Many correlated systems feature an insulator-to-metal transition that can be triggered by an electric field. Although it is known that metallization takes place through filament formation, the details of how this process initiates and evolves remain elusive. We use in-operando optical reflectivity to capture the growth dynamics of the metallic phase with space and time resolution. We demonstrate that filament formation is triggered by nucleation at hotspots, with a subsequent expansion over several decades in time. By comparing three case studies (VO2, V3O5 and V2O3), we identify the resistivity change across the transition as the crucial parameter governing this process. Our results provide a spatiotemporal characterization of volatile resistive switching in Mott insulators, key for emerging technologies such as optoelectronics or neuromorphic computing.

Until recently, there was no efficacious treatment for acute cerebral ischemia. As a result, the role of neuroimaging and the radiologist was peripheral in the diagnosis and management of this disease. The demonstration of efficacy using thrombolysis has redefined this role, with the success of intervention becoming increasingly dependent on timely imaging and accurate interpretation. The potential benefits of intervention have only begun to be realized. In this State-of-the-Art review of imaging of acute stroke, the role of imaging in the current and future management of stroke is presented. The role of computed tomography is emphasized in that it is currently the most utilized technique, and its value has been demonstrated in prospective clinical trials. Magnetic resonance techniques are equally emphasized in that they have the potential to provide a single modality evaluation of tissue viability and vessel patency in an increasingly rapid evaluation.

Background and Purpose-Cerebral ischemia has been proposed as contributing mechanism to secondary neuronal injury after intracerebral hemorrhage (ICH). Possible tools for investigating this hypothesis are diffusion-weighted (DWI) and proton magnetic resonance spectroscopic imaging ( 1 H-MRSI). However, magnetic field inhomogeneity induced by paramagnetic blood products may prohibit the application of such techniques on perihematoma tissue. We report on the feasibility of DWI and 1 H-MRSI in the study of human ICH and present preliminary data on their contribution to understanding perihematoma tissue functional and metabolic profiles. Methods-Patients with acute supratentorial ICH were prospectively evaluated using DWI and 1 H-MRSI. Obscuration of perihematoma tissue with both sequences was assessed. Obtainable apparent diffusion coefficient (Dav) and lactate spectra in perihematoma brain tissue were recorded and analyzed. Results-Nine patients with mean age of 63.4 (36 to 87) years were enrolled. Mean time from symptom onset to initial MRI was 3.4 (1 to 9) days; mean hematoma volume was 35.4 (5 to 80) cm 3 . Perihematoma diffusion values were attainable in 9 of 9 patients, and 1 H-MRSI measures were obtainable in 5 of 9 cases. Dav in perihematoma regions was 172.5 (120.0 to 302.5)ϫ10Ϫ5 mm 2 /s and 87.6 (76.5 to 102.1)ϫ10 Ϫ5 mm 2 /s in contralateral corresponding regions of interest (Pϭ0.002). One patient showed an additional area of reduced Dav with normal T 2 intensity, which suggests ischemia.1 H-MRSI revealed lactate surrounding the hematoma in 2 patients. Conclusions-DWI and 1 H-MRSI can be used in the study of ICH patients. Our preliminary data are inconsistent with ischemia as the primary mechanism for perihematoma tissue injury. Further investigation with advanced MRI techniques will give a clearer understanding of the role that ischemia plays in tissue injury after ICH. (Stroke. 2000;31:726-732.)

Dielectric-resonator-based nanophotonic devices show promise owing to their low intrinsic losses, support of multipolar resonances, and efficient operation in both reflection and transmission configurations. A key challenge is to make such devices dynamically switchable, such that optical behavior can be instantaneously reconfigured. In this work we experimentally demonstrate large, broadband, and continuous electrical tuning of reflection resonances in hybrid dielectric–VO2 devices. Our calculations, in strong agreement with experimental reflectance measurements, also indicate the presence of large transmission and absorption modulation. We additionally demonstrate independent modulation of both reflection amplitude and phase at Fabry–Pérot anti-nodes and nodes, respectively, a key requirement for metasurface design. We conclude with a temporal characterization, in which we achieve rapid electronic modulation rates of approximately 3 kHz, substantially faster than other recent approaches. These findings greatly expand the potential of designing nanophotonic devices that exploit the tunable behavior of hybrid dielectric–VO2 resonators.