Comparative study of graphene and metal-based grating-gate terahertz plasmonic structures

We present a terahertz (THz) Fourier spectroscopy study of plasmonic structures formed by metal- and graphene-based large active area (∼4 mm2) gratings integrated with AlGaN/GaN heterostructures of identical grating geometry. We show that the structures with metal gratings exhibit electrically tunable plasmon resonances in the THz spectral range. On the contrary, those structures with graphene gratings obtained using chemical vapor deposited (CVD) graphene transferred from a copper foil show broadband (0.5–6 THz) absorption with characteristics resembling free carrier absorption without distinct plasmonic resonances. Electrodynamic simulations confirm the excitation of the observed plasmon resonances in the metal-based grating-gate structures and the absence of plasmons in the structures with CVD graphene-based gratings. The latter is due to the relatively low conductivity of graphene transferred from the copper foil. The simulations also predict that for graphene with higher carrier mobility, the emergence of different plasmon modes is possible. The formation of a graphene-based grating-gate plasmonics crystal can be expected. This type of plasmonic structure is characterized by an additional THz plasmon mode unique to the graphene 2D grating itself. These findings advance the understanding of 2D plasmon resonances and provide practical insights into the optimization of grating-gate structures for THz radiation modulation, detection, and amplification.