Attosecond science based on high harmonic generation from gases and solids

Abstract

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

Geometrical Reorganization of a Methane Cation upon a Sudden Ionization: An Isotope Effect in Electronic Non Equilibrium Quantum Dynamics

“…Primarily this is due to the variation of the photoionization matrix elements with the geometry of the ground state in the Franck-Condon region that lead to different localization of the initial state resulting from the sudden ionization process. Such ultrafast electronic and structural rearrangements in cations ionized to multiple interfering channels are ideally probed by HHG spectroscopy 17,18 which provides the needed sub to few fs time resolution. Recent developments in 3D two electron angular streaking 19 and time-resolved X-Ray Transient Absorption spectroscopy 20,21 could also shed light on such ultrafast processes.…”

High-energy few-cycle pulses: post-compression techniques

“…The shortest events the mankind could ever generate are light pulses. In the visible to near-infrared spectral range, they have a duration of a few femtoseconds [1], which paved the way to the emergence of attosecond science by enabling the generation of XUV pulses of several tens of attoseconds duration via high-harmonic generation [2,3]. They are the tools of choice [4] for exploring electron dynamics inside atoms, molecules and solids [5][6][7][8][9][10] or in nanostructures [11].…”

Four-wave mixing with multi-color laser in extreme ultraviolet region

“…The combination of an attosecond pulse train and cascaded wave-mixing in the XUV region provides a novel tool to access the electron dynamics of the interaction of atoms and molecules with intense laser fields 13 . Moreover, all of the interactions occurring in our single gas cell chamber allows us to simplify the experimental setup while still being able to probe the time-evolution of the spectral shape of the electron wave-packet in the XUV cascaded wave-mixing process.…”