We demonstrate that the slot between parallel metal gates placed above a two-dimensional electron system (2DES) forms a plasmonic cavity with unconventional mode quantization. The resonant plasmon modes are excited when the slot width $L$ and the plasmon wavelength $\lambda$ satisfy the condition $L=\lambda /8+n\times \lambda /2$, where $n=0,1,2\dots$. The lowest resonance occurs at a surprisingly small cavity size, specifically one eighth of the plasmon wavelength, which contrasts with the conventional half-wavelength Fabry-Perot cavities in optics. This unique quantization rule arises from a nontrivial phase shift of $-\pi /4$ acquired by the 2D plasmon upon reflection from the edge of the gate. The slot plasmon modes exhibit weak decay into the gated 2DES region, with the decay rate being proportional to the square root of the separation between the gate and the 2DES. The absorption cross section by such slots reaches $\sim 50\%$ of the fundamental dipole limit without any matching strategies and is facilitated by field enhancement at the metal edges.

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• Electrical conductivity
• Plasma optics
• Plasmonics
• Refraction
• Two-dimensional electron system
• Electromagnetic wave theory
• Finite-element method
• Optical conformal mapping