Imaging Nanometer Phase Coexistence at Defects During the Insulator-Metal Phase Transformation in VO2 Thin Films by Resonant Soft X-ray Holography.

We use resonant soft X-ray holography to image the insulator-metal phase transition in vanadium dioxide with element and polarization specificity and nanometer spatial resolution. We observe that nanoscale inhomogeneity in the film results in spatial-dependent transition pathways between the insulating and metallic states. Additional nanoscale phases form in the vicinity of defects which are not apparent in the initial or final states of the system, which would be missed in area-integrated X-ray absorption measurements. These intermediate phases are vital to understand the phase transition in VO2, and our results demonstrate how resonant imaging can be used to understand the electronic properties of phase-separated correlated materials obtained by X-ray absorption.

Tracking the insulator-to-metal phase transition in VO2 with few-femtosecond extreme UV transient absorption spectroscopy.

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.

Geometric constraints on phase coexistence in vanadium dioxide single crystals.

The appearance of stripe phases is a characteristic signature of strongly correlated quantum materials, and its origin in phase-changing materials has only recently been recognized as the result of the delicate balance between atomic and mesoscopic materials properties. A vanadium dioxide (VO2) single crystal is one such strongly correlated material with stripe phases. Infrared nano-imaging on low-aspect-ratio, single-crystal VO2 microbeams decorated with resonant plasmonic nanoantennas reveals a novel herringbone pattern of coexisting metallic and insulating domains intercepted and altered by ferroelastic domains, unlike previous reports on high-aspect-ratio VO2 crystals where the coexisting metal/insulator domains appear as alternating stripe phases perpendicular to the growth axis. The metallic domains nucleate below the crystal surface and grow towards the surface with increasing temperature as suggested by the near-field plasmonic response of the gold nanorod antennas.

Control of plasmonic nanoantennas by reversible metal-insulator transition.

We demonstrate dynamic reversible switching of VO2 insulator-to-metal transition (IMT) locally on the scale of 15 nm or less and control of nanoantennas, observed for the first time in the near-field. Using polarization-selective near-field imaging techniques, we simultaneously monitor the IMT in VO2 and the change of plasmons on gold infrared nanoantennas. Structured nanodomains of the metallic VO2 locally and reversibly transform infrared plasmonic dipole nanoantennas to monopole nanoantennas. Fundamentally, the IMT in VO2 can be triggered on femtosecond timescale to allow ultrafast nanoscale control of optical phenomena. These unique features open up promising novel applications in active nanophotonics.

Super-resolution in label-free photomodulated reflectivity.

We demonstrate a new, label-free, far-field super-resolution method based on an ultrafast pump-probe scheme oriented toward nanomaterial imaging. A focused pump laser excites a diffraction-limited spatial temperature profile, and the nonlinear changes in reflectance are probed. Enhanced spatial resolution is demonstrated with nanofabricated silicon and vanadium dioxide nanostructures. Using an air objective, resolution of 105 nm was achieved, well beyond the diffraction limit for the pump and probe beams and offering a novel kind of dedicated nanoscopy for materials.

Instantaneous band gap collapse in photoexcited monoclinic VO2 due to photocarrier doping.

Using femtosecond time-resolved photoelectron spectroscopy we demonstrate that photoexcitation transforms monoclinic VO2 quasi-instantaneously into a metal. Thereby, we exclude an 80 fs structural bottleneck for the photoinduced electronic phase transition of VO2. First-principles many-body perturbation theory calculations reveal a high sensitivity of the VO2 band gap to variations of the dynamically screened Coulomb interaction, supporting a fully electronically driven isostructural insulator-to-metal transition. We thus conclude that the ultrafast band structure renormalization is caused by photoexcitation of carriers from localized V 3d valence states, strongly changing the screening before significant hot-carrier relaxation or ionic motion has occurred.

Tunable optical antennas enabled by the phase transition in vanadium dioxide.

Optical antennas, subwavelength metallic structures resonating at visible frequencies, are a relatively new branch of antenna technology being applied in science, technology and medicine. Dynamically tuning the resonances of these antennas would increase their range of application and offer potential increases in plasmonic device efficiencies. Silver nanoantenna arrays were fabricated on a thin film of the phase change material vanadium dioxide (VO(2)) and the resonant wavelength of these arrays was modulated by increasing the temperature of the substrate above the critical temperature (approximately 68 °C). Depending on the array, wavelength modulation of up to 110 nm was observed.

Ultra-compact silicon photonic devices reconfigured by an optically induced semiconductor-to-metal transition.

Vanadium dioxide (VO(2)) is a promising reconfigurable optical material and has long been a focus of condensed matter research owing to its distinctive semiconductor-to-metal phase transition (SMT), a feature that has stimulated recent development of thermally reconfigurable photonic, plasmonic, and metamaterial structures. Here, we integrate VO(2) onto silicon photonic devices and demonstrate all-optical switching and reconfiguration of ultra-compact broadband Si-VO(2) absorption modulators (L < 1 µm) and ring-resonators (R ~ λ(0)). Optically inducing the SMT in a small, ~0.275 µm(2), active area of polycrystalline VO(2) enables Si-VO(2) structures to achieve record values of absorption modulation, ~4 dB µm(-1), and intracavity phase modulation, ~π/5 rad µm(-1). This in turn yields large, tunable changes to resonant wavelength, |Δλ(SMT)| ~ 3 nm, approximately 60 times larger than Si-only control devices, and enables reconfigurable filtering and optical modulation in excess of 7 dB from modest Q-factor (~10(3)), high-bandwidth ring resonators (>100 GHz). All-optical integrated Si-VO(2) devices thus constitute platforms for reconfigurable photonics, bringing new opportunities to realize dynamic on-chip networks and ultrafast optical shutters and modulators.

Photothermal optical modulation of ultra-compact hybrid Si-VO2 ring resonators.

We demonstrate photothermally induced optical switching of ultra-compact hybrid Si-VO2 ring resonators. The devices consist of a sub-micron length ~70 nm thick patch of phase-changing VO2 integrated onto silicon ring resonators as small as 1.5 µm in radius. The semiconductor-to-metal transition (SMT) of VO2 is triggered using a 532 nm pump laser, while optical transmission is probed using a tunable cw laser near 1550 nm. We observe optical modulation greater than 10dB from modest quality-factor (~10³) resonances, as well as a large -1.26 nm change in resonant wavelength Δλ, resulting from the large change in the dielectric function of VO2 in the insulator-to-metal transition achieved by optical pumping.