Modeling terahertz wave propagation and production in out-of-equilibrium complex heterostructure devices

• Main Author: Yang, Yingshu
• Other Authors: Marco Battiato
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2023
• Subjects: Science::Physics
• Online Access: https://hdl.handle.net/10356/168638
• Institution: Nanyang Technological University

THz spectroscopy is one of the main technical tools to study the physics of THz devices and ultrafast dynamics. Therefore, the development of THz experimental techniques in studying the properties of materials and their dynamics is gaining increasing popularity. However, the complexity of the structure of THz emitting devices with multilayer structures and heterogeneous interfaces leads to subtle changes in the THz spectral properties. Thus, the simulation of THz propagation processes in these complicated structures becomes significant in exploring the correlation between device structure and performance. For this reason, an accurate and efficient theoretical model for the description of THz propagation in heterostructure systems under different circumstances has become an important topic. In this thesis, we have demonstrated several different modified Transfer Matrix Methods (TMM) that can be used for theoretical description and subsequent evaluation of various THz spectroscopic methods. We first modified the standard TMM and produced an analytical form that can remove the spectrum distortion induced by echoes caused by thick substrates. This removed echo TMM can provide a direct comparable theoretical spectrum to the experimentally measured data, often obtained by time-windowing methods in THz time-domain spectroscopy experiments. It can reduce the numerical complexity compared to traditional methods used for echo removal in theoretical calculations. Then, we extended the standard TMM by including the effect of a volume source current term by considering one of the layers in a multilayer system as a source layer. This method can combine the description of THz emission and subsequent transmission through heterostructures and gives a detailed description to THz emitting devices like spintronic THz emitters. We have also included general spin diffusion geometry models in the different layers of the spintronic THz emitter to describe the whole THz emission process. We applied the TMM-with-source in several analyses of experiments. We utilized the TMM-with-source and developed a general model for spintronic THz emission as a function of heavy metal (HM) layer thickness. Using this model, we demonstrated that besides the FM excitation that serves as a primary source of THz emission, HM excitation serves as a secondary enhancement for THz emission. The HM excitation can account for up to 40% of total emission at high HM thicknesses (e.g. dpt > 4 nm). We then utilized the TMM-with-source to explain another set of experiments on the unexpected THz emission from Co/Si structures. We demonstrated that the THz emission was generated by the spontaneous creation of silicides at the interface. Using the TMM-with-source and the Poynting theorem, we determined the thickness and spin Hall angle of the newly formed CoSi at the Co/Si interface. The TMM-with-source is also used to assess the connection between the charge current amplitude and the emitted THz amplitude. We then used it to show that the spintronic THz emitter can be used as a magnetometer with performance comparable to commercially available vibrating sample magnetometers. Then, we developed a Perturbative Transfer Matrix Method (PTMM) based on the TMM-with-source by modifying the source term as a perturbation expansion of the fields and the material properties. We included the dielectric response of the material using a time-dependent generalized Drude function. We explicitly accounted for the out-of-equilibrium carrier densities and scattering rates induced by the optical pump. This PTMM can analyze sub-picosecond time-dependent material characteristics in optical pump and THz probe experiments. In this thesis, we demonstrated the efficacy and accuracy of established modified TMMs in modeling diverse spectroscopic techniques. In addition, we also showed that our technique is effective in calculating meaningful information for diverse materials, which will be crucial in the future research and optimization of particular STE devices when combined with experimental data.