Photoelectron spectroscopy in intense laser fields has proven to be a powerful tool for providing detailed insights into molecular structure. The ionizing molecular orbital, however, has not been reconstructed from the photoelectron spectra, because its phase information is difficult to access. Here, we propose a method to retrieve the phase information of the ionizing molecular orbital. By analyzing the interference pattern in the photoelectron spectrum, the weighted coefficients and the relative phases of the constituent atomic orbitals for a molecular orbital can be extracted. With this information, we reconstruct the highest occupied molecular orbital of N2. Our work provides a reliable and straightforward approach for reconstructing molecular orbitals with the photoelectron spectroscopy.

Atomic time scale imaging, opening a new era for studying dynamics in microcosmos, is presently attracting immense research interest on the global level due to its powerful ability. On the atom level, physics, chemistry, and biology are identical for researching atom motion and atomic state change. The light possesses twoness, the information carrier and the research resource. The most fundamental principle of this imaging is that light records the event-modulated light field by itself, so-called all-optical imaging. This paper can answer what is the essential standard to develop and evaluate atomic time scale imaging, what is the optimal imaging system, and what are the typical techniques to implement this imaging, up to now. At present, the best record in the experiment, made by multistage optical parametric amplification (MOPA), is realizing 50-fs resolved optical imaging with a spatial resolution of ~83 lp/mm at an effective framing rate of 15 × 1012 fps for recording an ultrafast optical lattice with its rotating speed up to 13.5 × 1012 rad/s.

Surface plasmon resonances (SPRs) are often regarded as the collective oscillations of charge carriers localized at the dielectric–metal interface that display an ultrafast response upon light excitation. The recent developments in the fabrication and characterization of plasmonic nanostructures have stimulated continuous effects in the search for their potential applications in the photonic fields. Concentrating on the role of plasmonics in photonics, this review covers recent advances in ultrafast plasmonic materials with a prime focus on all-optical switching. Fundamental phenomena of plasmonic light–matter interaction and plasmon dynamics are discussed by elaborating on the ultrafast processes unraveled by both experimental and theoretical methods, along with a comprehensive illustration of leveraging ultrafast plasmonics for all-optical switching and pulse laser generation with a focus on device design and performance. This review is concluded with a brief highlight of the current progress and the potential future directions in ultrafast plasmonics.

Quantum interference occurs frequently in the interaction of laser radiation with materials, leading to a series of fascinating effects such as lasing without inversion, electromagnetically induced transparency, Fano resonance, etc. Such quantum interference effects are mostly enabled by single-photon resonance with transitions in the matter, regardless of how many optical frequencies are involved. Here, we report on quantum interference driven by multiple photons in the emission spectroscopy of nitrogen ions that are resonantly pumped by ultrafast infrared laser pulses. In the spectral domain, Fano resonance is observed in the emission spectrum, where a laser-assisted dynamic Stark effect creates the continuum. In the time domain, the fast-evolving emission is measured, revealing the nature of free-induction decay arising from quantum radiation and molecular cooperativity. These findings clarify the mechanism of coherent emission of nitrogen ions pumped with mid-infrared pump laser and are found to be universal. The present work opens a route to explore the important role of quantum interference during the interaction of intense laser pulses with materials near multiple photon resonance.

Single-shot 2-dimensional optical imaging of transient phenomena is indispensable for numerous areas of study. Among existing techniques, compressed ultrafast photography (CUP) using a chirped ultrashort pulse as active illumination can acquire nonrepetitive time-evolving events at hundreds of trillions of frames per second. However, the bulky size and conventional configurations limit its reliability and application scopes. Superdispersive metalenses offer a promising solution for an ultracompact design with a stable performance by integrating the functions of a focusing lens and dispersive optical components into a single device. Nevertheless, existing metalens designs, typically optimized for the full visible spectrum with a relatively low spectral resolution, cannot be readily applied to active-illumination CUP. To address these limitations, here, we propose single-shot compressed ultracompact femtophotography (CUF) that synergically combines the fields of nanophotonics, optical imaging, compressed sensing, and deep learning. We develop the theory of CUF’s data acquisition composed of temporal–spectral mapping, spatial encoding, temporal shearing, and spatiotemporal integration. We also develop CUF’s image reconstruction via deep learning. Moreover, we design and evaluate CUF’s crucial components—a static binary transmissive mask, a superdispersive metalens, and a 2-dimensional sensor. Finally, using numerical simulations, CUF’s feasibility is verified using 2 synthetic scenes: an ultrafast beam sweeping across a surface and the propagation of a terahertz Cherenkov wave.

