Bidirectional terahertz frequency conversion via structural resonances at a plasma time boundary.

A time boundary, an abrupt temporal change in material refractive index that conserves wave vector while shifting optical frequency, offers a powerful route to dynamic spectral control. Prior demonstrations of time boundaries in Drude-like materials have predominantly produced simple, unidirectional frequency shifts. Here, we experimentally realize an ultrafast time boundary for terahertz waves at an interface between air and a laser-induced plasma that exhibits a Lorentzian resonant response via a localized surface-plasmon mode. The boundary is created by converting air into a cylindrical plasma column within 100 femtoseconds, enabling strong, sub-cycle index modulation. We observe time refraction with unconventional bidirectional (simultaneous red- and blue-shifted) frequency conversion at a single boundary. A simple model that couples a tunneling-ionization description of plasma formation with Lorentzian dispersion quantitatively reproduces the measured spectra. By precisely tuning the delay between plasma creation and terahertz-wave arrival, we resolve the spectral evolution in time and directly confirm time refraction as a fundamental sub-cycle effect. These results establish Lorentzian time boundaries as an experimental platform for rich temporal light-matter interactions and lay groundwork for dynamic terahertz photonics, including temporally reconfigurable spectral elements, amplification schemes, and building blocks for photonic time crystals and spatiotemporal metamaterials.