Optical control of lattice dynamics with high spatiotemporal precision offers a route to manipulate local quantum states—such as magnetic, spin, and topological states—by exploiting the coupling between the lattice and other degrees of freedom. Here, deterministic strain engineering is demonstrated with spatial and temporal characteristics in van der Waals materials using spatially structured femtosecond optical fields. By confining structural oscillations at a submicron scale, phonon cavities with programmable dimensions, oscillation periods, and symmetries are engineered. Through ultrafast electron microscopy analysis and finite-element simulations the dominant cavity modes, out-of-plane confined oscillations, and in-plane Lamb waves are directly imaged and identified. It is shown that the properties of these phonon cavities are programmable via the spatial profile of the optical excitation, enabling localized modulation of strain and lattice displacement at nanometer and picosecond scales. This work establishes a general framework for spatiotemporal phonon engineering, bridging structured light excitation with atomic-scale control of lattice dynamics.

The ability to manipulate magnetization at femtosecond and nanometer scales is crucial for future advancements in spintronics and high-speed magnetic data storage. Ultrafast magnetization dynamics, driven by femtosecond laser pulses, have garnered significant attention due to their potential applications in energy-efficient magnetic switching, ultrafast spintronic devices, and high-density data storage, particularly relevant for the development of next-generation memory and logic devices, where speed, miniaturization, and energy efficiency are critical.  Recent studies have demonstrated that ultrafast laser pulses can induce phenomena such as femtosecond demagnetization, all-optical switching, and coherent spin wave excitation, providing new avenues for magnetic control at ultrafast timescales. However, despite these advances, key challenges remain in understanding the fundamental mechanisms governing ultrafast magnetization dynamics, particularly in complex magnetic systems and magnetization dynamics influenced by confined light, structured light, and geometric constraints, where spatially localized interactions play a crucial role. While existing optical techniques such as the magneto-optical Kerr effect (MOKE) and X-ray magnetic circular dichroism (XMCD) have been widely used to investigate ultrafast magnetization dynamics, their spatial resolution is fundamentally limited by the diffraction limit, preventing detailed real-space observation of localized ultrafast magnetic phenomena. On the other hand, Lorentz electron microscopy has proven to be a powerful technique for imaging static magnetic structures such as domain walls and skyrmions, but its application to ultrafast magnetization dynamics has been relatively underdeveloped. The emergence of ultrafast Lorentz electron microscopy (UEM), which combines transmission electron microscopy with femtosecond pump-probe techniques, overcomes these limitations by providing both high temporal and spatial resolution. This approach enables the direct visualization of real-space magnetization dynamics beyond the constraints of optical-probe techniques, offering a novel perspective on localized investigation on ultrafast magnetization phenomena.

Previous studies utilizing UEM have primarily focused on ultrafast dynamics of topological magnetic textures, leaving many aspects of ultrafast magnetization dynamics unexplored. To address this gap, this thesis expands the application of UEM to investigate laser-driven ultrafast magnetization dynamics beyond topological structures, including plasmon-mediated ultrafast demagnetization, all-optical domain wall writing, and optically engineered spin resonance modes. By utilizing structured light excitation and ultrafast Lorentz imaging, this work provides new insights into the fundamental mechanisms governing ultrafast magnetization dynamics, enabling precise control of magnetization at sub-picosecond timescales and sub-micrometer spatial resolution.

The experimental investigations in this thesis focus on three key areas. First, ultrafast demagnetization dynamics in permalloy (Ni₈₀Fe₂₀) thin films are studied in the presence of surface plasmon polaritons (SPPs), revealing a strong spatial modulation of demagnetization amplitude. By coupling SPPs to a ferromagnetic thin film Ni80Fe20, localized enhancements of demagnetization are observed near the sample edge, where plasmonic hotspots significantly influence the spatial profile of magnetization quenching. The results also demonstrate that SPP-induced demagnetization exhibits a characteristic beat pattern with a periodicity of approximately 17 μm, suggesting an interplay between propagating SPP waves and magnetization dynamics. These findings establish a direct link between plasmonic excitations and ultrafast magnetic responses, highlighting the potential for nanoscale magnetoplasmonic control.

Second, we present the first direct visualization in real space of ultrafast, all‑optical domain‑wall (DW) writing together with its nonequilibrium formation pathway. We tracked, both spatially and temporally, how a disordered magnetization configuration evolves into a well‑defined stripe‑like DW configuration within only a few picoseconds. Strikingly, during this transformation we observe an intermediate regime, manifested as a highly asymmetric and transient DW contrast. We also reveal a narrow fluence range where a short‑lived DW structure briefly forms and spontaneously collapses within roughly 10 ps, indicating the presence of a previously unidentified nonequilibrium in‑plane spin texture. We established a theoretical framework supported by multiscale micromagnetic simulations to interpret the observed DW nucleation dynamics. We found that DW nucleation is driven by both localized spin textures and the global drive generated by partial magnetization reversal through local all‑optical switching. The results of multiscale micromagnetic simulations are able to reproduce the Lorentz contrast of the intermediate state observed in the experiments. This DW formation pathway differs fundamentally from previously reported DW writing mechanisms and points to a promising strategy for achieving DW writing at speeds hundreds of times faster than approaches driven by magnetic fields, spin torques, or strain.

Finally, the thesis demonstrates the ultrafast creation of antiferromagnetic (AFM) coupling in the all-optical switching induced synthetic ferrimagnetic (SyF)-like configuration which can remain stable for at least nanoseconds, revealing the presence of coexisting spin resonance modes. Experimental results show the emergence of an acoustic resonance mode, whose frequency varies with applied magnetic field, alongside an optical resonance mode, which remains constant at approximately 4 GHz. These spin resonance modes are indicative of an engineered synthetic SyF-like state, wherein neighboring spin-up and spin-down domains are optically induced and AFM coupled. Furthermore, we can tune the coupling and decoupling of these two modes by adjusting the magnetic field and fluence, significantly influencing the amplitude and frequency of the two spin oscillation modes. The ability to manipulate this AFM coupled spin dynamics using ultrafast optical excitation opens new possibilities for engineered spintronic devices and magnonic applications.

