High harmonic generation by the initial displacement and velocity of photoionized electrons

• Applicants: Dr. Anton Husakou; Dr. Peter Jürgens
• Subject Area: Optics, Quantum Optics and Physics of Atoms, Molecules and Plasmas
• Term: since 2024
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 533828881

Optical nonlinearity and harmonic generation are ubiquitous in modern optics and represent key technologies for many applications. Up to now, three main mechanisms of harmonic generation were recognised and investigated: harmonics due to Kerr-type nonlinearity, three-step Lewenstein-type harmonics, and Brunel harmonics that are associated with a rapid change of the carrier density. On the contrary, harmonics which originate from the initial displacement as well as from the initial velocity just after the ionization event, went almost unnoticed up to now. However, as shown in our recent Nature Physics paper and preliminary investigations for this project, the abovementioned ionization-related harmonics can potentially be more powerful than the three previously known mechanisms. These findings motivate the current project, which encompasses the extensive investigation of the ionization-related harmonic generation mechanisms.In the framework of the current project, we will first design and implement an experimental platform for the investigation of ionization-related harmonics in gases and solids. Characterization of the harmonics, in particular its relative phase as a function of pump laser parameters, will allow us to differentiate between the different mechanisms. Simultaneously and in close cooperation with the experiment, we will concept and implement a comprehensive theoretical and numerical description of ionization-related harmonics. Different levels of approximation will be used, such as an ADK-type formalism, strong-field approximation, as well as first-principle solutions of the time-dependent Schrödinger equation. Finally, in a joint experimental-theoretical effort, we will perform a mapping of the parameter space according to the dominant harmonic generation mechanism.The output of the project will include the theoretical-numerical description of the ionization-related harmonic generation mechanisms, a comprehensive experimental setup designed for the characterization of the harmonic generation in both solids and gases, as well as deep understanding of the ionization-related mechanisms including the above mentioned mapping of the parameter space. These deliverables will considerably contribute to the state of the art in the fields of harmonic generation and strong-field ionization and will be a basis for further studies and applications (e.g. in the field of laser-micromachining).

• DFG Programme: Research Grants