Structuring high-order harmonic generation with the angular momentum of light

Resumen

Ultrashort laser pulses are a unique tool to explore the fastest dynamics in matter. Remarkably, the shortest laser pulses to date are produced from the non-linear frequency upconversion phenomenon of high-order harmonic generation (HHG), which results in the emission of pulses of attosecond durations. Importantly, such attosecond pulses can exhibit a very exciting property, the angular momentum, which presents two different forms, the spin angular momentum (SAM) and the orbital angular momentum (OAM), and that brings new scenarios for the light-matter interactions at the nanometric spatial and ultrashort temporal scales. In this thesis work, we develop a compilation of schemes for the creation of high-order harmonics and attosecond pulses with novel angular momentum properties by structuring the HHG process through the characteristics of the driving beams. For that purpose, we first address the description of the fundamental physical mechanisms of HHG. In particular, we study the tunnel ionization in molecules, finding that it is site-specific—its rate depends on the position of the electronic wavefunction at the ion sites—, due to the extended nature of the molecules. This characteristic leaves important signatures in the HHG and photoelectron spectra. Therefore, we provide a recipe for implementing the site-specificity in the existing strong-field models. Afterwards, we theoretically predict and describe the creation of extreme-ultraviolet (XUV) beams with novel angular momentum properties, which, in most of the cases, are experimentally generated and characterized by our collaborators from the Kapteyn-Murnane group in JILA, at the University of Colorado (USA) and from the group of Prof. M.-Ch. Chen at the Institute of Photonics Technologies of the Tsing Hua University (Taiwan). To begin with, we demonstrate the generation, for the first time, of light beams with time-varying OAM, a property which we denote as the self-torque of light. Importantly, selftorqued beams arise naturally in the XUV regime from HHG driven by two time-delayed infrared vortex beams. Under this configuration, the OAM of the high-order harmonics changes along time in the attosecond time-scale, being the amount of self-torque controlled through the temporal properties of the driving pulses. Thus, we believe that self-torqued beams can serve as unprecedented tools for laser-matter manipulation. In addition, we show how the OAM can serve as an instrument to manipulate the spectral and divergence properties of the high-order harmonics. By driving HHG with two vortex beams with properly selected OAM, we obtain high-order harmonic frequency combs with tunable line-spacing and low divergence. Such control is particularly interesting for XUV/soft-X-ray spectroscopy and imaging. Moreover, we present several schemes for the ellipticity control of the high-order harmonics and attosecond pulses. Using the non-collinear counter-rotating scheme, we extract the scaling of the ellipticity of the high-order harmonics with that of the driving beams’ and we unveil the information about the non-perturbative dipole response hidden in that connection. Also, we show the generation of circularly polarized vortex beams from HHG driven by a bi-circular vortex field. Interestingly, by properly selecting the OAM of the driving field we can obtain either circularly polarized attosecond pulses, or high-order harmonics with low topological charge. Finally, we theoretically demonstrate the generation of attosecond pulse trains with time-ordered polarization states by combining two time-delayed bi-circular vortex driving fields. We believe that the generation of attosecond pulses with controlled ellipticity can be employed for the study of ultrafast spin dynamics in chiral molecules or magnetic materials.