in situ PbSe surface treatment and show a significantly reduced two-phase region by stabilizing the Pnma layered structure out to Pb0.45Sn0.55Se, beyond the bulk limit around Pb0.25Sn0.75Se at low temperatures. Pushing further, we directly access metastable two-phase films of layered and rocksalt grains that are nearly identical in composition around Pb0.50Sn0.50Se and entirely circumvent the miscibility gap. We present microstructural and compositional evidence for an incomplete displacive transformation from a rocksalt to layered structure in these films, which we speculate occurs during the sample cooling to room temperature after synthesis. In situ temperature-cycling experiments on a Pb0.58Sn0.42Se rocksalt film reproduce characteristic attributes of a displacive transition and show a modulation in electronic properties. We find well-defined orientation relationships between the phases formed and reveal unconventional strain relief mechanisms involved in the crystal structure transformation using transmission electron microscopy. Overall, our work adds a scalable thin film integration route to harness the dramatic contrast in material properties in PbSnSe across a potentially ultrafast crystalline-crystalline structural transition.

Numerical study of transient absorption saturation in single-layer graphene for optical nanoscopy applications

Transient absorption, or pump–probe microscopy is an absorption-based technique that can explore samples ultrafast dynamic properties and provide fluorescence-free contrast mechanisms. When applied to graphene and its derivatives, this technique exploits the graphene transient response caused by the ultrafast interband transition as the imaging contrast mechanism. The saturation of this transition is fundamental to allow for super-resolution optical far-field imaging, following the reversible saturable optical fluorescence transitions (RESOLFT) concept, although not involving fluorescence. With this aim, we propose a model to numerically compute the temporal evolution under saturation conditions of the single-layer graphene molecular states, which are involved in the transient absorption. Exploiting an algorithm based on the fourth order Runge–Kutta (RK4) method, and the density matrix approach, we numerically demonstrate that the transient absorption signal of single-layer graphene varies linearly as a function of excitation intensity until it reaches saturation. We experimentally verify this model using a custom pump–probe super-resolution microscope. The results define the intensities necessary to achieve super-resolution in a pump–probe nanoscope while studying graphene-based materials and open the possibility of predicting such a saturation process in other light-matter interactions that undergo the same transition.

Trends in GeTe Thermoelectrics: From Fundamentals to Applications

AbstractGermanium telluride (GeTe) with ultrafast ferroelectric transition, Rashba‐like electronic transport, and anomalous phonon anharmonicity are historically studied for potential memorizing and thermoelectric applications. Due to recent breakthroughs in spintronics, valleytronics, orbitronics, pre‐eminent GeTe thermoelectrics have re‐attracted enormous interest from both academia and industries, with increasing reports of significant figure‐of‐merit over 2.7 and the maximum efficiency of up to 17.0%. Here, the emerging trends in advancing GeTe thermoelectrics, starting from fundamentals of phase transformation, crystal structure, bonding mechanisms, and transport characteristics, with a highlight on the roles of Ge_4s2 lone pairs, are timely overviewed. Technical insights in synthesis, characterization, property measurement, and computation are then summarized. After that, several innovative strategies for increasing the figure‐of‐merit, including entropy engineering, nanostructuring, and hybridization, which will further benefit near‐room‐temperature and n‐type performance, are examined. Moreover, high‐density and high‐efficiency devices with broad working temperatures are discussed as a result of rational configurational and interfacial design. In the end, perspective remarks on the challenges and outlook envisaging for next‐generation GeTe thermoelectrics, which will play a prominent role in future energy and environmental landscapes, are provided.

Electronic Population Reconstruction from Strong-Field-Modified Absorption Spectra with a Convolutional Neural Network

We simulate ultrafast electronic transitions in an atom and corresponding absorption line changes with a numerical, few-level model, similar to previous work. In addition, a convolutional neural network (CNN) is employed for the first time to predict electronic state populations based on the simulated modifications of the absorption lines. We utilize a two-level and four-level system, as well as a variety of laser-pulse peak intensities and detunings, to account for different common scenarios of light–matter interaction. As a first step towards the use of CNNs for experimental absorption data in the future, we apply two different noise levels to the simulated input absorption data.