Advances in producing tailored ultrashort laser pulses have enabled the generation and control of molecular dissociative Rydberg excitation along the polarization axis of the laser field. Here, we exploit the orthogonally polarized two-color femtosecond laser fields and achieve an unprecedented two-dimensional control of Rydberg fragment emission in the dissociative frustrated single ionization of oxygen. The Rydberg fragments are collected over the 4π solid angle, whose momentum distribution is manifested in a characteristic four-lobe pattern. Through precise scanning of the relative phase of the orthogonal two-color laser fields, we demonstrate control over asymmetric directional emission of the Rydberg fragments. Our experimental findings are well supported by classical trajectory Monte Carlo simulations, which suggest an efficient emission control achieved through the manipulation of charge localization upon ionization.

The field of ultrafast science is dependent on either ultrashort laser pulse technology or ultrafast passive detection. While there exists a plethora of sub-picosecond laser pulse solutions, streak cameras are singular in providing sub-picosecond passive imaging capabilities. Therefore, their use in fields ranging from medicine to physics is prevalent. Streak cameras attain such temporal resolutions by converting signal photons to electrons. However, the Coulomb repulsion force spreads these electrons spatiotemporally aggravating streak cameras’ temporal resolution and dynamic range—an effect that increases in severity in ultrafast applications where electrons are generated nearly instantaneously. While many electro-optical solutions have been proposed and successfully implemented, this issue remains as a challenge for all sub-picosecond streak camera technology. Instead of resorting to electro-optical solutions, in this work, we present an all-optical approach based on the combination of photon tagging and spatial lock-in detection with a technique called periodic shadowing—that is directly applicable to all generations of streak cameras. We have demonstrated that this accessible all-optical solution, consisting of a single externally applied optical component, results in (a) a >3× improvement in dynamic range, (b) a 25% increase in temporal resolution, and (c) a reduction of background noise levels by a factor of 50, which, when combined, allows for a markedly improved accuracy in the measurement of ultrafast signals.

Singlet fission (SF) is a spin-conserving process converting 1 singlet exciton into 2 triplet excitons. This exciton multiplication mechanism offers an attractive route to solar cells that circumvent the single-junction Shockley–Queisser limit. However, it remains unclear how intermolecular coupling, which is subject to the aggregation extent in thin-film morphology, controls SF pathways and dynamics. The prototype molecule 6,13-bis(triisopropylsilylethynyl)-pentacene (TIPS-pentacene) has been extensively studied to investigate SF mechanisms. However, previous literature reports have presented divergent SF mechanisms and pathways in TIPS-pentacene films. In this study, solvent vapor annealing treatment is used to deliberately adjust the aggregation extent in TIPS-pentacene films. This enables us to reproduce various SF pathways reported in the literature under the same experimental conditions, with the only variation being the level of aggregation. These results shed light on the crucial role that molecular aggregation plays in modulating both the SF mechanism and pathway and reconciles the previously contentious SF mechanisms and pathways reported in TIPS-pentacene films. Our study offers substantial insights into the understanding of the SF mechanism and provides a potential avenue for future control of SF pathways in accordance with specific application requirements.

Dynamic phenomena occurring on the ultrafast time scales are inherently difficult to image. While pump–probe techniques have been used for decades, probing nonrepeatable phenomena precludes this form of imaging. Additionally, many ultrafast phenomena, such as electron dynamics, exhibit low amplitude contrast in the optical wavelength range and thus require quantitative phase imaging. To better understand the underlying physics involved in a plethora of ultrafast phenomena, advanced imaging techniques must be developed to observe single events at an ultrafast time scale. Here, we present, to the best of our knowledge, the first ptychographic imaging system capable of observing ultrafast dynamics from a single event. We demonstrate ultrafast dynamic imaging by observing the conduction band electron population from a 2-photon absorption event in ZnSe pumped by a single femtosecond pulse. We verify experimental observations by comparing them to numeric solutions of a nonlinear envelope equation. Our imaging method represents a major step forward in ultrafast imaging, bringing the capabilities of ptychography to the ultrafast regime.

