Ultrafast Light Research Articles

Wafer-Scale Transfer and Integration of Tungsten-Doped Vanadium Dioxide Films.

Modern optoelectronic devices trend toward greater flexibility, wearability, and multifunctionality, demanding higher standards for fabrication and operation temperatures. Vanadium dioxide (VO2), with its metal-insulator transition (MIT) at 68 °C, serves as a crucial functional layer in many optoelectronic devices. However, VO2 usually needs to grow at >450 °C in an oxygen-containing atmosphere and to function across its MIT temperature, leading to low compatibility with most optoelectronic devices, especially on flexible substrates. In this work, we report a layer-by-layer transfer method of wafer-scale tungsten-doped VO2 films, which enables sequential integration of the VO2 films with low MIT temperatures (down to 40 °C) onto arbitrary substrates. Notably, by stacking multiple VO2 films with different doped levels, a quasi-gradient-doped VO2 architecture can be achieved, effectively broadening the MIT temperature window and reducing the hysteresis of VO2. These integrated VO2 films find a wide scope of applications in flexible temperature indicator strips, infrared camouflage devices, nonreciprocal ultrafast light modulators, and smart photoactuators. Our work promotes the development of more flexible and tunable optoelectronic devices integrated with VO2.

Subcycle modulation of light's orbital angular momentum via a Fourier space-time transformation.

Achieving highly tailored control over both the spatial and temporal evolution of light's orbital angular momentum (OAM) on ultrafast timescales remains a critical challenge in photonics. Here, we introduce a method to modulate the OAM of light on a femtosecond scale by engineering a space-time coupling in ultrashort pulses. By linking azimuthal position with time, we implement an azimuthally varying Fourier transformation to dynamically alter light's spatial distribution in a fixed transverse plane. Our experiments demonstrate self-torqued wave packets that exhibit spiraling motions and rapid temporal OAM changes down to a few femtoseconds. We further extend this concept to generate angularly self-accelerating wave packets that adjust their OAM by redistributing their energy density across their spectral bandwidth, without external forces. The ability to tune the spatial-temporal properties of light on demand opens the possibility for exploring ultrafast light dynamics at fundamental timescales, with far-reaching implications for ultrafast spectroscopy, nano- and microstructure manipulation, condensed matter physics, and other related areas.

Solitary wave solutions, bifurcation theory, and sensitivity analysis in the modified unstable nonlinear Schrödinger equation

This study examines the modified unstable nonlinear Schrödinger model, a fundamental nonlinear physical model vital for depicting optical solitary wave solutions and their evolution in dynamic fiber optics. The propagation of waves in nonlinear dispersive media is of great significance, presenting new opportunities for data processing in communication systems and generating ultrafast light pulses. Applying a wave transformation reduces the nonlinear Schrödinger equation to an ordinary differential equation, and an extended direct algebraic method is used to derive various soliton solutions. The novelty lies in using this method to uncover various soliton forms, which are then explored through graphical representations in 2D, 3D, and contour plots. The phase portraits and bifurcation theory also qualitatively assess the undisturbed planar system. Sensitivity analysis under different initial conditions indicates how the soliton solutions adapt to changes in system parameters. The outcomes confirm that the presented approach effectively evaluates soliton solutions across diverse nonlinear models, contributing to the understanding of wave propagation in slightly stable and unstable media.

A Space‐Time Knife‐Edge in Epsilon‐Near‐Zero Films for Ultrafast Pulse Characterization

AbstractEpsilon‐near‐zero (ENZ) materials have shown strong refractive nonlinearities that can be fast in an absolute sense. While continuing to advance fundamental science, such as time varying interactions, the community is still searching for an application that can effectively make use of the strong index modulation offered. Here, the effect of strong space‐time index modulation in ENZ materials is combined with the beam deflection technique to introduce a new approach to optical pulse characterization that is termed a space‐time knife edge. It is shown that in this approach, temporal and spatial information of a Gaussian beam can be extracted with only two time resolved measurements. The approach achieves this without phase‐matching requirements (<1 µm thick) and can achieve a high signal to noise ratio by combining the system with lock‐in detection, facilitating the measurement of weak refractive index changes (Δn 10−5) for low intensity beams. Thus, the space‐time knife edge can offer a new avenue for ultrafast light measurement and demonstrates a use case of ENZ materials. In support of this, temporal dynamics for refractive index changes in non‐colinear experiments opening avenues are outlined for better theoretical understanding of both the spatial and temporal dynamics of emerging ENZ films.

