Oxides exhibiting insulator–metal transitions are promising candidates for next-generation ultrafast electronic switching devices. However, critical gaps remain in understanding the onset of strain and its dynamics as these materials undergo structural transitions, particularly in nanostructured configurations. Here, we present ultrafast four-dimensional scanning transmission electron microscopy enabling virtual imaging and strain mapping at every point in space and time. Using this technique, we directly probe a laser-excited phase transition in the prototypical material vanadium dioxide (VO₂). This direct imaging capability reveals the dynamics of the structural phase transition and connects it to the resulting strain formation on picosecond time scales. We find that the transient in-plane strain reaches ∼1% within ∼20 ps, an order of magnitude larger than expected from thermal expansion of the monoclinic phase. This indicates that the dominant strain contribution originates from the evolving structural phase transformation. Our results reveal the coupling between electronic, structural, and mechanical responses in correlated oxides under nonequilibrium conditions.

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.

Strain provides a powerful route for manipulating quantum states in topological materials by coupling lattice distortions to spin, charge, and topological degrees of freedom. While static approaches such as strain engineering, stacking, and twisting have expanded access to new quantum phases, reversible and programmable control of such states on ultrafast timescales remains a central challenge. This thesis addresses this gap by investigating ultrafast structural dynamics and strain modulation in van der Waals materials using ultrafast electron microscopy (UEM), which combines nanometer spatial resolution with picosecond temporal resolution.

We begin by studying strain wave propagation and interference in defective samples. The interaction between static strain fields and photoexcited coherent acoustic phonons is studied in the Weyl semimetal WTe₂. Local standing waves are generated at defect sites where static and dynamic strain couple, enabling deterministic modulation of strain at tens of nanometer scales. Finite-element simulations complement UEM imaging and elucidate the mechanisms of light-induced transient strain engineering. By employing spatially structured femtosecond optical fields, we further confine lattice oscillations into programmable phonon cavities with tunable dimensions, symmetries, and frequencies. UEM directly images these phonon dynamics, including confined out-of-plane oscillations and in-plane Lamb waves, establishing a platform for coherent phonon engineering and programmable transient lattice control at the nanoscale.

We then extend the study into the metastable structural phase control. We demonstrate a photoinduced ultrafast structural transition in PdSe2, occurring within tens of picoseconds and followed by long-lived metastable states persisting on nanosecond timescales. This behavior highlights the potential of pentagonal-layered materials as optically switchable platforms for phase control. We further realize femtosecond laser-driven topological phase patterning in WTe₂ by engineering transient optical gratings. This approach enables selective and reversible transitions between topological and trivial phases, with real-space imaging revealing strain-mediated interface dynamics. These studies of metastable structure control pave the way for optically addressable quantum and topological devices.

Finally, we report the direct observation of moiré pattern dynamics under transient strain in twisted van der Waals homostructure. Time-resolved dark-field imaging reveals coherent oscillations of moiré contrast at phonon frequencies, spatially correlated with bent regions. These results demonstrate the coupling between lattice vibrations and emergent functionality in van der Waals junctions.

Together, this work establishes a general framework for spatiotemporal ultrafast structural control in layered materials. By bridging structured light excitation with lattice dynamics, it opens new pathway toward optically reconfigurable quantum phases, topological textures, and novel device functionalities.

Strain utgör en kraftfull metod för att manipulera kvanttillstånd i topologiska material genom att koppla gitterdeformationer till spinn-, laddnings- och topologiska frihetsgrader. Även om statiska metoder såsom strain-ingenjörskonst, stapling och vridning har utökat tillgången till nya kvantfaser, kvarstår reversibel och programmerbar kontroll av sådana tillstånd på ultrafasta tidsskalor som en central utmaning. Denna avhandling adresserar detta genom att undersöka ultrafasta strukturdynamiker och strain-modulering i van der Waals-material med hjälp av ultrafast elektronmikroskopi (UEM), som kombinerar rumslig upplösning på nanometernivå med temporal upplösning på pikosekundnivå.

Arbetet inleds med studier av utbredning och interferens av strainvågor i defekta prover. Samspelet mellan statiska strainfält och fotoexciterade koherenta akustiska fononer undersöks i Weyl-semimetallen WTe₂. Lokala stående vågor genereras vid defektplatser där statisk och dynamisk strain kopplas samman, vilket möjliggör deterministisk modulering av strain på längdskalor om tiotals nanometer. Finita element-simuleringar kompletterar UEM-avbildning och klargör mekanismerna bakom ljusinducerad transient strain-ingenjörskonst. Genom att använda rumsligt strukturerade femtosekundoptiska fält kan gitteroscillationer dessutom begränsas till programmerbara fononkaviteter med justerbara dimensioner, symmetrier och frekvenser. UEM avbildar direkt dessa fonondynamiker, inklusive begränsade ut-ur-planet-oscillationer och in-plan Lamb-vågor, vilket etablerar en plattform för koherent fonon-ingenjörskonst och programmerbar transient gitterkontroll på nanoskala.

