Unraveling the exact nature of nonequilibrium and correlated interactions is paramount for continued progress in many areas of condensed matter science. Such insight is a prerequisite to develop an engineered approach for smart materials with targeted properties designed to address standing needs such as efficient light harvesting, energy storage, or information processing. For this goal, it is critical to unravel the dynamics of the energy conversion processes between carriers in the earliest time scales of the excitation dynamics. We discuss the implementation and benefits of attosecond soft x-ray core-level spectroscopy up to photon energies of 600 eV for measurements in solid-state systems. In particular, we examine how the pairing between coherent spectral coverage and temporal resolution provides a powerful new insight into the quantum dynamic interactions that determine the macroscopic electronic and optical response. We highlight the different building blocks of the methodology and point out the important aspects for its application from condensed matter studies to materials as thin as 25 nm. Furthermore, we discuss the technological developments in the field of tabletop attosecond soft x-ray sources with time-resolved measurements at the near and extended edge simultaneously and investigate the exciting prospective of extending such technique to the study of 2-dimensional materials.

The gamma-ray vortex burst in the nonlinear Thomson scattering when the laser wakefield accelerated electron bunch collides with an ultra-intense Laguerre–Gaussian laser that was reflected from the refocusing spiral plasma mirror. The orbit angular momentum of the scattering laser would be transferred to the gamma radiation through the scattering process. The 3-dimensional particle-in-cell simulations gave the electron dynamics in the scattering, which determines the characteristics of the vortical radiation. The radiation calculation results illustrated the burst of gamma-ray vortex and surprisingly revealed the radiation pattern distortion phenomenon due to the nonlinear effect. This scheme can not only simplify the experimental setup for the generation of twisted radiation but also boost the yield of vortical gamma photons. The peak brightness of the gamma-ray vortex was estimated to be 1 × 1022 photons/s/mm2/mrad2/0.1% BW at 1 MeV, which might pave the way for the researches on angular momentum-related nuclear physics.

Manganese dioxide (MnO2) is a widely used and well-studied 3-dimensional (3D) transition metal oxide, which has advantages in ultrafast optics due to large specific surface area, narrow bandgap, multiple pores, superior electron transfer capability, and a wide range of light absorption. However, few studies have considered its excellent performance in ultrafast photonics. γ-MnO2 photonics devices were fabricated based on a special dual-core, pair-hole fiber (DCPHF) carrier and applied in ultrafast optics fields for the first time. The results show that the soliton molecule with tunable temporal separation (1.84 to 2.7 ps) and 600-MHz harmonic solitons are achieved in the experiment. The result proves that this kind of photonics device has good applications in ultrafast lasers, high-performance sensors, fiber optical communications, etc., which can help expand the prospect of combining 3D materials with novel fiber for ultrafast optics device technology.

We report on an ultrafast nonequilibrium phase transition with a strikingly long-lived martensitic anomaly driven by above-threshold single-cycle terahertz pulses with a peak field of more than 1 MV/cm. A nonthermal, terahertz-induced depletion of low-frequency conductivity in Nb3Sn indicates increased gap splitting of high-energy Γ12 bands by removal of their degeneracies, which induces the martensitic phase above their equilibrium transition temperature. In contrast, optical pumping leads to a Γ12 gap thermal melting. Such light-induced nonequilibrium martensitic phase exhibits a substantially enhanced critical temperature up to ∼100 K, i.e., more than twice the equilibrium temperature, and can be stabilized beyond technologically relevant, nanosecond time scales. Together with first-principle simulations, we identify a compelling terahertz tuning mechanism of structural order via Γ12 phonons to achieve the ultrafast phase transition to a metastable electronic state out of equilibrium at high temperatures far exceeding those for equilibrium states.

Breakdown spectroscopy is a valuable tool for determining elements in solids, liquids, and gases. All materials in the breakdown region can be ionized and dissociated into highly excited fragments and emit characteristic fluorescence spectra. In this sense, the elemental composition of materials can be evaluated by detecting the fluorescence spectrum. This paper reviews the recent developments in laser-induced breakdown spectroscopy. The traditional laser-induced breakdown spectroscopy, filament-induced breakdown spectroscopy, plasma grating, and multidimensional plasma grating-induced breakdown spectroscopy are introduced. There are also some proposals for applications of plasma gratings, such as laser ablation, laser deposition, and laser catalysis of chemical reactions in conjunction with research on the properties of plasma gratings.