Manipulating hyperbolic transient plasmons in a layered semiconductor

Anisotropic materials with oppositely signed dielectric tensors support hyperbolic polaritons, displaying enhanced electromagnetic localization and directional energy flow. However, the most reported hyperbolic phonon polaritons are difficult to apply for active electro-optical modulations and optoelectronic devices. Here, we report a dynamic topological plasmonic dispersion transition in black phosphorus via photo-induced carrier injection, i.e., transforming the iso-frequency contour from a pristine ellipsoid to a non-equilibrium hyperboloid. Our work also demonstrates the peculiar transient plasmonic properties of the studied layered semiconductor, such as the ultrafast transition, low propagation losses, efficient optical emission from the black phosphorus’s edges, and the characterization of different transient plasmon modes. Our results may be relevant for the development of future optoelectronic applications.

Ultrafast Negative Capacitance Transition for 2D Ferroelectric MoS2/Graphene Transistor.

Negative capacitance gives rise to subthreshold swing (SS) below the fundamental limit by efficient modulation of surface potential in transistors. While negative-capacitance transition is reported in polycrystalline Pb(Zr0.2Ti0.8)O3 (PZT) and HfZrO2 (HZO) thin-films in few microseconds timescale, low SS is not persistent over a wide range of drain current when used instead of conventional dielectrics. In this work, the clear nano-second negative transition states in 2D single-crystal CuInP2S6 (CIPS) flakes have been demonstrated by an alternative fast-transient measurement technique. Further, integrating this ultrafast NC transition with the localized density of states of Dirac contacts and controlled charge transfer in the CIPS/channel (MoS2/graphene) a state-of-the-art device architecture, negative capacitance Dirac source drain field effect transistor (FET) is introduced. This yields an ultralow SS of 4.8mV dec-1 with an average sub-10 SS across five decades with on-off ratio exceeding 107, by simultaneous improvement of transport and body factors in monolayer MoS2-based FET, outperforming all previous reports. This approach could pave the way to achieve ultralow-SS FETs for future high-speed and low-power electronics.

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 abinitio 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.

Ultrafast and slow luminescence decays at energy transfer from impurity-bound excitons

Different types of ultrafast radiative transitions are considered. The most interesting among them is the case when the radiative transition is accelerated by the configurational transformation of a structural unit where it occurs. Impurity-induced VUV excitation bands of doped Li2B4O7 are attributed to the creation of impurity-bound excitons. When Mn2+ is involved into exciton recombination, the radiative transition in the Mn2+3d5 configuration is accelerated and occurs on a sub-nanosecond time scale. Excitation within the UV bands is connected with energy transfer from the structural units formed by the sensitizers (Cu, Sn) and oxygen to Mn2+. In this case, Mn2+ transitions are not accelerated since its excited state appears after complete relaxation of excitation in the corresponding sensitizer’s unit. Pulsed cathodoluminescence decays are rather slow due to very slow transport of electron–hole pairs and excitons in Li2B4O7.

An efficient green-emitting quantum dot with near-unity quantum yield and suppressed Auger recombination for high-performance light-emitting diodes

