Strong-field physics in tailored light

Mitra, Sambit (2023): Strong-field physics in tailored light. Dissertation, LMU München: Fakultät für Physik

Abstract

This thesis reports on three distinct scenarios of strong tailored light fields interacting with matter in its gaseous and two-dimensional solid phases. Tailored light fields in the near- and mid-infrared (NIR, MIR) region facilitate precise control over electronic motion on an atomic level at atto- to femtosecond timescales. The first scenario deals with high-harmonic generation (HHG), a source of coherent and ultra-short XUV light pulses. The obtained XUV spectra often require modification for various applications which are primarily carried out using highly absorptive filters or custom-designed dielectric mirrors which drastically reduce the flux of the XUV light. This thesis reports on a novel scheme enabling the suppression of individual harmonics of such an extreme ultraviolet (XUV) comb, simultaneously affecting specific even and odd orders in the high-harmonic spectra generated by strongly tailored, two-colour (ω-2ω), multi-cycle laser pulses in neon. Realistic macroscopic strong-field approximation calculations confirm the experimental observations and correlate the effect to the use of symmetry-broken laser fields. Semi-classical calculations further corroborate the effect and reveal their underlying mechanism, where a nontrivial spectral interference between subsequent asymmetric half-cycles is found to be responsible for the suppression. This scheme particularly benefits future molecular time-resolved spectroscopy studies relying on narrow-band XUV light excitation around different photon energies with high flux. The second scenario deals with isolated soft-X-ray attosecond pulses generated via HHG in neon and driven by polarization-tailored few-cycle MIR pulses, commonly referred to as polarization gating. Accurate estimation of the attosecond soft-X-ray pulses in such experiments remains challenging given their higher photon energies and broad spectral bandwidth. Numerical results, based on strong-field approximation, are presented in this thesis to estimate the soft-X-ray pulse durations achievable using realistic polarization-gated MIR laser pulse parameters including macroscopic propagation effects. The intrinsic dispersion (attochirp) of such a pulse in the calculations is compensated by traversing it through a 90 cm plasma column after which a pulse duration of 110 as is obtained while the Fourier limit lay at 31 as. This theoretical estimate serves as a reference in judging the accuracy of new retrieval techniques while also highlighting the need for new compression schemes which compensate for higher-order dispersion in the soft-X-ray region. The third and final scenario presented in this thesis deals with using strong and far-off-resonant trefoil-shaped tailored light fields to alter time-reversal symmetry and induce valley selective bandgap modification (K and K' points in momentum space). The material chosen is monolayer hBN, an inversion-symmetry broken two-dimensional insulator. Such a scheme has only been predicted theoretically but not yet experimentally realized. Here, a novel apparatus is developed to produce trefoil light fields by combining two-colour (ω-2ω) counter-rotating pulses. Further, a controlled delay between the colours induces a rotation w.r.t. the hBN lattice, dynamically altering the band-gap and thereby electron population at each valley. A third linearly polarized pulse is then used to probe the induced valley polarization through a correlated change in the detected helicity of its intraband harmonics (third harmonic). The results presented here open further avenues in the direction of ultra-fast band engineering at petahertz frequencies which could revolutionize the future of electronics.

• Dokumententyp: Dissertationen (Dissertation, LMU München)
• Themengebiete: 500 Naturwissenschaften und Mathematik ; 500 Naturwissenschaften und Mathematik > 530 Physik
• Fakultäten: Fakultät für Physik
• Sprache der Hochschulschrift: Englisch
• Datum der mündlichen Prüfung: 22. Juni 2023
• 1. Bericht­erstatter:in: Kling, Matthias