Strong-field driven charge and spin dynamics in solids

Gessner, Julia Anthea (2021): Strong-field driven charge and spin dynamics in solids. Dissertation, LMU München: Fakultät für Physik

Abstract

Modern laser technology enables the generation of optical pulses which are only a few femtoseconds long. The use of such ultrashort laser pulses to produce electron currents could increase the speed of information processing by at least three orders of magnitude. For this purpose, the fundamentals of light-matter interaction need to be investigated. The application of attosecond metrology provides access to the electronic motion on its natural time scale. Therefore, it can be used to study the interaction between an ultrashort optical laser pulse and electrons inside a solid. In the work of this thesis, three attosecond spectroscopic techniques are applied to study the coherent control of the electronic and magnetic properties of solids of different natures. Particular emphasis is given to the optically driven charge dynamics in silicon (Si) and nickel (Ni). Silicon is an indirect band-gap semiconductor: it has an energetically forbidden region between the valence and the conduction band (band-gap) where the Fermi level is located. In contrast, nickel is a metal: the valence and the conduction band overlap and the Fermi level resides above the conduction band edge. Studying the interaction between ultrashort optical pulses and solids with different electronic band structures could determine whether there is a universal response of solids to optical excitation. By applying attosecond Transient Absorption (TA) spectroscopy, it is shown, for the first time, how the electronic response of both materials can be manipulated faster than a half cycle of the exciting laser pulse. However, the experimental results show an important dissimilarity in the response of the two materials. While silicon’s carrier response is given only by the transient population transfer of electrons from the valence to the conduction band, nickel’s carrier response is strongly influenced by the presence of electron currents. This is due to the optically-induced acceleration of the free electrons residing in the conduction band of a metallic material. Polarisation Sampling (PS) spectroscopy is applied for the time-resolved study of the nonlinearities occurring in the two solids. When the optical pulse propagates through the medium, it changes its electronic properties, i.e. the material gets polarised. The induced polarisation provides information about the reversible and irreversible amount of energy transferred from the pulse to the material. For future optoelectronic applications, the dissipated energy is an important factor which should be investigated, and ideally suppressed. The study of silicon reveals two-photon absorption and the Kerr effect as the two main third-order nonlinear effects induced by the optical pulse. These two nonlinear effects, which change the refractive index of silicon in opposite directions, are observed to balance each other out. However, the nonlinear polarisation exhibits a weaker intensity scaling than what is expected for a third-order nonlinearity. This behaviour indicates that higher-order nonlinearities are setting in with similar magnitude to the lowest order (third-order) nonlinearities, suggesting tunneling through the direct band gap (3.4 eV) of silicon as the main excitation channel for electrons. The amount of energy transferred to Si contains a reversible and an irreversible component, corresponding to the linear and nonlinear transferred energy. By comparing silicon’s nonlinear features observed in the present experiment with the nonlinearities of fused silica studied by Sommer et al. [1], it is demonstrated that the two band gap materials react to strong-field excitation in a different manner. In conclusion, the PS experiment proves silicon’s strong-field driven transition from being a linear absorber to being a nonlinear, direct band-gap absorber. The PS experiment on nickel shows intriguing results. The main nonlinearity induced by the optical pulse is saturable absorption, which manifests as an increase in nickel’s transmission with increasing intensity. This effect can be interpreted as a de-metallisation process induced by Pauli blocking within the conduction band. Ni’s transition from a conducting to a semi-conducting material is reflected in the increase in the phase velocity of the transmitted pulse with respect to the incident pulse. The measured phase shift is larger than π/5. The corresponding spatial delay is 10 times the layer thickness. As a result, the thin metallic film acts as an ultrafast switchable amplitude and phase modulator for ultrashort optical pulses. Finally, the ultrafast magnetisation dynamics of nickel are investigated. A novel technique called “atto-MCD” is applied to simultaneously track the charge and spin dynamics. The technique combines the attosecond time-resolution given by a transient absorption-based setup with the spin sensitivity given by the magnetic circular dichroism of a magnetic material. The results demonstrate the first measured process of demagnetisation on a sub-femtosecond time scale. The experiment is performed on a nickel film as well as on a stack of Nickel/Platinum (Ni/Pt) multilayers. The reduction of Ni’s magnetic moment, while still interacting with the pulse, is observed only in the Ni/Pt multilayer system, indicating that Optically-Induced Spin Transfer (OISTR) is the main channel of ultrafast demagnetisation. OISTR consists in the migration of spin-oriented electrons from the nickel to the platinum layer, causing the macroscopic reduction of Ni’s magnetic moment. This interface effect is first experimentally proven in the work reported in this thesis.

• Dokumententyp: Dissertationen (Dissertation, LMU München)
• Keywords: Charge dynamics, spin dynamics, light-matter interaction, optoelectronics, magnetism, atto-MCD, polarisation sampling, transient absorption
• Themengebiete: 500 Naturwissenschaften und Mathematik ; 500 Naturwissenschaften und Mathematik > 530 Physik
• Fakultäten: Fakultät für Physik
• Sprache der Hochschulschrift: Englisch
• Datum der mündlichen Prüfung: 17. September 2021
• 1. Bericht­erstatter:in: Krausz, Ferenc