Direct observation of the nucleation and growth of a new phase in solids is a significant challenge due to the wide range of length and timescales involved, from the atomic scale to the macroscale and from femtoseconds to microseconds. While diffraction-based probes have advanced our understanding of atomic-scale ultrafast dynamics, electronic probes are crucial for understanding the functionality of these states, particularly in quantum materials. In quantum materials, light can create transient states with electronic properties not found in equilibrium. These transient states are often believed to be heterogeneous at the nanoscale, due to both inhomogeneous excitation profiles and the intrinsic heterogeneity of many quantum materials. A major obstacle in understanding these phases is the need to isolate the photo-induced state and directly probe its properties at the nanoscale. Previous studies have used resonant coherent diffraction to infer statistical properties of domains, but real-space images have been lacking. This research employs time- and energy-resolved coherent resonant soft X-ray imaging to observe the ultrafast insulator-metal phase transition in vanadium dioxide (VO2), achieving sub-50 nm spatial resolution and 150 fs time resolution. The light-induced phase transition in VO2 is a well-studied system, with previous work utilizing time-resolved X-ray diffraction and ultrafast X-ray absorption techniques. However, questions remain about the relationship between bandgap collapse and the structural transition, and the role of heterogeneity in the transient state. This study directly addresses these questions using a powerful wide-field imaging technique that combines resonant X-ray spectroscopy to provide phase contrast and extract quantitative spectral information.

The light-induced phase transition in VO₂ has been extensively studied, serving as a model system for optically driven quantum materials. At room temperature, VO₂ exists in a monoclinic insulating (M1) phase, characterized by dimerized vanadium ions. Light excitation breaks these dimers, driving an ultrafast transition to the high-temperature rutile metallic (R) phase. This transition has been instrumental in the development of various time-resolved techniques, including X-ray diffraction and X-ray absorption spectroscopy, which have revealed the sub-100 fs timescale of structural and electronic changes. However, the precise sequence of bandgap collapse and structural transition remains unclear. Furthermore, the role of nanoscale heterogeneity in the dynamics has been a subject of ongoing debate. Analysis of terahertz conductivity suggests that the rutile metallic phase nucleates and grows on a timescale of tens to hundreds of picoseconds. Electron diffraction studies suggest the formation of a metastable, heterogeneous monoclinic metallic phase, distinct from the initial ultrafast transition, that persists for hundreds of picoseconds or even microseconds. The existence and properties of these non-equilibrium phases, however, remain controversial, due to the lack of direct measurements.

This study utilizes time- and spectrally resolved resonant soft X-ray coherent imaging at the Pohang Accelerator Laboratory X-ray Free Electron Laser (PAL-XFEL) to image the light-induced phase transition in VO₂ with nanometer spatial resolution and femtosecond time resolution. The technique employs two modes of operation: Fourier transform holography (FTH) and coherent diffractive imaging (CDI). FTH allows rapid data collection using single exposures but loses the absolute values of complex transmission. CDI uses multiple exposures to increase the dynamic range and provides quantitative absolute transmission via iterative phase retrieval algorithms. 75-nm-thick VO₂ layers were prepared on silicon nitride membranes by pulsed laser deposition. Pump-probe experiments were performed at the PAL-XFEL, using 800 nm laser pulses for excitation and time-resolved imaging at various photon energies around the vanadium L₃ and oxygen K edges. The X-ray polarization was perpendicular to the rutile *c* axis. Time-resolved images, averaging 9,000 XFEL shots per image, were acquired by alternating between positive and negative time delays. Principal component analysis (PCA) was used to analyze the spatial and temporal dependencies of the dynamics, separating out the contributions of different processes. Spectrally resolved CDI was employed to recover the full spectrum of the switched regions at a 20 ps delay, providing insights into the nature of the transient metallic state. X-ray absorption spectroscopy (XAS) simulations using density functional theory (DFT) aided in phase identification and interpretation of the spectral features.

The study reveals that the early-time dynamics (up to ~20 ps) of the light-induced insulator-to-metal transition in VO₂ are independent of initial spatial heterogeneity. A remarkably fast 200 fs switch to the metallic phase is observed. A heterogeneous response emerges only after hundreds of picoseconds. PCA analysis of the time-resolved images shows that only a single principal component is needed to describe the dynamics up to 20 ps, indicating a uniform temporal evolution across all regions. This finding is inconsistent with models proposing a direct M1 to R phase transition in some regions and a transition through a metastable monoclinic metallic phase in others, or a nucleation and growth scenario. The two time constants observed (203 ± 18 fs and 4.98 ± 0.04 ps) occur in the same regions of the sample. Spectrally resolved CDI at 20 ps post-excitation reveals that the transient metallic phase is highly orthorhombically strained. The spectral differences between initially insulating and metallic regions are small (less than 1% at 530.5 eV), indicating a transition to the R phase. These differences are attributed to strain generated during the ultrafast phase transition, where the loss of dimerization occurs before volume expansion. The picosecond timescale is explained by the propagation of strain waves launched from the surface-vacuum interface and the photo-generated R/M1 phase interfaces. The in-plane strain relaxation, which occurs on a much slower timescale (hundreds of picoseconds), is likely responsible for the spatial heterogeneity observed at longer delays. The results highlight the importance of spatially and spectrally resolved measurements in understanding ultrafast phase transitions.

The findings challenge existing interpretations of the light-induced phase transition in VO₂, demonstrating that the ultrafast dynamics are surprisingly homogeneous at early times. The observation of a single principal component governing the initial dynamics refutes models involving distinct pathways or nucleation and growth processes. The identification of orthorhombic strain in the transient metallic phase provides a new understanding of the structural changes during the transition. The proposed mechanism, involving ultrafast dimer breaking followed by slower strain relaxation, offers a coherent explanation for the observed time constants and spatial heterogeneity. This work underscores the limitations of spatially averaged probes in studying ultrafast phase transitions in complex materials and emphasizes the crucial role of spatially and spectrally resolved techniques for uncovering the intricacies of these processes.

This study provides a detailed, spatially and temporally resolved picture of the light-induced insulator-to-metal transition in VO₂, revealing a surprisingly homogeneous initial response followed by slower strain-driven heterogeneity. The results challenge existing models and highlight the importance of advanced imaging techniques for understanding ultrafast phenomena in quantum materials. Future research could focus on exploring the dynamics of strain propagation in more detail and investigating the influence of defects and interfaces on the phase transition.

The study primarily focuses on a specific sample thickness (75 nm) and excitation fluence. Extending the study to different thicknesses and fluences would enhance the generalizability of the findings. The analysis relies on the assumption of a single process dominating the initial ultrafast dynamics, and while the PCA strongly supports this, subtle contributions from other processes might be overlooked. Additionally, the interpretation of the strain effects is based on theoretical modeling, and further experimental validation, perhaps through techniques like ultrafast electron diffraction, could strengthen the conclusions.