Attosecond science based on high harmonic generation from gases and solids

Abstract

Recent progress in high power ultrafast short-wave 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 gas-phase 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.

Fig. 1. Atto-chirp compensation in the water window.

Comparison of group delay dispersion (a) and X-ray transmission (b) between a 2-μm-thick uranium foil and 8.26 cm long, 1 atm H₂ gas from 250 to 550 eV.

Fig. 2. Attosecond transient-absorption spectroscopy at the N K-edge.

a Experimental setup. b Attosecond X-ray spectrum. c Molecular orbitals of NO. d Absorption spectrogram. e Static absorbance of NO without the IR pump pulse. Reproduced from ref. , Copyright 2019, Optical Society of America. f Electronic response Adapted from ref. Copyright 2019, Optical Society of America. g Molecular vibration. Reproduced from ref. Copyright 2019, Optical Society of America.

Fig. 3. Charge migration in molecules.

a The reconstructed electron–hole density from HHG spectroscopy as a function of time after strong-field ionization of HCCI. Reproduced from ref. with permission from AAAS. b Dipole moment oscillations of C₂HCI⁺ ionized from a HOMO/HOMO-1 superposition of states. Reproduced from ref. (2017), AIP. c Charge migration measured using attosecond X-ray transient absorption. The mixed hole state in C₂HI⁺ is formed by strong-field ionization. The valence hole is probed by core-to-valence transitions. Adapted from ref. (2017), AIP.

Fig. 4. Charge transfer in organic photovoltaic materials.

a Schematics describing 2-picolinic acid adsorbed onto TiO₂ with femtosecond UV as the pump and attosecond SXR as the probe. b Energy diagram describing the electron transfer from LUMO of adsorbate to conduction band of TiO₂. Schematics showing transitions from C K-edge and N K-edge to unoccupied states for adsorbate and those from Ti L-edge to conduction band for TiO₂. c Expected attosecond transient-absorption signals at various probe energies (C K-edge, N K-edge, and Ti L-edge).

Fig. 5. High-harmonic spectroscopy of solid materials.

a Schematics for HHS of solid-state materials in transmission geometry. Few-cycle laser pulses excite the sample at the V/Å level without damage. Attosecond pulses are produced on subcycle timescales, which interfere in the far field and produce discrete peaks in high-harmonic spectrum. b Measured crystal orientation-dependent high-harmonic spectrum from wide-bandgap cubic MgO crystal. c Real-space electron trajectory model showing how harmonic signal increases (decreases) when the electron trajectories connect (miss) neighboring atomic sites. If trajectories connect the first nearest neighbors (O–Mg), they produce the strongest signal, as seen along cubic directions. If they connect the second nearest-neighbor (O–O) secondary maxima are produced, as seen along diagonal direction of the cube. Reproduced with permission from ref. , Copyright 2017, Springer Nature.

Fig. 6. Momentum-space interpretation for HHG in solid materials.

The microscopic mechanism for solid-state HHG showing two major channels semiclassically. The laser field promotes an electron from valence to conduction band leaving behind a hole in the valence band, as shown by the vertical solid line. Both the electron and hole are accelerated in their respective bands. Channel 1: the nonlinear intraband current in the conduction band can radiate. Channel 2: the electron can recombine with the hole at a later time, as shown by the dashed line, and emit a high-energy photon. Adapted with permission from ref. , Copyright 2019, Springer Nature.

Fig. 7. Attosecond interferometry in transmission and reflection configuration.

Experimental setup in a transmission and b reflection, where high harmonics from two sources interfere in the far field. Reproduced with permission from ref. , Copyright 2019, Springer Nature. A representative interferogram is shown in c. Fringe shift as a function of relative change in laser intensities, in transmission. d and in reflection (e). Reflection geometry closely represents the microscopic effects. Fringe shifts in transmission are overwhelmed by propagation effects. Adapted with permission from ref. , Copyright 2019, Springer Nature.