This dissertation investigates the nonequilibrium physics of VO2 phase transition using a dual-probe approach combining ultrafast electron diffraction (UED) and ultrafast optical differential transmittance measurements, enabled by advancements in RF compression techniques for ultrafast electron microscopy and diffraction. By simultaneously tracking the structural and electronic responses with enhanced momentum resolution, this work provides new insights into the cooperativity and competing mechanisms underlying the photoinduced phase transition (PIPT) in VO2.The development of a cascade RF control system, featuring a two-level PID feedback loop, significantly reduces noise and instabilities in the RF system. The experimental validation of this upgraded RF system demonstrates a temporal resolution of ≈ 50 fs (FWHM) and a spatial resolution of 10 femtometers, pushing the limits of ultrafast electron probe technology. Leveraging these advancements, the dual-probe measurements reveal a multi-threshold nonequilibrium phenomenology in VO2 that deviates from the behavior of thermally-induced phase transitions. A critical fluence threshold Fc ≈ 4.5 mJ/cm2 is identified for ultrafast insulator-to-metal transition (IMT) mediated by polaron formation localized to the V-V sublattice, establishing a transient polaronic metallic (pM) state. This ultrafast IMT pathway is distinct from the thermally driven process and unaffected by lattice strain, indicating a different organizing principle at the initial stage. However, at longer timescales, the system converges back to the thermal phases governed by strong electron-phonon coupling, where IMT and structural phase transition (SPT) remain tightly coupled. The dual-probe measurements, supported by an effective medium theory, disentangle the competing effects of photoexcitation, polaron formation, and metallic domain growth, providing a unified picture connecting the nonequilibrium and equilibrium regimes. These findings advance the understanding of PIPT in VO2 and highlight the power of combining UED and ultrafast optical probes for unraveling complex phase transition dynamics in strongly correlated materials.
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