Title: Graphene polaritonic crystal
Description: Photonic crystals are media with periodically varying optical properties. Photonic crystals enable exquisite control of light propagation in integrated optical circuits and also emulate advanced physical concepts. However, common photonic crystals directly pattern the optical medium and thus are unfit for in-operando on/off controls. In this dissertation, we introduced, fabricated, and studied the properties of graphene polaritonic crystals. Our polaritonic crystal system consists of a pristine sheet of graphene in a back-gated platform with nano-structured gate insulators. We employed scattering-type scanning near-field optical microscopy (s-SNOM) to study the novel properties of polaritons propagating in the polaritonic crystal. We demonstrated the formation of a polaritonic bandgap, variations of the polaritonic local density of states, and the emergence of polaritonic domain wall states. We also revealed the programmable control of the polariton propagation direction and reconstructed the polaritonic bandstructure from real-space polariton images.

The exploration of topological polaritonic phenomena in the polaritonic crystal relies on the selective excitation of topologically non-trivial modes using a chiral polariton launcher. We searched for the design of an efficient chiral polariton launcher. Throughout the journey, we visualized the polaritonic vortex mode of hBN phonon-polaritons. We discovered that the optical spin angular momentum of hBN phonon-polaritons resembles nano-scale meron spin textures. The meron spin texture possesses a half-integer topological charge determined by the handedness of the incident beam. The polaritonic crystal platform studied in this dissertation sheds light on the exploration of topologically non-trivial polaritonic states, such as valley plasmons and topological edge states. In addition, our electrostatically-tunable polaritonic crystals are derived from standard metal oxide semiconductor field-effect transistor technology and pave a way for practical on-chip light manipulation.