The identification of the decay pathway of the nucleobase uracil after being photoexcited by ultraviolet light has been a long-standing problem. Various theoretical models have been proposed but yet to be verified. Here, we propose an experimental scheme to test the theoretical models of gas phase uracil decay mechanism by a combination of ultrafast x-ray spectroscopy, x-ray diffraction, and electron diffraction methods. Incorporating the signatures of multiple probing methods, we demonstrate an approach that can identify the dominant mechanism of the geometric and electronic relaxation of the photoexcited uracil molecule among several candidate models.

An all-fiber Mamyshev oscillator with a single amplification arm is experimentally demonstrated to achieve high-energy and high-average-power ultrafast pulse output, with the initiating of an external seed pulse. In the high-energy operation, a maximum single-pulse energy of 153 nJ is achieved at a repetition rate of 9.77 MHz. After compression with a pair of diffraction gratings, a measured pulse width of 73 fs with a record energy of 122.1 nJ and a peak power of 1.7 MW is obtained. In the high-average-power operation, up to 5th harmonic mode locking of the oscillator is realized via slightly adjusting the output coupling ratio and the cavity length. The achieved maximum output power is 3.4 W at a repetition rate of 44.08 MHz, while the corresponding pulse width is compressed to around ~100 fs. Meanwhile, the system is verified to be operated reliability in both high-energy and -average-power operation regimes through assessing its short- and long-term stabilities. To the best of our knowledge, these are the highest records in pulse energy and average power delivered from a single all-fiber ultrafast laser oscillator with picosecond/femtosecond pulse duration. It is believed that even higher-energy and -average-power ultrafast laser can be realized with the proposed laser scheme through further increasing the core diameter of the all-fiber cavity, providing promising sources for advanced fabrication, biomedical imaging, laser micromachining, and other practical applications, as well as an unprecedented platform for exploring undiscovered nonlinear dynamics.

Optical beams carrying orbital angular momentum (OAM) play an important role in micro-/nanoparticle manipulation and information multiplexing in optical communications. Conventional OAM generation setups require bulky optical elements and are unsuitable for on-chip integration. OAM generators based on metasurfaces can achieve ultracompact designs. However, they generally have limited working bandwidth and require complex designs and multistep time-consuming fabrication processes. In comparison, graphene metalenses based on the diffraction principle have simple designs and can be fabricated by laser nanoprinting in a single step. Here, we demonstrate that a single ultrathin (200 nm) graphene OAM metalens can integrate OAM generation and high-resolution focusing functions in a broad bandwidth, covering the entire visible wavelength region. Broadband graphene OAM metalenses with flexibly controlled topological charges are analytically designed using the detour phase method considering the dispersionless feature of the graphene material and fabricated using ultrafast laser nanoprinting. The experimental results agree well with the theoretical predictions, which demonstrate the accuracy of the design method. The broadband graphene OAM metalenses can find broad applications in miniaturized and integrated photonic devices enabled by OAM beams.

All-dielectric metasurfaces offer low material loss and strong field localization and are, therefore, well suited for ultrathin and compact optical devices for electomagnetic wave manipulation at the nanoscale. All-silicon dielectric metasurfaces, in particular, may additionally offer the desired compatibility with complementary metal-oxide semiconductor technology and, hence, are ideal candidates for large-scale monolithic integration on a photonic chip. However, in conventional silicon microfabrication approaches, the combination of mask photolithography with reactive ion etching usually involves expensive masks and multiple preprocessing stages leading to increased cost and fabrication times. In this work, a single-step lithographical approach is proposed for the realization of all-silicon dielectric resonant metasurfaces that involves femtosecond laser processing of silicon below ablation threshold in combination with subsequent wet chemical etching. The method exploits the different etching rate between laser-modified and untreated regions, enabling large-area fabrication of patterned silicon surfaces in a facile and cost-efficient manufacturing approach. It is presented how two-dimensional silicon micro/nanostructures with controllable features, such as nanocones, can be effectively generated and, as a proof of concept, an all-silicon dielectric metasurface device supporting antiferromagnetic order is experimentally demonstrated.