Intense terahertz (THz) radiation in free space has immense potential for regulating material state, accelerating electrons, producing biological effects, and so on. However, the high cost and challenges involved in constructing strong-field THz sources have limited their developments, making it difficult for the potential applications of strong-field THz radiation to be widely adopted. Spintronic THz emitters (STEs) with numerous merits such as high efficiency, ultrabroadband, ease of integration, and low cost have become ubiquitous, but the majority of these emitters require stable operation in the presence of external magnets, limiting their applications, particularly in generating strong fields that necessitate large-sized samples. Here, we demonstrate the feasibility of generating strong-field THz radiation in 4-inch antiferromagnetic material–ferromagnetic metal (IrMn3 [2 nm]/Co20Fe60B20 [2 nm]/W [2 nm]) without external magnetic field driving. Under the excitation of a Ti:sapphire femtosecond laser amplifier with a 35-fs pulse duration and a 1-kHz repetition rate, we obtain strong-field THz radiation from our STEs with a pulse duration of ~110 fs, and a spectrum covering up to ~10 THz. Further scaling up the pump laser energy up to 55 mJ with a pulse duration of ~20 fs and a repetition rate of 100 Hz provided by the Synergetic Extreme Condition User Facility, the radiated THz electric field strength from the external-magnetic-free 4-inch STEs can exceed 242 kV/cm with a pulse duration of ~230 fs, a spectrum covering up to ~14 THz, and a single pulse energy of 8.6 nJ measured by a calibrated pyroelectric detector. Our demonstrated external-magnetic-field-free high-field STEs have some unique applications such as producing sub-cycle ultrashort strong THz fields in huge size emitters under the excitation of high-energy light sources, accelerating the development of THz science and applications.

Soliton molecules in optical resonators have attracted remarkable attention in nonlinear dynamics, driven by their compelling analogies with matter molecules. So far, while extensive research has been conducted on their generation, pulsations, and dissociation behaviors, the investigation of their quasi-periodic dynamics has been relatively limited. Here, we present a systematic exploration of the quasi-periodic dynamics of soliton molecules using advanced balanced optical cross-correlation techniques. The incommensurable quasi-period bifurcations constituted of cascaded Hopf bifurcations are found, providing an unambiguous pathway toward chaotic soliton molecules. The chaotic intramolecular dynamics are analyzed by time series, radio frequency spectra, phase portraits, and Lyapunov exponent analysis. In addition, we reveal an intrinsic frequency entrainment phenomenon experimentally. Such frequency entrainment provides a novel perspective on synchronization in optical resonators, encompassing the competition and interaction of oscillations across multiple temporal scales. Our experimental findings offer clear proof that the gain dynamics serve as the origin of the binding forces between solitons within the molecule, which are well supported by the numerical simulations. By advancing the understanding of sub-femtosecond resolved quasi-period dynamics of optical soliton molecules, this study contributes to the broader field of complex nonlinear dynamics, paving the way for future explorations into the intricate behaviors of solitons within optical resonators and relevant fields.

High-quality ultrafast light sources are critical for developing advanced time- and angle-resolved photoemission spectroscopy (TrARPES). While the application of high harmonic generation (HHG) light sources in TrARPES has increased substantially over the past decade, the optimization of the HHG probe beam size and selective control of the light polarization, which are important for TrARPES measurements, have been rarely explored. In this work, we report the implementation of high-quality HHG probe source with an optimum beam size down to 57 μm × 90 μm and selective light polarization control, together with mid-infrared (MIR) pumping source for TrARPES measurements using a 10-kHz amplifier laser. The selective polarization control of the HHG probe source allows to enhance bands with different orbital contributions or symmetries, as demonstrated by experimental data measured on a few representative transition metal dichalcogenide materials as well as topological insulator Bi2Se3. Furthermore, by combining the HHG probe source with MIR pumping at 2-μm wavelength, TrARPES on a bilayer graphene shows a time resolution of 140 fs, allowing to distinguish 2 different relaxation processes in graphene. Such high-quality HHG probe source together with the MIR pumping expands the capability of TrARPES in revealing the ultrafast dynamics and light-induced emerging phenomena in quantum materials.

