Symmetries and correlations in solids probed by ultrafast high-harmonic spectroscopy

by Tobias Gutberlet
Cumulative thesis
Date of Examination:2023-08-17
Date of issue:2023-11-13
Advisor:Prof. Dr. Claus Ropers
Referee:Prof. Dr. Claus Ropers
Referee:Prof. Dr. Stefan Mathias
Referee:Prof. Dr. Alexander Egner
Referee:Dr. Salvatore Manmana
Referee:Prof. Dr. Thomas Weitz
Referee:PD Dr. Martin Wenderoth
Persistent Address: http://dx.doi.org/10.53846/goediss-10194

 

 

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Abstract

English

Novel material systems with correlated and tunable properties promise to push the boundaries of current information processing and energy management technologies. Their investigation requires simultaneous access to multiple degrees of freedom at the fundamental timescales of electronic and lattice dynamics. High-harmonic generation (HHG) in gases offers such capabilities via absorption spectroscopy while solid-state HHG can probe the material properties via the nonlinear generation process. Although promising intriguing insights, studies of correlated solid systems with HHG remain challenging. The typically high noise floor of gas-phase HHG prohibits experiments of subtle dynamics in solids. Moreover, in the field of solid-state HHG, the access to fundamental observables such as chirality requires further development. In this cumulative thesis, we address both challenges by realizing and applying high-sensitivity absorption spectroscopy with gas phase HHG as well as introducing circularly polarized HHG in solids with threefold driving fields. In the first publication we extend solid-state HHG to chiral symmetry probing. By employing threefold driving fields, we achieve a material-independent generation of circularly polarized harmonics. This enables the investigation of surface magnetism in MgO and crystalline chirality in quartz. Additionally, by rotating the optical field, we gain access to the solid’s space group, enabling the probing of prominent symmetry-breaking effects like phase transitions. In the second publication we introduce an advanced referencing method for noise reduction in transient absorption experiments with a gas-phase HHG source. In comparison to other machine learning approaches, an artificial neural network best captures the non-polynomial noise of the HHG source and increases the sensitivity to absorption changes by one order of magnitude. With its general applicability for broadband sources, the presented approach is highly transferable to other beamlines including attosecond HHG, plasma sources, and free-electron lasers. In the final study presented as part of this thesis, we apply the novel referencing technique to study absorption spectra in the correlated material system 1T-TiSe2 with high-sensitivity. Combined with ab-initio calculations the spectra allow deducing various electronic and structural components with high precision. Laser excitation of the charge-density-wave phase in this material triggers two coherent phonon modes. By analyzing the spectral response of the atomic vibrations, the amplitude mode is distinguished from an optical phonon mode and linked to electronic screening. In their entirety, these results pave the way for future time-resolved studies of novel material systems. The presented circularly polarized HHG in solids holds potential applications in ultrafast magnetism or valleytronics. Moreover, the highly transferable referencing scheme enables investigations of weak absorption changes associated with correlated electronic, lattice, and spin degrees of freedom.
Keywords: Optics; CDW; transition metal dichalcogenide; ultrafast dynamics; structural dynamics; phase transition; charge-density wave; machine learning; high harmonic generation; spectroscopy
 
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