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.

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 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.

Amplitude mode is collective excitation emerging from frozen lattice distortions below the charge-density-wave (CDW) transition temperature T_CDW 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 ω_AM. 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 ω_AM 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.

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.

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.