The motion of electrons in the microcosm occurs on a time scale set by the atomic unit of time—24 attoseconds. Attosecond pulses at photon energies corresponding to the fundamental absorption edges of matter, which lie in the soft X-ray regime above 200 eV, permit the probing of electronic excitation, chemical state, and atomic structure. Here we demonstrate a soft X-ray pulse duration of 53 as and single pulse streaking reaching the carbon K-absorption edge (284 eV) by utilizing intense two-cycle driving pulses near 1.8-μm center wavelength. Such pulses permit studies of electron dynamics in live biological samples and next-generation electronic materials such as diamond.

Recent progress in high power ultrafast shortwave and mid-wave infrared lasers has enabled gas-phase high harmonic generation (HHG) in the water window and beyond, as well as the demonstration of HHG in condensed matter. In this Perspective, we discuss the recent advancements and future trends in generating and characterizing soft X-ray pulses from gasphase HHG and extreme ultraviolet (XUV) pulses from solid-state HHG. Then, we discuss their current and potential usage in time-resolved study of electron and nuclear dynamics in atomic, molecular and condensed matters. T abletop attosecond light sources in the soft X-ray (SXR) spectral region based on highharmonic generation are highly desirable in chemical and material sciences since they can spectroscopically identify specific elements, as well as the oxidation states, charge states and even the spin states of those elements 1. One of the important spectral regions is the "water window" (282-533 eV), which covers the atomic K-shell excitation of carbon and oxygen. Although high harmonics in the water window were first generated with Ti:Sapphire lasers centered at 800 nm more than 20 years ago 2,3 , the X-ray photon flux was too low for timeresolved applications. The mechanism of HHG in gases can be explained by the semiclassical three-step model 4-6. When driving laser-field strength reaches~10 8 V m −1 , the bound electron in the atomic gas can tunnel through the Coulomb potential barrier and become a free electron. In the oscillating laser field, the free-electron wave packet may return to its parent ion with the right time of birth. At recombination, the interference between the wave packets of the returning and bound electrons produces an oscillating dipole that emits attosecond radiation. Returning electrons with various kinetic energy will recombine at different times giving rise to the chirp in the attosecond radiation 7. This process repeats twice for every optical cycle. The temporal beating of attosecond pulses results in the high-harmonic combs in the spectral domain. Empowered by the advances in driving lasers with center wavelengths around 1.8 μm, soft X-ray high harmonics can be generated with a moderate intensity of 10 14 W cm −2 (see Box 1 for details). Significant progress has recently been made in developing attosecond light within the water window 8. By spectrally broadening pulses from an Optical Parametric Amplifier (OPA) using a gas-filled hollow-core fiber 9 or by broadband phase matching in an Optical Parametric Chirped Pulse Amplifier (OPCPA) 10 , two-cycle, mJ-level pulses centered with 1 kHz repetition

High-harmonic generation in isolated atoms and molecules has been widely utilized in extreme ultraviolet photonics and attosecond pulse metrology. Recently, high-harmonic generation has been observed in solids, which could lead to important applications such as all-optical methods to image valance charge density and reconstruct electronic band structures, as well as compact extreme ultraviolet light sources. So far these studies are confined to crystalline solids; therefore, decoupling the respective roles of long-range periodicity and high density has been challenging. Here we report the observation of high-harmonic generation from amorphous fused silica. We decouple the role of long-range periodicity by comparing harmonics generated from fused silica and crystalline quartz, which contain the same atomic constituents but differ in long-range periodicity. Our results advance current understanding of the strong-field processes leading to high-harmonic generation in solids with implications for the development of robust and compact extreme ultraviolet light sources.

Solid-state high-harmonic sources offer the possibility of compact, high-repetition-rate attosecond light emitters. However, the time structure of high harmonics must be characterized at the sub-cycle level. We use strong two-cycle laser pulses to directly control the time-dependent nonlinear current in single-crystal MgO, leading to the generation of extreme ultraviolet harmonics. We find that harmonics are delayed with respect to each other, yielding an atto-chirp, the value of which depends on the laser field strength. Our results provide the foundation for attosecond pulse metrology based on solid-state harmonics and a new approach to studying sub-cycle dynamics in solids.

Photoinduced quantum dynamics in molecules have hierarchical temporal structures with different energy scales that are associated with electron and nuclear motions. Femtosecond-to-attosecond transient absorption spectroscopy (TAS) using high-harmonic generation (HHG) with a photon energy below 300 eV has been a powerful tool to observe such electron and nuclear dynamics in a table-top manner. However, comprehensive measurements of the electronic, vibrational, and rotational molecular dynamics have not yet been achieved. Here, we demonstrate HHG-based TAS at the nitrogen K-edge (400 eV) for the first time, and observe all the electronic, vibrational, and rotational degrees of freedom in a nitric oxide molecule at attosecond to sub-picosecond time scales. This method of employing core-to-valence transitions offers an all-optical approach to reveal complete molecular dynamics in photochemical reactions with element and electronic state specificity.