Discovery of Slot Plasma Excitations in a AlGaN/GaN Plasmonic Crystal

TL;DR

This study uncovers and characterizes slot plasma excitations in AlGaN/GaN plasmonic crystals, revealing their unique dispersion and quantization, with potential applications in room-temperature plasmonic devices.

Contribution

The paper reports the first experimental observation of unscreened slot plasmons with unconventional dispersion and quantization in AlGaN/GaN plasmonic crystals, supported by an analytical model.

Findings

Discovery of slot plasma excitations with square-root dispersion.

Unconventional wave-vector quantization rule for slot plasmons.

Slot plasmons persist up to room temperature.

Abstract

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 the plasmonic crystals, the existence of unscreened modes localized in the slots between the gates has remained unobserved until now. We discover this slot plasma excitation exhibiting square-root dispersion. It turned 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 condition for excitation $q_u h \ll 1$, where $h$ is the gate-to-2DES distance and $l_u$ is the slot width. We develop an analytical model that accurately captures the found dispersion and relaxation, revealing a non-trivial $-\pi/4$ phase shift upon plasmon reflection at the gate edge. Experiments demonstrate that the slot plasmons persist up to room temperature, thereby enabling a broad range of opportunities for the advancement of plasmonic devices.