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Pulse-duration-sensitive high harmonics and attosecond locally-chiral light from a chiral topological Weyl semimetal

Alba de las Heras, Ofer Neufeld, Angel Rubio

Abstract

High harmonic generation (HHG) in solids results from an interplay between intraband acceleration and electron-hole recombination driven by a high-intensity laser pulse. Here, we theoretically reveal that the driving pulse duration can play a major role in extending HHG to higher photon energies by promoting higher conduction band excitations. The effect is present in a conventional semiconductor as Si, restricted in a large-gap insulator as MgO, and most prominent in RhSi, a prototypical chiral Weyl semimetal presenting numerous band crossings. Further, we elucidate the HHG selection rules in RhSi required for the synthesis of attosecond locally chiral light. The chiral crystal structure enables the generation of a local 3D electric field exhibiting an asymmetric instantaneous torsion on attosecond timescales. A pronounced circular dichroism emerges when the driving helicity is either aligned with or opposite to the crystal handedness. Our findings motivate future experiments in chiral Weyl semimetals to track high-energy band crossings and in-situ locally chiral light, paving the way for chiral compact light sources and light-wave driven topological electronics.

Pulse-duration-sensitive high harmonics and attosecond locally-chiral light from a chiral topological Weyl semimetal

Abstract

High harmonic generation (HHG) in solids results from an interplay between intraband acceleration and electron-hole recombination driven by a high-intensity laser pulse. Here, we theoretically reveal that the driving pulse duration can play a major role in extending HHG to higher photon energies by promoting higher conduction band excitations. The effect is present in a conventional semiconductor as Si, restricted in a large-gap insulator as MgO, and most prominent in RhSi, a prototypical chiral Weyl semimetal presenting numerous band crossings. Further, we elucidate the HHG selection rules in RhSi required for the synthesis of attosecond locally chiral light. The chiral crystal structure enables the generation of a local 3D electric field exhibiting an asymmetric instantaneous torsion on attosecond timescales. A pronounced circular dichroism emerges when the driving helicity is either aligned with or opposite to the crystal handedness. Our findings motivate future experiments in chiral Weyl semimetals to track high-energy band crossings and in-situ locally chiral light, paving the way for chiral compact light sources and light-wave driven topological electronics.
Paper Structure (6 sections, 2 equations, 3 figures)

This paper contains 6 sections, 2 equations, 3 figures.

Table of Contents

  1. Introduction
  2. Results
  3. Pulse duration dependence of high harmonic generation from bulk crystals
  4. Symmetry selection rules and attosecond locally chiral light in RhSi
  5. Conclusions
  6. Author contributions

Figures (3)

  • Figure 1: Pulse duration dependence of high harmonic generation in bulk crystals. Computed band structure (left), high harmonic spectrum (center) and change of the density of occupied states (right) for three different materials: (A) RhSi, (B) Si and (C) MgO. The Fermi energies are indicated at 0 eV (dashed violet line). The full duration of the driving laser pulse is set to 4 (yellow), 12 (blue) and 20 (black) optical cycles. In RhSi and Si, longer durations of the driving pulse are associated with the occupation of higher-energy states and a substantial extension of the emission towards higher photon energies. The insets show the unit cell and the direction of the linearly polarized electric field.
  • Figure 2: Time-resolved spectrogram of the high harmonic emission in RhSi, Si and MgO. The emission of high photon energies requires long interaction times in RhSi and Si. In both crystals, the energy cutoff increases with the interaction time. In contrast, the emission in MgO reaches similar photon energies independently of the pulse duration. The driving pulses of 4 (left), 12 (center) and 20 optical cycles (right) are shown in grey lines at the top.
  • Figure 3: Chiral high harmonic generation in RhSi driven by a RCP pulse. The interaction of RhSi with a right circularly polarized (RCP) driver results in a complex emission with in-plane and out-of-plane polarization components. (A) The high harmonic spectrum driven by a RCP pulse in the (111) crystal plane contains alternate harmonic orders of counterrotating circular polarization (RCP in blue and LCP in violet) followed by a longitudinal component (yellow). The interaction geometry is represented in the inset. The harmonic orders 3, 4, and 9 selected for the spectral synthesis are indicated with grey stars. (B) Time evolution of the 3D synthesized electric field. Its local chirality is quantified via the instantaneous torsion (light blue) and the torsion envelope (shaded area) plotted in (C) time and (D) frequency domains. The dotted line represents the electric field intensity, $I(t)$, in arbitrary units as a reference. (E-H) Equivalent plots are shown for a RCP driving laser pulse in the (001) crystal plane.