Optical control of magnons in two-dimensional (2D) materials promises new functionalities for spintronics and magnonics in atomically thin devices. Here, we report control of magnon dynamics, using laser polarization, in a ferromagnetic van der Waals (vdW) material, Fe3.6Co1.4GeTe2. The magnon amplitude, frequency, and lifetime are controlled and monitored by time-resolved pump-probe spectroscopy. We show substantial (over 25%) and continuous modulation of magnon dynamics as a function of incident laser polarization. Our results suggest that the modification of the effective demagnetization field and magnetic anisotropy by the pump laser pulses with different polarizations is due to anisotropic optical absorption. This implies that pump laser pulses modify the local spin environment, which enables the launch of magnons with tunable dynamics. Our first-principles calculations confirm the anisotropic optical absorption of different crystal orientations. Our findings suggest a new route for the development of opto-spintronic or opto-magnonic devices.

With the development of conformable photonic platforms, particularly those that could be interfaced with the human body or integrated into wearable technology, there is an ever-increasing need for mechanically flexible optical photonic elements in soft materials. Here, we realize mechanically flexible liquid crystal (LC) waveguides using a combination of ultrafast direct laser writing and ultraviolet (UV) photo-polymerization. Results are presented that demonstrate that these laser-written waveguides can be either electrically switchable (by omitting the bulk UV polymerization step) or mechanically flexible. Characteristics of the waveguide are investigated for different fabrication conditions and geometrical configurations, including the dimensions of the waveguide and laser writing power. Our findings reveal that smaller waveguide geometries result in reduced intensity attenuation. Specifically, for a 10-μm-wide laser-written channel in a 14-μm-thick LC layer, a loss factor of -1.8 dB/mm at λ = 650 nm was observed. Following the UV polymerization step and subsequent delamination of the glass substrates, we demonstrate a free-standing flexible LC waveguide, which retains waveguide functionality even when bent, making it potentially suitable for on-skin sensors and other photonic devices that could interface with the human body. For the flexible LC waveguides fabricated in this study, the loss in a straight waveguide with a cross-sectional area of 20 μm × 20 μm was recorded to be -0.2 dB/mm. These results highlight the promising potential of electrically responsive and mechanically moldable optical waveguides using laser writing and UV-assisted polymer network formation.

We investigate the strong field ionization of argon using counter-rotating 2-color dual-elliptical phase-of-phase spectroscopy. We perform a multidimensional control experiment over the 2-color dual-elliptical laser field to modulate the photoelectron momentum spectra generated under strong fields. Incorporating the classical trajectory Monte Carlo calculation, we study the behavior of the relative phase contrast and the phase of phase and clarify how the enhancement of temporal precision in angular resolution measurements is achieved when the relative phase corresponds to the frequency of 3ω. Our results uncover the potential for using quantitatively controlled 2-color elliptical fields to realize precise probes of the attosecond ionization and scattering dynamics.

Phase control of random lasing processes has been a challenge both in physics and in the device/materials design. Although conventional saturable absorbers can be integrated with random lasers to conceive mode-locking scheme, low intensity and random directions of the lasing radiation reduce largely the possibility. In such considerations, we put forth a new mode-locking mechanism, which is defined as cascaded absorption and stimulated emission (CASE), and have it achieved in multicrystalline microdisk structures of a hybrid perovskite. This scheme applies only to lasing materials with strong overlap between the absorption and emission spectra. In this work, we employed 2-photon pumping at 800 nm with a pulse duration of about 150 fs to realize phase-locked random lasing in MAPbBr3 microdisks in donut shapes, which are produced by micro-imprinting using a flexibly transferred template of tricyclo[5.2.1.02,6] decanedimethanol diacrylate. The phase-locking performance is identified by the narrow-band lasing lines with equal separations. The constant phase shift for initializing phase locking is determined by the internal conversion lifetime in the MAPbBr3 molecules. Two-photon pumping enables large penetration depth into the microdisks and consequently large numbers of phase-locked lasing modes, producing much narrowed and high-contrasted spectral lines. Lasing lines with a bandwidth as narrow as 0.26 to 0.3 nm and an equal separation ranging from 1.7 to 4.8 nm have been achieved for different microdisk schemes. These results imply marked progress in new random lasing physics and potential applications in ultrafast laser technology.

