Mid-infrared long-range surface polaritons in two-dimensional van der Waals materials.

The discovery of graphene plasmons (GPs) and hyperbolic phonon polaritons (HPhPs) in two-dimensional (2D) van der Waals (vdW) materials has enabled extreme light confinement and enhanced light-matter interactions, holding the promise for miniaturized mid-infrared (mid-IR) photonic devices. However, GPs and HPhPs suffer from limited propagation lengths, hindering their impact in various applications. Here, we demonstrate long-range surface polaritons (LRSPs) in 2D vdW materials, featuring much longer propagation lengths and faster group velocities, which may offer complementary opportunities for high-speed on-chip photonic applications in the mid-IR regime. The demonstrated LRSPs are supported by deep-subwavelength heterostructures consisting of a hexagonal boron nitride (h-BN) flake, a high-index germanium (Ge) layer, and a gold (Au) film. Within such geometry, we experimentally demonstrate propagation distances exceeding 80 micrometers crossing an entire h-BN flake without substantial decay. A theoretical prediction of ∼925-micrometer propagation length is obtained. These polaritons may be ideally suited for realizing polaritonic interconnects that bridge different components of compact and planar photonic systems, enabling mid-IR information transport and seamless integration with other vdW material-based mid-IR photonic devices.