We experimentally investigate the terahertz spectrum of plasma excitations in a plasmonic crystal based on AlGaN/GaN two-dimensional electron system (2DES). While screened plasmon modes with linear dispersion are readily observed in such structures, unscreened modes localized in the slots between the gates have remained experimentally elusive. We discover this slot plasma excitation exhibiting square-root dispersion. It turns out that these slot plasmons follow an unconventional wave-vector quantization rule, $q_u=(N+1/4)\times \pi /l_u$ for even integers $N$, and require the excitation condition $q_{u}h\ll 1$, where $h$ is the gate-to-2DES distance and $l_u$ is the slot width. We develop an analytical model that accurately captures the observed dispersion and relaxation, revealing a nontrivial $-\pi /4$ phase shift upon plasmon reflection at the gate edge. Remarkably, the slot plasmons persist up to room temperature, thereby enabling a broad range of opportunities for the advancement of plasmonic devices.

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• Plasma oscillations
• Plasma waves
• 2-dimensional systems
• Terahertz time-domain spectroscopy