Quantum interference (QI) has been widely studied in advanced materials and can be exploited to control the nonlinear response by varying the relative phase between the incident optical pulses. However, the contribution of the coherent injected photocurrent by QI from the indirect gap materials is still unclear because of the much weaker phonon-assisted absorption compared with that from the direct gap materials. Here, we investigate the coherent injected photocurrent in mono- and multilayer MoS2 with thickness at the nanometer scale under 2-color light excitation by detecting the generated coherent terahertz (THz) wave. We observe that the THz radiation can be controlled by the relative phase. Besides, we obtain similar experimental results of the THz wave generation from mono- and multilayer MoS2 when we change the relative polarization angle between 𝜔 and 2𝜔 pulses, in comparison to the case of direct gap materials. Thus, these experimental results further verify that, in multilayer MoS2 with an indirect gap, QI in the direct gap region is the dominant process for the THz wave generation. Furthermore, we demonstrate that QI can be a more effective mechanism to induce THz radiation than optical rectification under single-color light excitation. This study enhances the understanding of QI in indirect gap materials and highlights the potential of 2-color light excitation for investigating third-order nonlinear processes in advanced materials.

Exploiting the infinite-order continuous dynamical rotational symmetry of circularly or elliptically polarized classical light pulses, we establish the conservation law between the angular momentum and energy in strong-field ionization that is applicable at the subcycle level. We illustrate the conservation law through the correlated spectrum of angular momentum and energy of photoelectrons, both at the tunnel exit and in the asymptotic region. Moreover, we propose a protocol based on electron vortices to directly visualize the existence of the subcycle conservation law. Our work paves the pathway toward a deeper understanding of fundamental light–matter interactions on the subcycle scale.

The blue light using flavin (BLUF) domain is one of nature’s smallest photoswitching protein domains, yet a cross-species photoactivation mechanism is lacking. Its photoactivation involves an intricate bidirectional proton-coupled electron transfer (PCET) reaction; however, the key reverse PCET route remains largely elusive, with its elementary steps undissected. Here, we resolved the light-state photoreaction cycles of the BLUF domains in 3 species, i.e., AppA from Rhodobacter sphaeroides, OaPAC from Oscillatoria acuminata, and SyPixD from Synechocystis sp. PCC6803, with a unified kinetic model. Using mutant design and femtosecond spectroscopy, we captured the spectroscopic snapshots of a key proton-relay intermediate in all species, revealing that the light-state photoreaction cycle consists of 4 elementary steps including a forward concerted electron-proton transfer (CEPT), a 2-step proton rocking, and a reverse CEPT. We emphasize that the last reverse CEPT step (1.5 to 3.7 ps) is shared by both the light-state and dark-state photocycles and is essential to the photoswitching functionality.

Coupled nuclear and electronic dynamics within a molecule are key to understanding a broad range of fundamental physical and chemical processes. Although probing the coupled vibrational and electronic dynamics was demonstrated, it has so far been challenging to observe the coupling interactions between the rotational and electronic degrees of freedom. Here, we report the first observation of Coriolis coupling, a coupling interaction between nuclear rotational angular momentum and electronic axial angular momentum, during laser-induced molecular fragmentation by tracing the electronic structure of a dissociating O2+ molecule. We observe that the electron density changes its shape from that of a molecular σ orbital to a nearly isotropic shape as the internuclear distance goes up to ∼20 Å, which results from the transition between nearly degenerate electronic states associated with different rotational angular momenta. Our experiment demonstrates that the breaking of a chemical bond does not occur suddenly during molecular dissociation. Instead, it lasts for a long time of several hundred femtoseconds due to the Coriolis coupling interaction. Our experiment can be extended to complicated molecules, holding the potential of revealing yet unobserved electron–nuclear coupling interactions during ultrafast processes.

