GaN Mid‐IR Plasmonics: Low‐Loss Epsilon‐Near‐Zero Modes

ABSTRACT Epsilon‐near‐zero (ENZ) materials, defined by ∣Re(ε)∣ < 1, enable unique light propagation characteristics, including confinement within sub‐wavelength regions. To reduce losses in this regime, materials with both near‐zero permittivity and n < 1 refractive index, known as near‐zero‐index (NZI) materials, are desired. When both conditions are satisfied, the resulting region is classified as a low‐loss ENZ medium combining strong light confinement with reduced optical losses. To achieve this behavior in the mid‐IR, heavily doped semiconductors are required, and those compatible with current technologies are most desirable. This work provides the first in‐depth study, supported by experimental demonstrations, of the plasmonic properties of highly doped GaN thin films on Si, exhibiting low optical losses and low‐loss ENZ characteristics up to 3 µm. From the extracted optical parameters, the ENZ and NZI regions are determined and compared with the existing literature. As a result of the large polar character of nitrides, a hybridization of the surface plasmon and phonon polaritons is observed, accompanied by a flat dispersion of the high‐energy mode (pinned near the plasma frequency) indicative of its ENZ character. Establishing GaN as a viable platform for mid‐IR ENZ‐based plasmonics paves the way for integration into future infrared photonic technologies.