Observation of Polaritonic Topological Nodal Line States in Natural Materials

Photonic topological systems have displayed significant potential for various applications, including robust wave propagation, topological lasers, non-Hermitian photonics, and nonlinear topological phases. A promising direction in the field of topological photonics is the exploration of interactions between photons and a diverse range of material excitations, such as excitons, plasma, and optical phonons. These interactions can give rise to half-light-half-matter quasiparticles called polaritons. Recent studies have unveiled the formation of topological exciton/phonon polaritons resulting from the strong coupling between photonic topological modes and excitons or optical phonons. However, most investigations into topological polaritons have focused on artificial structures or a combination of artificial topological structures with natural materials. These studies often require precise fabrication of sophisticated artificial structures, thus posing challenges for their practical implementation in real-world applications.<br/><br/>In this study, we report the observation of polaritonic topological nodal lines supported by unpatterned natural materials and experimentally characterize the associated surface polaritonic waves using scattering-type scanning near-field optical microscopy (s-SNOM). In electronic materials, topological nodal lines are manifested as one-dimensional lines in the electronic band structure, where the energy levels are doubly degenerate in three-dimensional momentum space, exhibiting a Berry phase of π. These nodal lines are topologically protected under specific spatial symmetries, rendering them robust against perturbations when these symmetries are preserved. The discovery of topological nodal lines has opened exciting opportunities for designing and synthesizing materials with intriguing electronic and optical properties. However, due to technical limitations and the delicate nature of their topology, the exploration of topological nodal lines in condensed matter experiments has been limited. More recently, the concept of topological nodal lines has been extended to classical wave systems, leading to a surge of interest in characterizing these modes and investigating their unique properties. Notably, the study of topological nodal lines in photonics and metamaterials has emerged as a rapidly growing field within topological photonics in recent years.<br/><br/>Through theoretical exploration and simulations, we have discovered that calcite crystal offers an outstanding platform for demonstrating polaritonic topological nodal line states in natural materials. The topological nodal line originates from a two-fold degeneracy within the upper Reststrahlen band of calcite when the crystal surfaces are oriented along the plane (001), forming angles <i>θ</i> of 90° with respect to the optical axis. At the interface of calcite, this topological nodal line gives rise to a novel type of topological surface polaritons, whose dispersion in momentum-energy space can be effectively characterized using s-SNOM. Additionally, we experimentally demonstrate optical spin-momentum locking for these new surface polaritons, with their propagation direction steered effortlessly through the handedness of circularly polarized excitations.