Warm-White Light Emission of Lead-free CsAg2I3 Single Crystal Scintillator with a One-Dimensional Electronic Structure.

Presently, the exploration of novel inorganic lead-free perovskite scintillators has emerged as a prominent topic in the field of perovskite materials. Extensive attention has been garnered by materials such as Cs3Cu2I5 due to their notable advantage in scintillation intensity, but the response time constants in the microsecond or even millisecond range severely constrain their potential applications in scintillators. In this study, large-sized (5-6 mm) CsAg2I3 single crystals with an ultrafast warm-white light emission on a nanosecond time scale are presented. Specifically, upon X-ray excitation, the single crystal demonstrates a broad-spectrum white light emission with a color temperature as high as 5129 K, attributed to its self-trapped exciton emission. The 137Cs energy spectrum reveals that CsAg2I3 possesses an ultrafast response for γ rays with a time constant of 15 ns, which is significantly faster than that of Cs3Cu2I5. Furthermore, time-resolved photoluminescence unveils a subnanosecond component with a response time of 0.9 ns. The characteristics of ultrafast warm-white light emission exhibit the significant potential of CsAg2I3 in radiation scintillation detection and its probability of playing a pivotal role in future radiation detection technology.

Achieving a comparable transverse magneto-optical Kerr effect by spin–orbit field driven magnetoplasmonic

In this study, we propose and simulate a magnetoplasmonics heterostructure that utilizes spin–orbit fields to generate an internal magnetic field and create a significant magneto-optical effect. Our approach offers a new way to overcome the challenges of using permanent magnets or magnetic coils in conventional magnetoplasmonics, such as high-power consumption and non-scalability. We demonstrate that it is possible to create an appropriate amount of magnetic field using spin–orbit fields induced by the spin-Hall effect, such that the consumption power becomes reasonable and the dimensions could be miniaturized. This approach will be an important development in the field of magneto-optics, as it can lead to enhanced transverse magneto-optical Kerr effect in the present of surface plasmon polaritons. The proposed nanostructure consists of a ferromagnetic film adjacent to a heavy metal layer, both sandwiched between two noble metal films, and deposited on a dielectric prism. The strength of the Kerr signal strongly depends on the thickness of the ferromagnetic layer, with the maximum effect observed at a thickness of 5nm. This concept has potential for various nanophotonic and spintronic applications, particularly for developing high-speed active plasmonic devices for ultrafast light modulation.

Excited‐State Dynamics and Optical Properties of Silica Under Ultrafast Laser Irradiation

AbstractExcited by intense infrared ultrafast light pulses, a wide bandgap material undergoes nonlinear ionization, generating a high density of free electrons in conduction states. As a result, the electronic band structure is critically modified and the bandgap shrinks. This induces rapid changes in optical properties, dramatically affecting the absorption spectrum during light coupling to the dielectric surface or during nonlinear propagation inside the bulk. This study analyzes the structural behavior and the modification of the optical properties of laser‐excited silica glass at the molecular cluster level through first‐principles simulations. Employing density functional theory and the GW approximations for band structure under nonequilibrium conditions, alongside the Bethe–Salpeter equation, the dynamics of the optical properties of fused silica are comprehensively explored. The behavior of excited fused silica in a wide photon energy range (from a few to 20 eV) is thus predicted. Laser‐induced electron excitation triggers a redistribution of charges between oxygen and silicon atoms, accompanied by a significant increase in electronic pressure, local atomic structure rearrangement, and material expansion. Molecular dynamics simulations offer a temporal perspective on the excited state dynamics, unveiling the intricate interplay between electronic and atomic effects on bandgap evolution. The analysis sheds light on excitonic resonances, intraband and interband transitions in fused silica under ultrafast laser irradiation, providing valuable insights into its excited state behavior and optical properties.