Near 100% photoluminescence quantum yield (PL QY) is the first target for designing luminescent quantum dots (QDs). Here, an alloyed QD with a core/shell structure of Cd0.194Zn0.806Se0.406S0.594/ZnS is synthesized and characterized by the feature of large core size (about 10 nm) and thin shell (around three ZnS monolayers). It exhibits an efficient green emission with the PL QY of 99.8% and ultra-fast radiative transition rate (kr) of 4.16 × 107 s−1. It demonstrates that the excited state will be rapidly decayed to its ground state through a radiative channel and less vulnerable to non-radiative processes. Under a high pump intensity (254.65 μJ cm−2), the Cd0.194Zn0.806Se0.406S0.594/ZnS QD still owns a long biexciton decay lifetime (τxx) of 1.05 ns and high biexciton QYxx of 25.1%, corresponding to a suppressed non-radiative Auger recombination. The Cd0.194Zn0.806Se0.406S0.594/ZnS QD based light-emitting diode exhibits a peak external quantum efficiency of 20.1% and brightness of over 105 cd m−2 at 5.2 V. We believe that the Cd0.194Zn0.806Se0.406S0.594/ZnS QD not only provides a near-unity QY but also ensures its QD based light-emitting diode (QLED) with an excellent performance, which could be an excellent QD material for QLED applications.

Transition Routes of Electrokinetic Flow in a Divergent Microchannel with Bending Walls.

Electrokinetic flow can be generated as a highly coupled phenomenon among velocity fields, electric conductivity fields, and electric fields. It can exhibit different responses to AC electric fields in different frequency regimes, according to different instability/receptivity mechanisms. In this investigation, by both flow visualization and single-point laser-induced fluorescence (LIF) method, the response of AC electrokinetic flow and the transition routes towards chaos and turbulence have been experimentally investigated. It is found, when the AC frequency ff>30 Hz, the interface responds at both the neutral frequency of the basic flow and the AC frequency. However, when ff≥30 Hz, the interface responds only at the neutral frequency of the basic flow. Both periodic doubling and subcritical bifurcations have been observed in the transition of AC electrokinetic flow. We hope the current investigation can promote our current understanding of the ultrafast transition process of electrokinetic flow from laminar state to turbulence.

Bimetal–Organic Frameworks for High Efficiency Catalytic Reduction

The design of efficient reduction catalysts is crucial for sustainable development in order to reduce waste and protect the environment. Here, an ultrathin bimetallic metal–organic framework (MOF) is synthesized and fabricated as an effective catalyst for the reduction of 4-nitrophenol (which is harmful to the environment) to 4-aminophenol (which is an important chemical and pharmaceutical intermediate). The rapid reduction time of the as-prepared bimetallic CuNi-MOFs is 15 s, and the apparent rate constant is 0.153 s–1, which is far beyond the single-metal MOFs. Ultrafast transition absorption spectra show that Cu doping can induce defect states, which can effectively trap and store long-lived electrons in the catalytic process. X-ray photoelectron spectra verify that Cu doping changes the electron density distribution of CuNi-MOFs. It is believed that the introduction of Cu sites leads to synergistic effects of Cu and Ni sites, which significantly improves the catalytic activity of 4-NP.

Photoinduced Ultrafast Transition of the Local Correlated Structure in Chalcogenide Phase-Change Materials.

Revealing the bonding and time-evolving atomic dynamics in functional materials with complex lattice structures can update the fundamental knowledge on rich physics therein, and also help to manipulate the material properties as desired. As the most prototypical chalcogenide phase change material, Ge_{2}Sb_{2}Te_{5} has been widely used in optical data storage and nonvolatile electric memory due to the fast switching speed and the low energy consumption. However, the basic understanding of the structural dynamics on the atomic scale is still not clear. Using femtosecond electron diffraction, structure factor calculation, and time-dependent density-functional theory molecular dynamic simulation, we reveal the photoinduced ultrafast transition of the local correlated structure in the averaged rocksalt phase of Ge_{2}Sb_{2}Te_{5}. The randomly oriented Peierls distortion among unit cells in the averaged rocksalt phase of Ge_{2}Sb_{2}Te_{5} is termed as local correlated structures. The ultrafast suppression of the local Peierls distortions in the individual unit cell gives rise to a local structure change from the rhombohedral to the cubic geometry within ∼0.3 ps. In addition, the impact of the carrier relaxation and the large number of vacancies to the ultrafast structural response is quantified and discussed. Our Letter provides new microscopic insights into contributions of the local correlated structure to the transient structural and optical responses in phase change materials. Moreover, we stress the significance of femtosecond electron diffraction in revealing the local correlated structure in the subunit cell and the link between the local correlated structure and physical properties in functional materials with complex microstructures.

