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Coulomb correlations can manifest in exotic properties in solids, but how these properties can be accessed and ultimately manipulated in real time is not well understood. The insulator-to-metal phase transition in vanadium dioxide (VO2) is a canonical example of such correlations. Here, few-femtosecond extreme UV transient absorption spectroscopy (FXTAS) at the vanadium M2,3 edge is used to track the insulator-to-metal phase transition in VO2 This technique allows observation of the bulk material in real time, follows the photoexcitation process in both the insulating and metallic phases, probes the subsequent relaxation in the metallic phase, and measures the phase-transition dynamics in the insulating phase. An understanding of the VO2 absorption spectrum in the extreme UV is developed using atomic cluster model calculations, revealing V3+/d2 character of the vanadium center. We find that the insulator-to-metal phase transition occurs on a timescale of 26 ± 6 fs and leaves the system in a long-lived excited state of the metallic phase, driven by a change in orbital occupation. Potential interpretations based on electronic screening effects and lattice dynamics are discussed. A Mott-Hubbard-type mechanism is favored, as the observed timescales and d2 nature of the vanadium metal centers are inconsistent with a Peierls driving force. The findings provide a combined experimental and theoretical roadmap for using time-resolved extreme UV spectroscopy to investigate nonequilibrium dynamics in strongly correlated materials.

Vanadium dioxide (VO2) is under intensive consideration for optical switching due to its reversible phase transition, which features a drastic and rapid shift in infrared reflectivity. Classified as an insulator-to-metal transition, the phase transition in VO2 can be induced thermally, electrically, and optically. When induced optically, the transition can occur on sub-picosecond time scales. It is interesting to dope VO2 with erbium ions (Er3+) and observe their combined properties. The first excited-state luminescence of Er3+ lies within the wavelength window of minimal transmission-loss in silicon and has been widely utilized for signal amplification and generation in silicon photonics. The incorporation of Er3+ into VO2 could therefore result in a novel photonic material capable of simultaneous optical switching and amplification. In this work, we investigate the optical switching and photoluminescence in Er-implanted VO2 thin films. Thermally driven optical switching is demonstrated in the Er-implanted VO2 by infrared reflectometry. Photoluminescence is observed in the thin films annealed at ∼800 °C or above. In addition, Raman spectroscopy and a statistical analysis of switching hysteresis are carried out to assess the effects of the ion implantation on the VO2 thin films. We conclude that Er-implanted VO2 can function as an optical switch and amplifier, but with reduced switching quality compared to pure VO2. [ABSTRACT FROM AUTHOR]

Hybrid material systems are a promising approach for extending the capabilities of silicon photonics. Given the weak electro‐optic and thermo‐optic effects in silicon, there is intense interest in integrating an ultrafast‐switching phase‐change material with a large refractive index contrast into the waveguide, such as vanadium dioxide (VO2). It is well established that the phase transition in VO2 thin films can be triggered by ultrafast, 800 nm laser pulses, and that pump‐laser fluence is a critical determinant of the recovery time of thin films irradiated by femtosecond pulses. However, thin‐film experiments are not reliable guides to a VO2:Si system for all‐optical, on‐chip switching because of the differences in VO2 optical constants in the telecommunication band, and the complex sample geometry and alignment issues in a waveguide geometry. This paper reports the first demonstration that the reversible, ultrafast photoinduced phase transition in VO2 can achieve sub‐picosecond response when small VO2 volumes are integrated into a silicon waveguide as the active element. The result suggests that VO2 can be pursued as a strong candidate for waveguide switching with sub‐picosecond on‐off times. [ABSTRACT FROM AUTHOR]

Coulomb correlations can manifest in exotic properties in solids, but how these properties can be accessed and ultimately manipulated in real time is not well understood. The insulator-to-metal phase transition in vanadium dioxide (VO2) is a canonical example of such correlations. Here, few-femtosecond extreme UV transient absorption spectroscopy (FXTAS) at the vanadium M2,3 edge is used to track the insulator-to-metal phase transition in VO2. This technique allows observation of the bulk material in real time, follows the photoexcitation process in both the insulating and metallic phases, probes the subsequent relaxation in the metallic phase, and measures the phase-transition dynamics in the insulating phase. An understanding of the VO2 absorption spectrum in the extreme UV is developed using atomic cluster model calculations, revealing V3+/d² character of the vanadium center. We find that the insulator-to-metal phase transition occurs on a timescale of 26 ± 6 fs and leaves the system in a long-lived excited state of the metallic phase, driven by a change in orbital occupation. Potential interpretations based on electronic screening effects and lattice dynamics are discussed. A Mott-Hubbard-type mechanism is favored, as the observed timescales and d² nature of the vanadium metal centers are inconsistent with a Peierls driving force. The findings provide a combined experimental and theoretical roadmap for using time-resolved extreme UV spectroscopy to investigate nonequilibrium dynamics in strongly correlated materials. [ABSTRACT FROM AUTHOR]

We demonstrate a new, label-free,far-field super-resolution method based on an ultrafast pump–probescheme oriented toward nanomaterial imaging. A focused pump laserexcites a diffraction-limited spatial temperature profile, and thenonlinear changes in reflectance are probed. Enhanced spatial resolutionis demonstrated with nanofabricated silicon and vanadium dioxide nanostructures.Using an air objective, resolution of 105 nm was achieved, well beyondthe diffraction limit for the pump and probe beams and offering anovel kind of dedicated nanoscopy for materials. [ABSTRACT FROM AUTHOR]