Attosecond vortex pulse trains

The landscape of ultrafast structured light pulses has significantly advanced thanks to the ability of high-order harmonic generation (HHG) to translate the spatial properties of infrared laser beams to the extreme-ultraviolet (EUV) spectral range. In particular, the up-conversion of orbital angular momentum (OAM) has enabled the generation of high-order harmonics whose OAM scales linearly with the harmonic order and the topological charge of the driving field. Having a well-defined OAM, each harmonic is emitted as an EUV femtosecond vortex pulse. However, the order-dependent OAM across the harmonic comb precludes the synthesis of attosecond vortex pulses. Here we demonstrate a method for generating attosecond vortex pulse trains, i.e., a succession of attosecond pulses with a helical wavefront, resulting from the coherent superposition of a comb of EUV high-order harmonics with the same OAM. By driving HHG with a polarization tilt-angle fork grating, two spatially separated circularly polarized high-order harmonic beams with order-independent OAM are created. Our work opens the route towards attosecond-resolved light-matter interactions with two extra degrees of freedom, spin and OAM, which are particularly interesting for probing chiral systems and magnetic materials.

Ultra-fast light repair, ultrasensitive, large strain detection range PDA@RGO/EVA composites fiber flexible strain sensor

Ultra-fast light repair, ultrasensitive, large strain detection range PDA@RGO/EVA composites fiber flexible strain sensor

A right-handed molecule is coaxed to behave like a left-handed one

Electrons in a chiral molecule, if excited by ultrafast light pulses, can give the molecule entirely different properties.

Direct space–time manipulation mechanism for spatio-temporal coupling of ultrafast light field

Traditionally, manipulation of spatiotemporal coupling (STC) of the ultrafast light fields can be actualized in the space-spectrum domain with some 4-f pulse shapers, which suffers usually from some limitations, such as spectral/pixel resolution and information crosstalk associated with the 4-f pulse shapers. This work introduces a novel mechanism for direct space-time manipulation of ultrafast light fields to overcome the limitations. This mechanism combines a space-dependent time delay with some spatial geometrical transformations, which has been experimentally proved by generating a high-quality STC light field, called light spring (LS). The LS, owing a broad topological charge bandwidth of 11.5 and a tunable central topological charge from 2 to −11, can propagate with a stable spatiotemporal intensity structure from near to far fields. This achievement implies the mechanism provides an efficient way to generate complex STC light fields, such as LS with potential applications in information encryption, optical communication, and laser-plasma acceleration.

Perspectives: Light Control of Magnetism and Device Development.

Accurately controlling magnetic and spin states presents a significant challenge in spintronics, especially as demands for higher data storage density and increased processing speeds grow. Approaches such as light control are gradually supplanting traditional magnetic field methods. Traditionally, the modulation of magnetism was predominantly achieved through polarized light with the help of ultrafast light technologies. With the growing demand for energy efficiency and multifunctionality in spintronic devices, integrating photovoltaic materials into magnetoelectric systems has introduced more physical effects. This development suggests that sunlight will play an increasingly pivotal role in manipulating spin orientation in the future. This review introduces and concludes the influence of various light types on magnetism, exploring mechanisms such as magneto-optical (MO) effects, light-induced magnetic phase transitions, and spin photovoltaic effects. This review briefly summarizes recent advancements in the light control of magnetism, especially sunlight, and their potential applications, providing an optimistic perspective on future research directions in this area.

Light-driven nanoscale vectorial currents.

