Coupling of far-infrared light to the surface phonon-polaritons in Transition metal dichalcogenides

Abstract

Enhanced light-matter interactions through a plethora of dipole-typepolaritonic excitations started to emerge in two-dimensional (2D) layered materials in recent years. 2D Van der Waals (vdWs) polar crystals sustaining phonon-polaritons (PhPs) have opened up new avenues for fundamental research and optoelectronic applications within the mid-infrared (mid-IR) to terahertz ranges. One of the fundamental hurdles in polaritonics is the trade-off between electromagnetic field confinement and the coupling efficiency with free-space light, a consequence of the large momentum mismatch between the excitation source and polaritonic modes. Our group recently demonstrated the fundamental problem of momentum mismatch can be overcome with a graphene acoustic plasmon resonator with nearly perfect absorption (94%) of incident mid-infrared light. This high efficiency is achieved by utilizing a two-stage coupling scheme: free-space light coupled to conventional graphene plasmons, which then couple to ultraconfined acoustic plasmons. To date, experimental demonstration of excitation of the surface phonon-polaritons (SPhPs) in transition metal dichalcogenides (TMDs) remains unexplored, so here we demonstrate novel strategies for dynamically controlling of far-infrared light using unique optical properties of TMDs in particular ZrS2. In comparison to other vdW materials, the phonon modes of these TMDs are much softer and exhibits phonon peaks within the far-infrared (far-IR) regime. Here we experimentally demonstrate the excitation of SPhPs in ZrS2 acoustic resonator by the far-IR absorption spectroscopy. The surface optical (SO) phonon modes of these TMDs were tuned and tailored to lie anywhere within the reststrahlen band, by controlling the ribbon width, which brings extreme tunability. In addition, the absorption can be pushed beyond 90% with the integration of gold reflectors and can become promising material for far-IR biosensing. The results demonstrate TMDs as a new platform for studying phonon-polaritons exhibiting good quality factors and excellent tunability which enable far-IR nanophotonics devices. Recently our group demonstrated ultra strong coupling (USC) between polar phonons and mid-IR light in coaxial nanocavities. Here we push the boundary further up to the far-IR range and we propose to demonstrate USC coupling between polar phonons and far-IR light using the coaxial nanocavity platform. Our numerical simulation predicts a level splitting of strongly coupled polaritons of 95% of the resonant frequency, enabled by epsilon-near-zero (ENZ) responses in the far-IR of the ZrS2 filled coaxial nanocavities. The ability to reach the USC regime in mass-produced nanocavity systems can open up new avenues to explore non-perturbatively coupled light-matter systems, multiphoton effects, as well as higher-order nonlinear effects, which may lead to novel applications in sensing, spectroscopy, and nanocavity optomechanics.