Tracking ultrafast solid-state dynamics using high harmonic spectroscopy

WWe establish time-resolved high harmonic generation (tr-HHG) as a powerful spectroscopy method for tracking photoinduced dynamics in strongly correlated materials through a detailed investigation of the insulator-to-metal phase transitions in vanadium dioxide. We benchmark the technique by comparing our measurements to established momentum-resolved ultrafast electron diffraction, and theoretical density functional calculations. Tr-HHG allows distinguishing of individual dynamic channels, including a transition to a thermodynamically hidden phase. In addition, the HHG yield is shown to be modulated at a frequency characteristic of a coherent phonon of the equilibrium monoclinic phase over a wide range of excitation fluences. These results demonstrate that tr-HHG is capable of tracking complex dynamics in solids through its sensitivity to the band structure

Tracking ultrafast solid-state dynamics using high harmonic spectroscopy

We establish time-resolved high harmonic generation (tr-HHG) as a powerful spectroscopy for photoinduced dynamics in strongly correlated materials through a detailed investigation of the insulator-to-metal transitions in vanadium dioxide. We benchmark our technique by comparing our measurements to established momentum-resolved ultrafast electron diffraction, and theoretical density functional calculations. Tr-HHG allows distinguishing of individual dynamic channels, including a transition to a thermodynamically hidden phase. In addition, the HHG yield is shown to be modulated at a frequency characteristic of a coherent phonon in the equilibrium monoclinic phase over a wide range of excitation fluences. These results demonstrate that tr-HHG is capable of tracking complex dynamics in solids through its sensitivity to the band structure.Comment: 20 pages and 4 figures main text, 8 pages and 4 figures supplementary informatio

Natural metamaterial behavior across the phase transition for W_xV_1−xO₂ films revealed by terahertz spectroscopy

International audienceMetamaterials, which are made of repeated patterns of appropriately arranged small discrete structures, display unusual electromagnetic properties that overwhelm those of conventional materials. The modification of their properties is generally achieved by arranging the structures mechanically or electrically and requires rather complex designs. We report on the study of the complex conductivity of epitaxially-grown tungsten-doped vanadium dioxide (WxV1−xO2) thin films through the semiconductor-to-metal phase transition (SMT) using terahertz time-domain spectroscopy. The modelling of the terahertz conductivity across WxV1−xO2 SMT provides clear insights about the gradual nucleation of VO2 metallic domains among the semiconducting host and evidences the presence of strong carrier confinement and enhanced absorption close to the transition temperature, leading to a strong capacitive response of the electrons. The evolution of the SMT is also strongly affected by W doping, which reduces the scattering time in the metallic state, lowers the transition onset temperature and extends the temperature range over which the transition occurs. The WxV1−xO2 films thus forms an effective medium in the vicinity of the SMT and display the signature of a disordered metamaterial, which has significantly enhanced functionality thanks to its readily thermally-tunable properties over a wide range of temperatures close to room temperature

Probing the role of thermal vibrational disorder in the SPT of VO 2_2 2 by Raman spectroscopy

Abstract Phase competition in transition metal oxides has attracted remarkable interest for fundamental aspects and technological applications. Here, we report a concurrent study of the phase transitions in undoped and Cr-doped VO $$_2$$ 2 thin films. The structural, morphological and electrical properties of our films are examined and the microstructural effect on the metal–insulator transition (MIT) are highlighted. We further present a distinctive approach for analyzing the Raman data of undoped and Cr-doped VO $$_2$$ 2 thin films as a function of temperature, which are quantitatively correlated to the electrical measurements of VO $$_2$$ 2 films to give an insight into the coupling between the structural phase transition (SPT) and the MIT. These data are also combined with reported EXAFS measurements and a connection between the Raman intensities and the mean Debye–Waller factors $$\sigma ^2$$ σ 2 is established. We found that the temperature dependence of the $$\sigma _{R}^{2}(V-V)$$ σ R 2 ( V - V ) as calculated from the Raman intensity retraces the temperature profile of the $$\sigma _{EXAFS}^{2}(V-V)$$ σ EXAFS 2 ( V - V ) as obtained from the EXAFS data analysis. Our findings provide an evidence on the critical role of the thermal vibrational disorder in the VO $$_2$$ 2 phase transitions. Our study demonstrates that correlating Raman data with EXAFS analysis, the lattice and electronic structural dynamics can be probed

Tracking ultrafast solid-state dynamics using high harmonic spectroscopy

International audienceWe establish time-resolved high harmonic generation (tr-HHG) as a powerful spectroscopy method for tracking photoinduced dynamics in strongly correlated materials through a detailed investigation of the insulatorto-metal phase transitions in vanadium dioxide. We benchmark the technique by comparing our measurements to established momentum-resolved ultrafast electron diffraction, and theoretical density functional calculations. Tr-HHG allows distinguishing of individual dynamic channels, including a transition to a thermodynamically hidden phase. In addition, the HHG yield is shown to be modulated at a frequency characteristic of a coherent phonon of the equilibrium monoclinic phase over a wide range of excitation fluences. These results demonstrate that tr-HHG is capable of tracking complex dynamics in solids through its sensitivity to the band structure

Ninety‐Day Stroke or Transient Ischemic Attack Recurrence in Patients Prescribed Anticoagulation in the Emergency Department With Atrial Fibrillation and a New Transient Ischemic Attack or Minor Stroke

Background For patients with atrial fibrillation seen in the emergency department (ED) following a transient ischemic attack (TIA) or minor stroke, the impact of initiating oral anticoagulation immediately rather than deferring the decision to outpatient follow‐up is unknown. Methods and Results We conducted a planned secondary data analysis of a prospective cohort of 11 507 adults in 13 Canadian EDs between 2006 and 2018. Patients were eligible if they were aged 18 years or older, with a final diagnosis of TIA or minor stroke with previously documented or newly diagnosed atrial fibrillation. The primary outcome was subsequent stroke, recurrent TIA, or all‐cause mortality within 90 days of the index TIA diagnosis. Secondary outcomes included stroke, recurrent TIA, or death and rates of major bleeding. Of 11 507 subjects with TIA/minor stroke, atrial fibrillation was identified in 11.2% (1286, mean age, 77.3 [SD 11.1] years, 52.4% male). Over half (699; 54.4%) were already taking anticoagulation, 89 (6.9%) were newly prescribed anticoagulation in the ED. By 90 days, 4.0% of the atrial fibrillation cohort had experienced a subsequent stroke, 6.5% subsequent TIA, and 2.6% died. Results of a multivariable logistic regression indicate no association between prescribed anticoagulation in the ED and these 90‐day outcomes (composite odds ratio, 1.37 [95% CI, 0.74–2.52]). Major bleeding was found in 5 patients, none of whom were in the ED‐initiated anticoagulation group. Conclusions Initiating oral anticoagulation in the ED following new TIA was not associated with lower recurrence rates of neurovascular events or all‐cause mortality in patients with atrial fibrillation