In the recent decade, single-shot ultrafast optical imaging by active detection, called single-shot active ultrafast optical imaging (SS-AUOI) here, has made great progress, e.g., with a temporal resolution of 50 fs and a frame rate beyond 10 trillion frames per second. Now, it has become indispensable for charactering the nonrepeatable and difficult-to-reproduce events and revealing the underlying physical, chemical, and biological mechanisms. On the basis of this delightful status, we would like to make a review of SS-AUOI. On the basis of a brief introduction of SS-AUOI, our review starts with discussing its characteristics and then focuses on the survey and prospect of SS-AUOI technology.

For terahertz wave applications, tunable and rapid modulation is highly required. When studied by means of optical pump–terahertz probe spectroscopy, single-walled carbon nanotube (SWCNT) thin films demonstrated ultrafast carrier recombination lifetimes with a high relative change in the signal under optical excitation, making them promising candidates for high-speed modulators. Here, combination of SWCNT thin films and stretchable substrates facilitated studies of the SWCNT mechanical properties under strain and enabled the development of a new type of an optomechanical modulator. By applying a certain strain to the SWCNT films, the effective sheet conductance and therefore modulation depth can be fine-tuned to optimize the designed modulator. Modulators exhibited a photoconductivity change of approximately 2 times of magnitude under the strain because of the structural modification in the SWCNT network. Stretching was used to control the terahertz signal with a modulation depth of around 100% without strain and 65% at a high strain operation of 40%. The sensitivity of modulators to beam polarization is also shown, which might also come in handy for the design of a stretchable polarizer. Our results give a fundamental grounding for the design of high-sensitivity stretchable devices based on SWCNT films.

Mid-infrared (MIR) ultra-short pulses with multiple spectral-band coverage and good freedom in spectral and temporal shaping are desired by broad applications such as steering strong-field ionization, investigating bound-electron dynamics, and minimally invasive tissue ablation. However, the existing methods of light transient generation lack freedom in spectral tuning and require sophisticated apparatus for complicated phase and noise control. Here, with both numerical analysis and experimental demonstration, we report the first attempt, to the best our knowledge, at generating MIR pulses with dual-wavelength spectral shaping and exceptional freedom of tunability in both the lasing wavelength and relative spectral amplitudes, based on a relatively simple and compact apparatus compared to traditional pulse synthesizers. The proof-of-concept demonstration in steering the high-harmonic generation in a polycrystalline ZnSe plate is facilitated by dual-wavelength MIR pulses shaped in both spectral and temporal domains, spanning from 5.6 to 11.4 μm, with multi-microjoule pulse energy and hundred- milliwatt average power. Multisets of harmonics corresponding to different fundamental wavelengths are simultaneously generated in the deep ultraviolet region, and both the relative strength of individual harmonics sets and the spectral shapes of harmonics are harnessed with remarkable freedom and flexibility. This work would open new possibilities in exploring femtosecond control of electron dynamics and light–matter interaction in composite molecular systems.

Ultrafast laser filamentation results from the interaction of ultrafast laser with Kerr media. During filamentary propagation, the transparent medium is altered by numerous linear and nonlinear effects of ultrashort laser pulses. Filamentation can cause material modification in solids through laser energy deposition and ionization processes, which creates a new opportunity for ultrafast laser processing of materials when combined with filamentary propagation characteristics, such as intensity champing and long propagation distance. This paper reviews the research on ultrafast laser filamentation in solids for micro- and nano-processing, including the fundamental physics, filamentation characteristics, and applications in solids for ultrafast laser filamentation-induced processing. Additionally highlighted are the difficulties and potential applications for solid-based filamentation-induced processing.

As a new energy source, hydrogen (H2) detection is a hot topic in recent years. Because of the weak absorption characteristic, laser spectroscopy-based H2 detection is challenging. In this paper, a highly sensitive H2 sensor based on light-induced thermoelastic spectroscopy (LITES) technique is demonstrated for the first time. A continuous-wave, distributed feedback diode laser with emission in the 2.1 μm region was adopted as the excitation source to target the strongest H2 absorption line of 4,712.90 cm-1. A Herriott multipass cell with an optical length of 10.1 m was chosen to further improve the H2 absorption. With the feature of processing the raw input data without data preprocessing and extracting the desired features automatically, the robust shallow neural network (SNN) fitting algorithm was brought in to denoise the sensor. For the LITES-based H2 sensor, the concentration response was tested, and an excellent linear response to H2 concentration levels was achieved. A minimum detection limit (MDL) of ~80 ppm was obtained. On the basis of implementation of the H2-LITES sensor, a heterodyne H2-LITES sensor was further constructed to realize a fast measurement of resonance frequency of quartz tuning fork and H2 concentration simultaneously. The resonance frequency can be retrieved in several hundred milliseconds with the measurement accuracy of ±0.2 Hz, and the result of 30,713.76 Hz is exactly same as the experimentally determined value of 30,713.69 Hz. After the SNN algorithm was applied, an MDL of ~45 ppm was achieved for this heterodyne H2-LITES sensor.