Controlled charge flows are fundamental to many areas of science and technology, serving as carriers of energy and information, as probes of material properties and dynamics1 and as a means of revealing2,3 or even inducing4,5 broken symmetries. Emerging methods for light-based current control5-16 offer particularly promising routes beyond the speed and adaptability limitations of conventional voltage-driven systems. However, optical generation and manipulation of currents at nanometre spatial scales remains a basic challenge and a crucial step towards scalable optoelectronic systems for microelectronics and information science. Here we introduce vectorial optoelectronic metasurfaces in which ultrafast light pulses induce local directional charge flows around symmetry-broken plasmonic nanostructures, with tunable responses and arbitrary patterning down to subdiffractive nanometre scales. Local symmetries and vectorial currents are revealed by polarization-dependent and wavelength-sensitive electrical readout and terahertz (THz) emission, whereas spatially tailored global currents are demonstrated in the direct generation of elusive broadband THz vector beams17. We show that, in graphene, a detailed interplay between electrodynamic, thermodynamic and hydrodynamic degrees of freedom gives rise to rapidly evolving nanoscale driving forces and charge flows under the extremely spatially and temporally localized excitation. These results set the stage for versatile patterning and optical control over nanoscale currents in materials diagnostics, THz spectroscopies, nanomagnetism and ultrafast information processing.

Feasibility verification of ultrafast FEL generation experimental scheme based on SXFEL

The photon energy in the soft X-ray range corresponds to the fundamental absorption edges of matter. Ultrashort X-ray pulses can be used to observe the breaking of chemical bonds in biochemical reactions and capture the transfer process of electrons in ultrafast physical phenomena. In this paper, the feasibility of ESASE experiments on Shanghai Soft X-ray Free Electron Laser Facility (SXFEL) is theoretically verified. The results show that the ESASE scheme can produce ultrafast light pulses on the order of attosecond, with a peak power of 450 MW. At the same time, the simulation results in this paper verify the feasibility of chirped enhanced SASE schenme based on SXFEL. The results show that compared with the ESASE scheme, the power of the radiation pulse can be greatly improved by this scheme. A relatively low energy electron beam (1.5 GeV) was used to generate about 40 GW of radiation, and the length of the radiation pulse was significantly shortened.

Annealing-free fabrication of high-quality indium tin oxide films for free-carrier-based hybrid metal–semiconductor nanophotonics

Recent discoveries have revealed that indium tin oxide (ITO), due to the presence of an epsilon-near-zero (ENZ) point and suitable carrier concentration and mobility, can be used to modulate the refractive index, confine fields in the nanoscale, enhance nonlinear effects, achieve ultrafast light switching or to construct so-called time-varying media. While this potential positions ITO as a key material for future nanophotonic devices, producing ITO films with precisely engineered properties remains a significant challenge. Especially when the device’s complex geometry or incorporated materials require the fabrication process to be conducted at substrate temperatures below 100 °C and without any post-annealing treatment. Here we present a comprehensive study on the low-temperature deposition of 70 nm thick ITO films using an e-beam PVD system. The nanolayers evaporated under different conditions were characterized by SEM and AFM microscopy, Hall effect measurement system as well as spectroscopic ellipsometry. We discuss the factors influencing the optical, electrical, and morphological properties of ITO films. We show that smooth nanolayers of similar quality to annealed samples can be obtained at 80 °C by controlling the oxygen plasma parameters, and the ENZ wavelength can be tuned throughout the NIR spectral range. Finally, we show that using the proposed methodology, we fabricated ITO films with resistivity as low as 5.2 × 10–4 Ω cm, smooth surface with RMS < 1 nm, high carrier concentration reaching 1.2 × 1021 cm−3 and high transmittance (85%) in the Vis/NIR spectrum.

