Strong Terahertz Nonlinearity in GaN-Based Two-Dimensional Plasmonic Structures

We report on the nonlinear optical response of AlGaN/GaN-based grating-gate plasmonic crystals under high- intensity THz pulses. For the THz pump-probe measurements, a free-electron laser (FEL) was tuned to the plasmon resonance frequency (1.84 THz). Our experiments revealed substantial pump-induced changes in THz transmission, with ΔT/T reaching up to 45% and rapid relaxation times not exceeding 15 ps. It makes these structures highly promising for fast THz switching applications. To analyze experimental results we performed the theoretical modeling grounded in a nonlinear theory that couples Maxwell's equations with electron transport equations within a viscous hydrodynamic framework. The model accurately predicts the observed experimental results accounting for key nonlinear processes, including electron acceleration, electron heating, and the spatial inhomogeneity of near-fields, which affect the plasmonic resonances. Numerical simulations, performed using a custom-developed finite-difference time-domain algorithm, showed good agreement with the experimental data.