Photonic crystals are commonly implemented in 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 are unfit for in-operando on/off controls. We overcome this limitation and demonstrate a broadly tunable two-dimensional photonic crystal for surface plasmon polaritons. Our platform consists of a continuous graphene monolayer integrated in a back-gated platform with nano-structured gate insulators. Infrared nano-imaging reveals the formation of a photonic bandgap and strong modulation of the local plasmonic density of states that can be turned on/off or gradually tuned by the applied gate voltage. We also implement an artificial domain wall which supports highly confined one-dimensional plasmonic modes. Our electrostatically-tunable photonic crystals are derived from standard metal oxide semiconductor field effect transistor technology and pave a way for practical on-chip light manipulation.

A valley plasmonic crystal for graphene surface plasmons is proposed. We demonstrate that a designer metagate, placed within a few nanometers of graphene, can be used to impose a periodic Fermi energy landscape on graphene. For specific metagate geometries and bias voltages, the combined metagate-graphene structure is shown to produce complete propagation band gaps for the plasmons, and to impart them with nontrivial valley-linked topological properties. Sharply curved domain walls between differently patterned metagates are shown to guide highly localized plasmons without any reflections owing to suppressed intervalley scattering. Our approach paves the way for nonmagnetic and dynamically reconfigurable topological nanophotonic devices.

Polarization is one of the important properties of light, and its detection is of significant interest for various fundamental and practical applications. We demonstrate a mid-infrared polarimetry device using a gatetunable graphene-integrated anisotropic metasurface. The Stokes parameters of the incident light are extracted by sweeping the gate voltage applied to the device and subsequent fitting of the measured reflected intensities. Considering subpicosecond carrier relaxation times in graphene, the polarization measurement rate of our device is governed only by the speed of the gate voltage sweep. Thus, our work serves as a proof-of-principle demonstration for high-speed microscale polarimetric devices.

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A range of tetrahydropyrans and piperidines were produced by Fe(III)-catalyzed intramolecular hydroalkoxylation and hydroamination reactions of allenes. Various Fe catalysts with different counterions were tested. Their activities toward allene and alkene activation depended sensitively on their counterion and reaction conditions. Mechanistic study of the reaction intermediates found a new reaction pattern involving the Fe catalysts and diene substrates.