Modulation of THz transmission in AlGaN/GaN-based plasmonic crystals

We investigate two-dimensional (2D) plasmon resonances in AlGaN/GaN grating-gate plasmonic crystals under both linear (electric field of incoming radiation E<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</inf> < 0.1 kV/cm) and nonlinear (E<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</inf> ≈ 1–5 kV/cm) excitation regimes. THz Fourier transform infrared spectroscopy demonstrates that in the linear regime, THz transmission spectra can be electrically modulated by the excitation of different plasmon resonance modes. We identify delocalized plasmon (DLP) and localized plasmon (LP) modes, which arise under weak and strong modulation of the 2D electron gas (2DEG) concentration profile, respectively. Both modes are electrically controllable by gate voltage applied to the grating electrode.In the nonlinear regime, we observe a strong optically selfinduced modulation of THz transmission, with a maximum relative transmission change of 45% at a pump fluence as low as 350 nJ/cm<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup>, occurring under resonant excitation of the LP mode. This modulation exhibits an ultrafast relaxation time of ~15 ps. Our experimental results are supported by electrodynamic modeling, which takes into account the near-field localization under the grating electrodes, 2DEG heating, and the temperaturedependent effective electron mass. These findings provide insight into the fundamental nonlinear dynamics of plasmonic crystals and their potential for ultrafast THz modulation applications.