High-efficiency amplitude and phase control of infrared light using topological polaritons.

Polaritons - material excitation coupled with light - are thought to hold the potential for the extreme control of light down to the atomic length scale because of their high field confinement and sub-wavelength scales. For practical applications, it is essential but still a formidable challenge to manipulate polaritons with high efficiency and a wide tunable range. These obstacles may be overcome by the topology of polaritons. In photonic systems composed of graphene/α-MoO heterostructures, the topology of the hybrid polariton characterized by the isofrequency curve can transform from open hyperbolas to closed ellipse-like curves, driven by the carrier concentrations of graphene. The electronic tunability of such topological polaritons offers a unique platform for two-dimensional energy transfer. Here, by introducing local gates to obtain a tunable spatial carrier density profile in the graphene/α-MoO heterostructure, the phase of the polariton is predicted to be efficiently tuned from 0 to 2π . Remarkably, the reflectance and transmittance through the gap between local gates can also be modulated from 0 to 1 with high efficiency, where the device length can be less than 100 nm. The modulation is achieved owing to the dramatic changes in the wave vector of polaritons near the topological transition point. The proposed structures not only have direct applications in two-dimensional optics such as total reflectors, phase (amplitude) modulators, and optical switches but also can serve as an important component for complex nano-optical devices.