The exploration of optical and photonic phenomena, particularly the modulation of pulse signals and the ultrafast control of light fields at extreme temporal and spatial scales, substantially enhances our understanding of light–matter interactions and broadens the scope of potential applications inspired by metamaterials and metasurfaces. In this perspective, we highlight advancements in ultrafast metaphotonics by introducing ultrafast pulse shaping and control using metadevices. We begin with a detailed exposition of the principles of metasurfaces and evaluate their role in manipulating light fields in high-frequency and terahertz bands, emphasizing the importance of metasurfaces in ultrafast optics. We then present several methods for controlling the output response of metadevices using external physical fields or phase-change materials to achieve active metadevices. Finally, we anticipate the prospects of this field in terms of fundamental research and practical applications. The integration of these 2 disciplines will drive vibrant developments across multiple fields, including biology, chemistry, and materials science.

Femtosecond laser filamentation has recently emerged as a promising technique to actively create a channel through clouds and fog, thereby providing a revolutionary opportunity to overcome the obstacle of fog-induced attenuation for free-space optical communication (FSOC) in atmosphere. However, the underlying physics remains elusive, which is critical for optimizing time window and efficiency of guiding light in this channel. In this work, the time evolution of the filament-induced channel is investigated under various laser pulse energies and repetition rates. The combined diffusion model is built to reveal the contributions of gas molecules and aerosol droplets in competition of guiding and defocusing effect of the filament-induced channel. The related findings can deepen our understanding on the underlying physics of the air channel induced by the filament, provide insight into the optimizing time window and efficiency of guiding light, and potentially contribute to the improvement of filament-assisted FSOC.

Raman scattering spectroscopy is widely used as an analytical technique in various fields, but its measurement process tends to be slow due to the low scattering cross-section. In the last decade, various broadband coherent Raman scattering spectroscopy techniques have been developed to address this limitation, achieving a measurement rate of 500 kSpectra/s. Here, we present a substantially increased measurement rate of 50 MSpectra/s, which is 100 times higher than the previous state-of-the-art, by developing time-stretch coherent Raman scattering spectroscopy. Our newly developed system, based on a mode-locked Yb fiber laser, enables highly efficient broadband excitation of molecular vibrations via impulsive stimulated Raman scattering with an ultrashort femtosecond pulse and sensitive time-stretch detection with a picosecond probe pulse at a high repetition rate of the laser. As a proof-of-concept demonstration, we measure broadband coherent Stokes Raman scattering spectra of organic compounds covering the molecular fingerprint region from 200 to 1,200 cm-1. This high-speed broadband vibrational spectroscopy technique holds promise for unprecedented measurements of sub-microsecond dynamics of irreversible phenomena and extremely high-throughput measurements.

Valleytronic devices based on all-optical ultrafast control are expected to increase the speed of information processing to petahertz and serve a new generation of quantum computers. However, the current difficulty in realizing this vision is the lack of a nondamaging means suitable for ultrafast lasers. We propose a robust scheme to control the valley polarization of monolayer materials, achieved through the quantum interference between 1- and 2-photon transition pathways. The scheme reveals that conventional circularly polarized light is unnecessary for resonantly induced valley polarization and, instead, only a parallel-polarized 2-color field is required. The interference dynamics enables the switch of valley to be manipulated within few femtoseconds without the necessity for extremely strong or single-cycle pulses. The disclosure of this interference scheme enables repetitive operations in valley devices for signal processing at petahertz clock rates without causing material damage. It sheds light on the practical manufacture of high-speed valleytronic devices.

Interferometric measurements of high harmonics induced by multiple laser fields represent a burgeoning field of research, offering prospects for optimized harmonic yield and enabling time- and space-resolved nonlinear spectroscopy. While most investigations have focused on controlling the time delay between pulses, our study introduces a novel approach. By manipulating an additional parameter—the phase difference between the fields—we unveil detailed insights into the physical mechanisms governing the ultrafast processes underlying high harmonic generation. Leveraging high harmonic 2-dimensional interferograms, our method facilitates the streamlined analysis of attosecond electron dynamics in complex molecules and solids, marking an important advancement in this rapidly evolving field.