Following the Nonthermal Phase Transition in Niobium Dioxide by Time-Resolved Harmonic Spectroscopy

Area of Science:

• Condensed Matter Physics
• Materials Science
• Ultrafast Spectroscopy

Background:

• Photoinduced phase transitions in correlated materials are crucial for applications like ultrafast switches and optoelectronics.
• Temporal resolution of these transitions is challenging for existing experimental methods.
• Extreme nonlinear optics offers high sensitivity and temporal resolution for probing phase changes.

Purpose of the Study:

• To investigate the ultrafast transition from semiconducting to metallic phases in polycrystalline thin-film NbO_{2}.
• To demonstrate femtosecond harmonic probing of nonthermal dynamics in solids.

Main Methods:

• Time-resolved harmonic spectroscopy was employed to study NbO_{2}.
• Excitation fluence was varied to identify transition thresholds.
• Ab initio simulations and a 1D chain model supported the experimental observations.

Main Results:

• A steplike suppression in harmonic emission was observed above a threshold fluence (∼11-12 mJ/cm^{2}).
• This suppression indicates a nonthermal emergence of the metallic phase within 100±20 fs.
• The results were consistent with theoretical simulations.

Conclusions:

• Femtosecond harmonic spectroscopy is a viable method for probing ultrafast phase transitions.
• Nonthermal dynamics in solids can be resolved with high temporal resolution.
• This research advances the understanding of photoinduced phase transitions in correlated materials.