In summary, this thesis advances ultrafast Lorentz electron microscopy for direct spatiotemporal visualization of ultrashort laser-driven magnetization dynamics, laying the groundwork for controlling localized ultrafast spin dynamics and potentially driving future innovations in spintronics and nanoscale magnetic manipulation.

Surface plasmons offer a promising avenue in the pursuit of swift and localized manipulation of magnetism for advanced magnetic storage and information processing technology. However, observing and understanding spatiotemporal interactions between surface plasmons and spins remains challenging, hindering optimal optical control of magnetism. Here, we demonstrate the spatiotemporal observation of patterned ultrafast demagnetization dynamics in permalloy mediated by propagating surface plasmon polaritons with sub-picosecond time- and sub-μm spatial- scales by employing Lorentz ultrafast electron microscopy combined with excitation through transient optical gratings. We discover correlated spatial distributions of demagnetization amplitude and surface plasmon polariton intensity, the latter characterized by photo-induced near-field electron microscopy. Furthermore, by comparing the results with patterned ultrafast demagnetization dynamics without surface plasmon polariton interaction, we show that the demagnetization is not only enhanced but also exhibits a spatiotemporal modulation near a spatial discontinuity (plasmonic hot spot). Our findings shed light on the intricate interplay between surface plasmons and spins, offer insights into the optimized control of optical excitation of magnetic materials and push the boundaries of ultrafast manipulation of magnetism.

MXene is a promising energy storage material for miniaturized microbatteries and microsupercapacitors (MSCs). Despite its superior electrochemical performance, only a few studies have reported MXene-based ultrahigh-rate (>1000 mV s−1) on-paper MSCs, mainly due to the reduced electrical conductance of MXene films deposited on paper. Herein, ultrahigh-rate metal-free on-paper MSCs based on heterogeneous MXene/poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS)-stack electrodes are fabricated through the combination of direct ink writing and femtosecond laser scribing. With a footprint area of only 20 mm2, the on-paper MSCs exhibit excellent high-rate capacitive behavior with an areal capacitance of 5.7 mF cm−2 and long cycle life (>95% capacitance retention after 10,000 cycles) at a high scan rate of 1000 mV s−1, outperforming most of the present on-paper MSCs. Furthermore, the heterogeneous MXene/PEDOT:PSS electrodes can interconnect individual MSCs into metal-free on-paper MSC arrays, which can also be simultaneously charged/discharged at 1000 mV s−1, showing scalable capacitive performance. The heterogeneous MXene/PEDOT:PSS stacks are a promising electrode structure for on-paper MSCs to serve as ultrafast miniaturized energy storage components for emerging paper electronics. 

Ultrafast optical control of magnetic textures offers new opportunities for energy-efficient, high-speed spintronic devices. While uniform magnetization reversal via all-optical switching is well established, the formation dynamics of non-uniform domain walls (DWs) under ultrafast excitation remain poorly understood. Here, we use Lorentz ultrafast electron microscopy combined with transient optical grating excitation to directly image the real-time formation of DWs in a ferrimagnetic GdFeCo film. We observe a rapid evolution from disordered spin contrast to ordered DW arrays within 10 ps, including a transient, strongly asymmetric DW state. In a narrow fluence window, short-lived DWs form and spontaneously vanish within picoseconds. Multiscale simulations combining atomistic spin dynamics and micromagnetics reveal a nonlinear nucleation pathway involving a hybrid transition state where localized, unstable spin textures coalesce into metastable DWs. This nonequilibrium mechanism explains the observed asymmetry and spatial ordering, and establishes a framework for controlling spin textures in magnetic materials on femtosecond timescales.

Direct imaging of spin waves is critical for advancing both fundamental understanding of magnetic dynamics and applied magnonics. However, achieving high spatiotemporal resolution of spin wave propagation dynamics remains a challenge. Here, we demonstrate spatiotemporal observation of laser excited spin waves in a Ni80Fe20 (permalloy) thin film using Lorentz ultrafast electron microscopy with a Gaussian pump spatially modulated by an optical transient grating (TG). Strikingly, we observe multiple concentric spin-wave wavefronts converging from the sample edges toward the center of the laser spot. These propagating wavefronts modulate the magnetic grating contrast induced by optical TG and are readily detected in the Lorentz images. Micromagnetic simulations reproduce the spatiotemporal propagation of the observed wavefronts. This work establishes structured optical excitation as a powerful and universal strategy for enhancing magnetic contrast and enabling real-space imaging of weak, long-wavelength spin wave dynamics, opening new opportunities for magnonic visualization.

All-optical control of magnetism provides a powerful platform for designing ultrafast and reconfigurable spintronic architectures. Here, we demonstrate the femtosecond-laser-induced formation of synthetic antiferromagnetic (SAF) order in a single-layer GdFeCo ferrimagnetic film, bypassing the need for multilayer engineering. By using transient optical gratings (TOGs), alternating spin domains are written via all-optical switching (AOS), giving rise to a SAF coupling effect with coexisting acoustic and optical spin resonance modes. The optical mode persists without damping and shows field-invariant behavior, unlike the field-sensitive acoustic mode. By tuning the laser fluence, external field, and grating periodicity, we control the coupling dynamics and mode frequencies, achieving magnon-magnon coupling near resonance. Our results establish a scalable, reconfigurable method for engineering synthetic AFM for next-generation spintronic devices.