Nonthermal melting of charge density wave order via nucleation in VTe2

Ultrafast optical control of phase change materials is of great importance from fundamental and practical points of view. Transition-metal dichalcogenides are of significant interest in this research field because of the photoinduced phase transition originating from the nonthermal melting of their charge density wave (CDW) orders. In this work, we investigated the ultrafast optical response of ${\mathrm{VTe}}_{2}$ by using broadband coherent optical phonon spectroscopy with subpicosecond time resolution. With an increase of laser-excitation fluence, a characteristic oscillation around 4.4 THz appeared in the transient reflectivity change. This suggests photoinduction of the high-temperature phase, i.e., a CDW-melt state, of ${\mathrm{VTe}}_{2}$ within a picosecond. Such an ultrafast transition would be induced by a purely electronic effect, but as with thermodynamic melting, it is strongly suggested that the phase transition dynamics induced spatially by laser irradiation proceeds inhomogeneously via a nucleation mode.

Density functional tight binding approach utilized to study X-ray-induced transitions in solid materials

Intense X-ray pulses from free-electron lasers can trigger ultrafast electronic, structural and magnetic transitions in solid materials, within a material volume which can be precisely shaped through adjustment of X-ray beam parameters. This opens unique prospects for material processing with X rays. However, any fundamental and applicational studies are in need of computational tools, able to predict material response to X-ray radiation. Here we present a dedicated computational approach developed to study X-ray induced transitions in a broad range of solid materials, including those of high chemical complexity. The latter becomes possible due to the implementation of the versatile density functional tight binding code DFTB+ to follow band structure evolution in irradiated materials. The outstanding performance of the implementation is demonstrated with a comparative study of XUV induced graphitization in diamond.

Multi-mode excitation drives disorder during the ultrafast melting of a C4-symmetry-broken phase

Spontaneous C4-symmetry breaking phases are ubiquitous in layered quantum materials, and often compete with other phases such as superconductivity. Preferential suppression of the symmetry broken phases by light has been used to explain non-equilibrium light induced superconductivity, metallicity, and the creation of metastable states. Key to understanding how these phases emerge is understanding how C4 symmetry is restored. A leading approach is based on time-dependent Ginzburg-Landau theory, which explains the coherence response seen in many systems. However, we show that, for the case of the single layered manganite La0.5Sr1.5MnO4, the theory fails. Instead, we find an ultrafast inhomogeneous disordering transition in which the mean-field order parameter no longer reflects the atomic-scale state of the system. Our results suggest that disorder may be common to light-induced phase transitions, and methods beyond the mean-field are necessary for understanding and manipulating photoinduced phases.

Isothermal phase transition across phase boundary in (Pb0.95Ba0.05)ZrO3 ceramics

Paraelectric-ferroelectric-antiferroelectric (PE-FE-AFE) phase transitions induced by external stimulus (temperature, electric field, and hydrostatic pressure) usually produce exotic performances in functional ceramics. This Letter reports an isothermal phase transition in (Pb0.95Ba0.05)ZrO3 ceramics in the proximity of separated PE-FE and FE-AFE phase boundaries as extended waiting time. Through an isothermal process, PE (Pm3¯m)-to-FE (R3cH) and FE-to-AFE (Pbam) phase transformations occur within several minutes (with different transition degrees). This is in contrast to the wide recognition of ultrafast and time-independent ferroelectric transitions. In situ Raman spectra analysis unveils the sluggish growth of FE and AFE phases out of the corresponding parent matrix. Finally, the Landau theory analysis is proposed to explain the isothermal phase transition among PE-FE-AFE phases. This work will refresh the wisdom of phase transitions in functional ceramics.