Femtosecond dual-comb lasers have revolutionized linear Fourier-domain spectroscopy by offering a rapid motion-free, precise, and accurate measurement mode with easy registration of the combs beat note in the radio frequency domain. Extensions of this technique already found application for nonlinear time-resolved spectroscopy within the energy limit available from sources operating at the full oscillator repetition rate. Here, we present a technique based on time filtering of femtosecond frequency combs by pulse gating in a laser amplifier. This gives the required boost to the pulse energy and provides the flexibility to engineer pairs of arbitrarily delayed wavelength-tunable pulses for pump–probe techniques. Using a dual-channel millijoule amplifier, we demonstrate programmable generation of both extremely short, fs, and extremely long (>ns) interpulse delays. A predetermined arbitrarily chosen interpulse delay can be directly realized in each successive amplifier shot, eliminating the massive waiting time required to alter the delay setting by means of an optomechanical line or an asynchronous scan of 2 free-running oscillators. We confirm the versatility of this delay generation method by measuring χ(2) cross-correlation and χ(3) multicomponent population recovery kinetics.

A universal mechanism of ultrafast 2-electron orbital swap is discovered through 2-photon sequential double ionization of Li. After a 1s electron in Li is ionized by absorbing an extreme ultraviolet photon, the other 2 bound electrons located on 2 different shells have either parallel or antiparallel spin orientations. In the latter case, these 2 electrons are in the superposition of the singlet and triplet states with different energies, forming a quantum beat and giving rise to the 2-electron orbital swap with a period of several hundred attoseconds. The orbital swap mechanism can be used to manipulate the spin polarization of photoelectron pairs by conceiving the attosecond-pump attosecond-probe strategy and thus serves as a knob to control spin-resolved multielectron ultrafast dynamics.

Ultrafast extreme ultraviolet (XUV) transient absorption spectroscopy measures the time- and frequency-dependent light losses after light–matter interactions. In the linear region, the matter response to an XUV light field is usually determined by the complex refractive index n˜. The absorption signal is directly related to the imaginary part of n˜, namely, the absorption index. The real part of n˜ refers to the real refractive index, which describes the chromatic dispersion of an optical material. However, the real refractive index information is usually not available in conventional absorption experiments. Here, we investigate the refractive index line shape in ultrafast XUV transient absorption spectroscopy by using a scheme that the XUV pulse traverses the target gas jet off-center. The jet has a density gradient in the direction perpendicular to the gas injection direction, which induces deflection on the XUV radiation. Our experimental and theoretical results show that the shape of the frequency-dependent XUV deflection spectra reproduces the refractive index line profile. A typical dispersive refractive index line shape is measured for a single-peak absorption; an additional shoulder structure appears for a doublet absorption. Moreover, the refractive index line shape is controlled by introducing a later-arrived near-infrared pulse to modify the phase of the XUV free induction decay, resulting in different XUV deflection spectra. The results promote our understanding of matter-induced absorption and deflection in ultrafast XUV spectroscopy.

Optical logic gates call for materials with giant optical nonlinearity to break the current performance bottleneck. Metal–organic frameworks (MOFs) provide an intriguing route to achieve superior optical nonlinearity benefitting from structural diversity and design flexibility. However, the potential of MOFs for optoelectronics has been largely overlooked and their applications in optical logic have not been exploited. Here, through temporally manipulating the nonlinear optical absorption process in porphyrin-based MOFs, we have successfully developed AND and XOR logic gates with an ultrafast speed approaching 1 THz and an on–off ratio above 90%. On this basis, all-optical information encryption is further demonstrated using transmittance as primary codes, which shows vast prospects in avoiding the disclosure of security information. To the best of our knowledge, this is the first exploration of MOFs for applications in ultrafast optical logic devices and information encryption.

