High-efficiency in situ 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-MoO3 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-MoO3 heterostructure, the phase of the polariton is predicted to be efficiently tuned from 0 to 2π in situ. Remarkably, the reflectance and transmittance through the gap between local gates can also be modulated in situ 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.

Erratum: Observation of ultrahigh mobility surface states in a topological crystalline insulator by infrared spectroscopy.

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Observation of ultrahigh mobility surface states in a topological crystalline insulator by infrared spectroscopy.

Topological crystalline insulators possess metallic surface states protected by crystalline symmetry, which are a versatile platform for exploring topological phenomena and potential applications. However, progress in this field has been hindered by the challenge to probe optical and transport properties of the surface states owing to the presence of bulk carriers. Here, we report infrared reflectance measurements of a topological crystalline insulator, (001)-oriented Pb1-x Sn x Se in zero and high magnetic fields. We demonstrate that the far-infrared conductivity is unexpectedly dominated by the surface states as a result of their unique band structure and the consequent small infrared penetration depth. Moreover, our experiments yield a surface mobility of 40,000 cm2 V-1 s-1, which is one of the highest reported values in topological materials, suggesting the viability of surface-dominated conduction in thin topological crystalline insulator crystals. These findings pave the way for exploring many exotic transport and optical phenomena and applications predicted for topological crystalline insulators.Probing optical and transport properties of the surface states in topological crystalline insulators remains a challenge. Here, Wang et al. demonstrate that the far-infrared conductivity of Pb1-x Sn x Se (x = 0.23-0.25) single crystals is dominated by the surface states where carriers show a high surface mobility of 40,000 cm2 V-1 s-1.