Extreme Terahertz Nonlinearity of AlGaN/GaN‐Based Grating‐Gate Plasmonic Crystals

Abstract A novel approach is presented to enhance terahertz (THz) nonlinear absorption by resonant excitation of 2D plasmons in grating‐gate plasmonic crystals. Using THz pump‐probe experiments, the nonlinear interaction of spectrally narrow THz pulses is investigated with plasmonic oscillations in a 2D electron gas of an AlGaN/GaN grating‐gate structures. Nonlinear effects are observed as ultrafast, pump‐induced changes in THz transmission, with relative transparency strongly dependent on plasmonic mode excitation and saturating at pump fluences of ≈200 nJ cm⁻². The strongest relative transparency, reaching 45% at 350 nJ cm⁻², occurs under resonant excitation of a localized plasmon mode at the strong electrostatic modulation of 2DEG concentration. Transient dynamics reveal ultrafast relaxation times of 15–20 ps, the effects can be observed even at elevated temperatures of up to 150 K. A nonlinear model of the plasmonic crystal, based on finite‐difference time‐domain electrodynamic simulations coupled with viscous hydrodynamic electron transport model, elucidates key nonlinear mechanisms, including near‐field effects under metallic gratings, electron heating, plasmon resonance broadening, and redshift. These results demonstrate that even conventional semiconductors such as AlGaN/GaN can achieve nonlinear THz responses comparable to, or exceeding, those of graphene, showing strong potential for ultrafast THz modulation and nonlinear photonic applications.