Unconventional quantization of 2D plasmons in cavities formed by gate slots

TL;DR

This paper reveals an unconventional mode quantization of 2D plasmons in gate slot cavities, showing a unique resonance condition and weak decay, with potential for efficient plasmonic absorption.

Contribution

It introduces a novel quantization rule for 2D plasmons in gate slots, differing from traditional optical cavities, due to a non-trivial phase shift upon reflection.

Findings

Resonant modes occur when slot width equals an eighth of the plasmon wavelength.

Lowest resonance is at one eighth of the wavelength, unlike conventional half-wavelength cavities.

Absorption cross-section reaches about 50% of the fundamental dipole limit without matching.

Abstract

We demonstrate that the slot between parallel metal gates placed above 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 $n=0, 1, 2 \ldots$. 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 non-trivial 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. 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.