Ultrafast transient microscopy is a key tool to study the photophysical properties of materials in space and time, but current implementations are limited to ≈1-μm fields of view, offering no statistical information for heterogeneous samples. Recently, we demonstrated wide-field transient imaging based on multiplexed off-axis holography. Here, we perform ultrafast microscopy in parallel around a hundred diffraction-limited excitation spots over a ≈60-μm field of view, which not only automatically samples the photophysical heterogeneity of the sample over a large area but can also be used to obtain a 10-fold increase in signal-to-noise ratio by computing an average spot. We apply our microscope to study the carrier diffusion processes in methylammonium lead bromide perovskites. We observe strong diffusion due to the presence of hot carriers during the first picosecond and slower diffusion afterward. We also describe how many-body kinetics can be misleadingly interpreted as strong diffusion at high excitation densities, while at weak excitation, real diffusion is observed. Therefore, the vast increase in sensitivity offered by this technique benefits the study of carrier transport not only by reducing data acquisition times but also by enabling the measurement of the much smaller signals generated at low carrier densities.

Recent advancements in photonic bound states in the continuum (BICs) have opened up exciting new possibilities for the design of optoelectronic devices with improved performance. In this perspective article, we provide an overview of recent progress in photonic BICs based on metamaterials and photonic crystals, focusing on both the underlying physics and their practical applications. The first part of this article introduces 2 different interpretations of BICs, based on far-field interference of multipoles and near-field analysis of topological charges. We then discuss recent research on manipulating the far-field radiation properties of BICs through engineering topological charges. The second part of the article summarizes recent developments in the applications of BICs, including chiral light and vortex beam generation, nonlinear optical frequency conversion, sensors, and nanolasers. Finally, we conclude with a discussion of the potential of photonic BICs to advance terahertz applications in areas such as generation and detection, modulation, sensing, and isolation. We believe that continued research in this area will lead to exciting new advancements in optoelectronics, particularly in the field of terahertz devices.

Recollision physics and attosecond pulse generation meld the precision of optics with collision physics. As a follow-up to our previous work, we reveal a new direction for the study of electronic structure and multielectron dynamics by exploiting the collision-physics nature of recollision. We show experimentally that, by perturbing recollision trajectories with an infrared field, photorecombination time delays can be measured entirely optically using the Cooper minimum in argon as an example. In doing so, we demonstrate the relationship between recollision trajectories and the transition moment coupling the ground and continuum states. In particular, we show that recollision trajectories are influenced by their parent ion, while it is commonly assumed they are not. Our work paves the way for the entirely optical measurement of ultrafast electron dynamics and photorecombination delays due to electronic structure, multielectron interaction, and strong-field-driven dynamics in complex molecular systems and correlated solid-state systems.

One of the main constraints for reducing the temporal duration of attosecond pulses is the attochirp inherent to the process of high-order harmonic generation (HHG). Though the attochirp can be compensated in the extreme-ultraviolet using dispersive materials, this is unfeasible toward x-rays, where the shortest attosecond or even sub-attosecond pulses could be obtained. We theoretically demonstrate that HHG driven by a circularly polarized infrared pulse while assisted by an strong oscillating ultrafast intense magnetic field enables the generation of few-cycle Fourier-limited few attosecond pulses. In such a novel scenario, the magnetic field transversally confines the ionized electron during the HHG process, analogously to a nanowire trapping. Once the electron is ionized, the transverse electron dynamics is excited by the magnetic field, acting as a high-energy reservoir to be released in the form of phase-locked spectrally wide high-frequency harmonic radiation during the electron recollision with the parent ion. In addition, the transverse breathing dynamics of the electron wavepacket, introduced by the magnetic trapping, strongly modulates the recollision efficiency of the electronic trajectories, thus the attosecond pulse emissions. The aftermath is the possibility of producing high-frequency (hundreds of eV) attosecond isolated few-cycle pulses, almost Fourier limited. The isolated intense magnetic fields considered in our simulations, of tens of kT, can be produced in finite spatial volumes considering structured beams or stationary configurations of counter-propagating state-of-the-art multi-terawatt/petawatt lasers.