Dispersion-dependent superluminal propagation and photon drag in GaAs/AlGaAs quantum dot molecule

Controlling the basic properties of light such as polarization, angular momentum, and speed through various optical media leads to interesting phenomena in various fields such as quantum optics, photonics, plasmonics, and quantum information technology. Few of the phenomenon include, for instance, light storage, ultra-slow and ultra-fast propagation, optical transparency, and quantum entanglement via quantum coherence or superposition. The understanding of these phenomenon of milestone implications paved the way for the invention of novel classical and quantum technology. In this article, we present detailed theoretical and numerical demonstration on the dispersion-dependent phenomenon of slow and fast light, optically induced transparency, and photon drag. For this, we employ a three-level scheme of an ensemble of GaAs\\AlGaAs double quantum dot molecule in cascade configuration. In particular, by tuning the medium dispersion we predict interesting interplay between ultra-slow and ultra-fast light propagation with vanishing, infinite, ultra-positive and ultra-negative group velocities, and optical-assisted quantum transparency. The quantum coherence of the dressed states modifies considerably the optical properties of the system in view of dispersion, absorption, electron energy loss, transmission, and photon drag. An interesting interplay between ultra-slow to ultra-fast propagation is found by tuning various optical parameters. The calculated superluminal value of group velocity is m s−1. We also predict light propagation at vanishing and infinite group velocity. The calculated light drag is rad/m.

A random optical parametric oscillator

Synchronously pumped optical parametric oscillators (OPOs) provide ultra-fast light pulses at tuneable wavelengths. Their primary drawback is the need for precise cavity control (temperature and length), with flexibility issues such as fixed repetition rates and marginally tuneable pulse widths. Targeting a simpler and versatile OPO, we explore the inherent disorder of the refractive index in single-mode fibres realising the first random OPO – the parametric analogous of random lasers. This novel approach uses modulation instability (χ(3) non-linearity) for parametric amplification and Rayleigh scattering for feedback. The pulsed system exhibits high inter-pulse coherence (coherence time of ~0.4 ms), offering adjustable repetition rates (16.6–2000 kHz) and pulse widths (0.69–47.9 ns). Moreover, it operates continuously without temperature control loops, resulting in a robust and flexible device, which would find direct application in LiDAR technology. This work sets the stage for future random OPOs using different parametric amplification mechanisms.

Electrically-driven ultrafast out-of-equilibrium light emission from hot electrons in suspended graphene/hBN heterostructures

Nanoscale light sources with high speed of electrical modulation and low energy consumption are key components for nanophotonics and optoelectronics. The record-high carrier mobility and ultrafast carrier dynamics of graphene make it promising as an atomically thin light emitter, which can be further integrated into arbitrary platforms by van der Waals forces. However, due to the zero bandgap, graphene is difficult to emit light through the interband recombination of carriers like conventional semiconductors. Here, we demonstrate ultrafast thermal light emitters based on suspended graphene/hexagonal boron nitride (Gr/hBN) heterostructures. Electrons in biased graphene are significantly heated up to 2800 K at modest electric fields, emitting bright photons from the near-infrared to the visible spectral range. By eliminating the heat dissipation channel of the substrate, the radiation efficiency of the suspended Gr/hBN device is about two orders of magnitude greater than that of graphene devices supported on SiO2 or hBN. We further demonstrate that hot electrons and low-energy acoustic phonons in graphene are weakly coupled to each other and are not in full thermal equilibrium. Direct cooling of high-temperature hot electrons to low-temperature acoustic phonons is enabled by the significant near-field heat transfer at the highly localized Gr/hBN interface, resulting in ultrafast thermal emission with up to 1 GHz bandwidth under electrical excitation. It is found that suspending the Gr/hBN heterostructures on the SiO2 trenches significantly modifies the light emission due to the formation of the optical cavity and showed a ∼440% enhancement in intensity at the peak wavelength of 940 nm compared to the black-body thermal radiation. The demonstration of electrically driven ultrafast light emission from suspended Gr/hBN heterostructures sheds the light on applications of graphene heterostructures in photonic integrated circuits, such as broadband light sources and ultrafast thermo-optic phase modulators.

Revealing the buildup of dynamic mode-switchable frequency-shifted feedback laser based on photon–phonon interaction

Revealing the buildup of dynamic mode-switchable frequency-shifted feedback laser based on photon–phonon interaction

Generation of ultrafast tunable super-oscillation light fields

Generation of ultrafast tunable super-oscillation light fields