Studien utvidgas därefter till kontroll av metastabila strukturella faser. Vi demonstrerar en fotoinducerad ultrafast strukturell fasövergång i PdSe₂, som sker inom tiotals pikosekunder och följs av långlivade metastabila tillstånd som kvarstår på nanosekundtidsskalor. Detta beteende belyser potentialen hos pentagonalt skiktade material som optiskt switchbara plattformar för fasstyrning. Vidare realiseras topologisk fas-mönstring i WTe₂ driven av femtosekundlasrar genom att konstruera transienta optiska gitter. Detta möjliggör selektiva och reversibla övergångar mellan topologiska och triviala faser, där realrumsavbildning avslöjar strain-medierad dynamik vid gränsytor. Dessa studier av metastabil strukturkontroll banar väg för optiskt adresserbara kvant- och topologiska enheter.

Slutligen rapporterar vi direkt observation av moiré-mönsterdynamik under transient strain i tvistade van der Waals-homostrukturer. Tidsupplöst mörkfältsavbildning visar koherenta oscillationer i moiré-kontrasten vid fononfrekvenser, rumsligt korrelerade med böjda regioner. Dessa resultat demonstrerar kopplingen mellan gittervibrationer och framväxande funktionalitet i van der Waals-kopplingar.

Sammantaget etablerar detta arbete ett generellt ramverk för spatiotemporal ultrafast strukturell kontroll i skiktade material. Genom att sammanföra strukturerad ljusexcitation med gitterdynamik öppnas nya vägar mot optiskt rekonfigurerbara kvantfaser, topologiska texturer och nya enhetsfunktionaliteter.

The application of dynamic strain holds the potential to manipulate topological invariants in topological quantum materials. This study investigates dynamic structural deformation and strain modulation in the Weyl semimetal WTe2, focusing on the microscopic regions with static strain defects. The interplay of static strain fields, at local line defects, with dynamic strain induced from photo-excited coherent acoustic phonons results in the formation of local standing waves at the defect sites. The dynamic structural distortion is precisely determined utilizing ultrafast electron microscopy with nanometer spatial and gigahertz temporal resolutions. Numerical simulations are employed to interpret the experimental results and explain the mechanism for how the local strain fields are transiently modulated through light-matter interaction. This research provides the experimental foundation for investigating predicted phenomena such as the mixed axial-torsional anomaly, acoustogalvanic effect, and axial magnetoelectric effects in Weyl semimetals, and paves the road to manipulate quantum invariants through transient strain fields in quantum materials.

Two-dimensional palladium diselenide (PdSe₂) exhibits unique electronic andstructural properties arising from its puckered pentagonal lattice and strong interlayer coupling. The directional Pd–Se bonding and anisotropic phonon modes make PdSe₂ an ideal platform for exploring light–matter interactions and photoinduced structural dynamics. Here, we report a reversible photoinduced structural state transition in PdSe₂ occurring on the picosecond timescale. Using ultrafast transmission electron microscopy, we reveal a structural transformation characterized by an in-plane rotationof planar (PdSe₄)²- units, researching a metastable state that persists for nanoseconds before relaxing back to the ground state. Time- and angle-resolved photoemission spectroscopy reveals transient electronic excitations consistent with photodoping,which serves as the driving force for the transition. These results demonstrate coherent coupling between electronic and lattice degrees of freedom in PdSe₂, establishing it asa prototype system for optical control of metastable structural phases in low-dimensional materials.

Moiré superlattices in van der Waals materials provide a powerful platform for engineering emergent quantum phenomena, yet their dynamical response to ultrafast lattice perturbations remains largely unexplored. Here we directly visualize the ultrafast dynamics of a distorted Moiré superlattice in 1T′-TaTe_₂ using ultrafast transmission electron microscopy with nanometer–picosecond resolution. Static imaging reveals acomplex moiré pattern arising from a small interlayer twist angle combined with uniaxial heterostrain. Upon femtosecond laser excitation, we observe coherent oscillations of the moiré contrast at ~16 GHz, corresponding to cavity phonon modes in thickness direction. Space–time analysis demonstrates that the moiré fringes oscillate in-phase without measurable changes in periodicity, indicating transient global lattice distortion rather than interlayer rotation or sliding. The oscillations emerge after a delay of ~200 ps and are spatially correlated with bent regions of the sample, where in-plane strain is enhanced. These observations establish a direct link between coherent phonon excitation and dynamic modulation of Moiré superlattices, revealing Moiré patterns as sensitive probes of transient strain fields.

Microscopic and dynamic control over quantum states is essential for bridgingfundamental studies of material properties to device function. Realizing such controlat combined high spatial resolution and ultrafast temporal precision remains a majorchallenge. Here, we demonstrate femtosecond laser-driven patterning of topologicalquantum states in the Weyl semimetal WTe₂. By engineering the excitation field into atransient optical grating, we spatially selectively and reversibly drive phase transitionsbetween the topological Td and topologically trivial 1T* phases. Using ultrafast transmission electron microscopy, we directly visualize the formation of a periodic Td/1T* heterostructure, observe the propagation of a phase front, and analyzenanoscale confinement of coherently excited optical phonon modes. Our findingsestablish a platform for all-optical, spatially programmable, and reconfigurable controlof quantum states, paving the way for optically addressable topological devices.