Mode-order conversion in a Mach-Zehnder interferometer based on Chern insulators.

Mode-order conversion devices can provide a flexible platform to achieve mode coupling and optimizing in mode division multiplex (MDM) that can eliminate the restrain of capacity and density in photonic integration and communication. However, mode-order converters based on traditional photonic crystal (PC) waveguides are susceptible to defects, which always render device incapacitation in mode-order conversion. Herein, a mode converter designed by the Mach-Zehnder interferometer (MZI) structure is proposed to manipulate the conversion of topological edge states (TESs) based on Chern insulators consisting of gyromagnetic PCs. The back-and-forth conversion between fundamental and high-order modes is numerically demonstrated based on phase modulation in our proposed device, in which each mode can be immune to defects. This unique approach for converting the mode order of TES exploits a new perspective in MDM to design a high-performance multimode device, leading to potential applications in photonic integrated circuits (PIC), on-chip processors, and optical fiber communication.

Three-Dimensional Strawlike MoSe2-Ni(Fe)Se Electrocatalysts for Overall Water Splitting.

The development of efficient and low-cost transition-metal electrocatalysts is of great significance for hydrogen production from water splitting. Herein, we synthesized three-dimensional strawlike MoSe2-NiSe composed of microrods on nickel foam (NF) by a one-step hydrothermal reaction. The as-prepared MoSe2-NiSe/NF exhibited effective hydrogen evolution reaction (HER) activity (low overpotential of 79 mV at 10 mA cm-2 and stability of 21 h in 1 M KOH), benefiting from the large electrochemically active area provided by strawlike structures, proper Se content, and synergistic effect of active phases. The enhanced oxygen evolution reaction (OER) activity (the low overpotential of 217 mV at 10 mA cm-2 and maintaining stability for 47 h in 1 M KOH) was further observed for Fe-doped MoSe2-NiSe/NF (MoSe2-NiFeSe/NF) prepared by facile soaking, which can be mainly ascribed to optimized active phases formed on the OER process after Fe doping. The two-electrode system (MoSe2-NiSe/NF||MoSe2-NiFeSe/NF) requires a low cell voltage of 1.54 V to obtain a current density of 10 mA cm-2 in 1 M KOH, which provides an interesting idea for constructing an effective overall water splitting system.