A Study on Ultrafast Spin and Orbital Current Dynamics in Magnetic Heterostructures for THz Emission

Abstract

Terahertz (THz) radiation has emerged as a powerful tool for probing ultrafast carrier, spin, and orbital dynamics in condensed-matter systems. This dissertation investigates three distinct classes of femtosecond-laser-driven THz emitters—photoconductive, nonlinear-optical, and magnetic heterostructure–based sources—focusing especially on the ultrafast charge, spin, and orbital current dynamics encoded in the THz waveform. The first part examines THz time-domain spectroscopy (THz-TDS) as an optical probe of electronic transport in polycrystalline MAPbI₃ perovskite films. By systematically varying grain size, the study reveals that the Drude-like free- carrier conductivity, momentum scattering rate, and carrier relaxation dynamics are strongly governed by microstructural coherence. Larger grains lead to enhanced carrier density, reduced scattering, and longer relaxation times, confirming that grain boundaries act as dominant traps and scattering centers. These results demonstrate the utility of THz-TDS as a noncontact probe of carrier dynamics in emerging optoelectronic materials. The second part explores spintronic THz emission (STE) in ferromagnet/nonmagnet (FM/NM) heterostructures. Using optimized Pt/Co and W/Co/Pt geometries, the study identifies how layer thickness, interfacial transparency, and material-specific spin Hall angles govern ultrafast spin-to-charge conversion. By extracting the charge-current spectrum from the THz waveform, the work further uncovers the role of the hot-phonon bottleneck and electron–phonon 20172050 Ph.D/PH scattering in modulating spin current formation and temporal evolution. Notably, the trilayer W/Co/Pt structure exhibits both enhanced THz amplitude and a nonlinear fluence response associated with a transition from collision-limited to ballistic-like transport. Building on this spintronic framework, the final part extends the analysis to orbitronic THz emission, where orbital angular momentum (OAM) rather than spin acts as the primary carrier of angular momentum. Recent theoretical and experimental findings indicate that orbital currents can propagate over much longer distances than spin currents, and can be generated through mechanisms such as the orbital Hall effect (OHE) and the orbital Rashba–Edelstein effect (OREE). This work experimentally verifies spin-to-orbital conversion using Pt interlayers and demonstrates that orbital-current–driven THz emission can exceed the amplitude of conventional spintronic THz signals. Overall, this dissertation establishes a comprehensive picture of how ultrafast charge, spin, and orbital currents evolve under femtosecond excitation and how these dynamics manifest as THz radiation. The findings highlight orbitronics as a compelling pathway toward next-generation THz emitters, offering extended transport length, SOC-independent tunability, and new opportunities for probing nonequilibrium orbital physics.