Nonadiabatic dynamics around an avoided crossing or a conical intersection play a crucial role in the photoinduced processes of most polyatomic molecules. The present work shows that the topological phase in conical intersection makes the behavior of pump-probe high-order harmonic signals different from the case of avoided crossing. The coherence built up when the system crosses the avoided crossing will lead to the oscillatory behavior of the spectrum, while the geometric phase erodes these oscillations in the case of conical intersection. Additionally, the dynamical blueshift and the splitting of the time-resolved spectrum allow capturing the snapshot dynamics with the sub-femtosecond resolution.

Amplitude mode is collective excitation emerging from frozen lattice distortions below the charge-density-wave (CDW) transition temperature TCDW and relates to the order parameter. Generally, the amplitude mode is non-polar (symmetry-even) and does not interact with incoming infrared photons. However, if the amplitude mode is polar (symmetry-odd), it can potentially couple with incoming photons, thus forming a coupled phonon–polariton quasiparticle that can travel with light-like speed beyond the optically excited region. Here, we present the amplitude mode dynamics far beyond the optically excited depth of ∼150 nm in the CDW phase of ∼10-μm-thick single-crystal EuTe4 using time-resolved x-ray diffraction. The observed oscillations of the CDW peak, triggered by photoexcitation, occur at the amplitude mode frequency ωAM. However, the underdamped oscillations and their propagation beyond the optically excited depth are at odds with the observation of the overdamped nature of the amplitude mode measured using meV-resolution inelastic x-ray scattering and polarized Raman scattering. The ωAM is found to decrease with increasing fluence owing to a rise in the sample temperature, which is independently confirmed using polarized Raman scattering and ab-initio molecular dynamics simulations. We rationalize the above observations by explicitly calculating two coupled quasiparticles—phonon–polariton and exciton–polariton. Our data and simulations cannot conclusively confirm or rule out the one but point toward the likely origin from propagating phonon–polariton. The observed non-local behavior of amplitude mode thus provides an opportunity to engineer material properties at a substantially faster time scale with optical pulses.

The surface/interface species in perovskite oxides play essential roles in many novel emergent physical phenomena and chemical processes. With low eigen-energies in the terahertz region, such species at buried interfaces remain poorly understood due to the lack of feasible surface-specific spectroscopic probes to resolve the resonances. Here, we show that polarized phonons and two-dimensional electron gas at the interface can be characterized using surface-specific nonlinear optical spectroscopy in the terahertz range. This technique uses intra-pulse difference frequency mixing process, which is allowed only at the surface/interface of a centrosymmetric medium. Submonolayer sensitivity can be achieved using the state-of-the-art detection scheme for the terahertz emission from the surface/interface. Through symmetry analysis and proper polarization selection, background-free Drude-like nonlinear response from the two-dimensional electron gas emerging at the LaAlO3/SrTiO3 or Al2O3/SrTiO3 interface was successfully observed. The surface/interface potential, which is a key parameter for SrTiO3-based interface superconductivity and photocatalysis, can now be determined optically in a nonvacuum environment via quantitative analysis on the phonon spectrum that was polarized by the surface field in the interfacial region. The interfacial species with resonant frequencies in the THz region revealed by our method provide more insights into the understanding of physical properties of complex oxides.

In the last few decades, there have been several advances in ultrafast imaging of light propagation with light-in-flight recording by holography (LIF holography), which can capture light propagation as a motion picture with a single shot in principle. Here, we review the recent advances in LIF holography by considering the perspectives of various development of functional imaging techniques and evaluation of LIF holography with numerical simulation methods. The methods for recording multiple motion pictures such as a space-division multiplexing, a pixel-by-pixel-based space-division multiplexing, and an angular multiplexing technique are added extend the capability of LIF holography. The numerical simulation models used for investigating the image characteristics of LIF hologram are discussed. Finally, a summary and conclusion of recent advances in LIF holography is presented.

The 2023 Nobel Prize in Physics spotlights the techniques to generate attosecond light pulses. The generation of attosecond pulses heralds a new era in understanding electron dynamics. This perspective traces the evolution of ultrafast science, from early microwave electronics to the recent breakthroughs in attosecond pulse generation and measurement. Key milestones, such as high harmonic generation, the RABBITT method, attosecond streaking camera, etc, illuminate our journey toward capturing the transient electron motions in atoms. Recent discoveries, including zeptosecond delays in H2 single-photon double ionization and the potential of attosecond “electron” pulses despite challenges, etc., hint at an exciting future for ultrafast studies.