Speaker
Description
Metasurfaces, planar arrays of engineered subwavelength structures, enable unprecedented control over light’s amplitude, phase, and polarization, driving innovations in imaging, sensing, and active nanophotonics. For the optical characterization of these structures, tabletop optical parametric oscillators (OPOs) and amplifiers (OPAs) are widely used, as they provide rich information on linear light-matter interactions. However, pushing metasurfaces into extreme, non-perturbative regimes, particularly within the mid-infrared (mid-IR) and terahertz (THz) spectral windows, exposes fundamental physical bottlenecks in tabletop systems. Specifically, generating mid-IR/THz pulses via solid-state down-conversion is severely limited by the Manley-Rowe energy conservation constraint and the intrinsic phonon-absorption gaps (Reststrahlen bands) of the mixing crystals. Free-electron lasers (FELs) bypass these fundamental material limits. By utilizing relativistic free electrons undulating in a vacuum, FELs offer gapless tunability and natively high pulse energies across the entire polaritonic spectrum. Crucially, the unique macro/micropulse burst architecture of the FEL delivers the extreme peak fields required to trigger nonlinear phenomena, while allowing sufficient thermal relaxation to prevent the cumulative structural melting inherent to high-repetition-rate tabletop oscillators. These attributes make FELs uniquely suited for probing metasurfaces under extreme optical conditions. This perspective demonstrates how FEL-driven time-resolved pump-probe spectroscopy can be utilized to capture sub-picosecond carrier dynamics, plasmon dephasing, and biexponential electron-phonon relaxation. By leveraging these capabilities, researchers can map plasmonic hotspots, quantify fabrication-induced asymmetry, and assess the fundamental operational speed limits of ultrafast optical switches. Despite their limited global availability, FEL facilities offer transformative opportunities to advance metasurface science, providing a versatile platform for exploring the fundamental boundaries of light-matter interactions under extreme non-equilibrium conditions.
| Scientific Topics | Solid State Physics |
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