Probing ultrafast laser-induced spin dynamics in magnetic heterostructures using time domain terahertz spectroscopy

Abstract

Over the past decade, terahertz radiation (0.1-3 THz) has attracted much attention due to the wide range of applications in medical and industrial fields. Magnetic heterostructures consisting of ferromagnet and nonmagnetic metal layers have recently emerged as broadband THz sources that may have advantages over conventional sources such as photoconductive antennas (PCA) and nonlinear crystals. This thesis focuses on the influence of microstructures of spintronic emitter on the emission characteristics of its THz radiation and control of the THz polarization using the geometrical design of the spintronic emitter or in combination with conventional THz emitter PCA to form a hybrid THz emitter. The ultrafast spin injection and spin-to-charge conversion are also studied in ferromagnet/topological heterostructures using time-domain THz emission spectroscopy. ☐ First, the modification of the THz spectrum of the emitted radiation from microstructured spintronic emitters Fe/Pt patterned in stripes is demonstrated. By proper choices of the dimensions of the width and separation for the stripe obtained using a 1D simplified interference model, the peak position and bandwidth of the THz spectra of the microstructured spintronic emitter are shown to be modified compared to the spectrum of the extended films. ☐ Next, the influence of the magnetization texture dictated by the microstructure of the spintronic emitter W/Fe/Pt on the THz emission is investigated. Supported by micromagnetic simulations, the underlying mechanism for the generation of THz radiation from a magnetic heterostructure with a nonuniform magnetization state in the ferromagnetic layer is revealed, and the control of linearly polarized THz radiation using micropatterned "chopped" disks is demonstrated. ☐ Furthermore, a spintronic/III-V semiconducting material hybrid emitter, which benefits both from the broad bandwidth of spintronic emitters and the high-intensity THz emission at the low frequency of PCAs, is proposed and experimentally realized using a time domain THz spectroscopy system with dual-wavelength excitation. The unprecedented control of the THz pulse shape, polarization, and chirality of the combined THz pulse from the hybrid emitter is presented. ☐ Finally, the light-induced spin current generation, diffusion, and conversion to charge current, hence THz radiation, are studied in the emergent polycrystalline 3D topological insulator interfaced with an amorphous or epitaxial ferromagnet. The extracted spin diffusion parameters are compared to the results of the GHz spectroscopy measurement and to the theoretical calculations in the framework of the Kubo-Bastin formula and considering an atomistic tight-binding model. The remarkable consistency of the results suggests a great similarity of the spin inject and spin-to-charge conversion process across a very different time scale, pushing the studies on spin dynamics to a faster time scale and higher frequency regime. ☐ We discuss the preliminary results of the synchronizing time-resolved magneto-optic Kerr effect spectroscopy and time-domain THz emission spectroscopy, we foresee greater progress in understanding the microscopic process for the generation of THz radiation from magnetic heterostructures by correlating the time scale of the ultrafast demagnetization dynamics with THz radiation upon optical excitation. ☐ Our works, on the one hand, open fascinating opportunities for functional THz spintronics with tunable THz optical properties such as bandwidth, polarization, and chirality, on the other hand, provide a deeper understanding of the ultrafast spin and charge dynamics and push the frontier studies on spintronics to a faster time scale.