Infrared Nanoimaging Reveals the Surface Metallic Plasmons in Topological Insulator

Surface plasmons make a high degree of localization of electromagnetic fields achievable at the vicinity of metal surfaces. Topological insulators (TIs) are a family of materials which are insulating in the bulk but have metallic surfaces caused by the strong spin–orbit coupling. Surface plasmons supported by the surface state on topological insulators have attracted incredible interests from ultraviolet to mid-infrared frequencies. In this work, we experimentally investigate the near-field properties of Bi₂Te₃ nanosheets using scattering-type scanning near-field optical microscopy (s-SNOM). The s-SNOM tip enables to detect significantly enhanced intensity in its near field at precisely controlled positions with regards to Bi₂Te₃ structure. With the help of highly position-selective excitation and high-pixel real-space mapping, we discover near-field patterns of bright outside fringes which are associated with its surface-metallic, plasmonic behavior at mid-infrared frequency. Thereby, we experimentally demonstrate that the scattered signal responses and near-field amplitudes of outside fringes can be tailored via mechanical (sheet thickness of Bi₂Te₃), electric (electrostatic gating), and optical (incident wavelength) fashions. The discovery of outside fringes in TI nanosheets may enable the development of strongly enhanced light–matter interactions for quantum optical devices, mid-infrared (MIR) and terahertz detectors or sensors

Highly Efficient and Air-Stable Infrared Photodetector Based on 2D Layered Graphene–Black Phosphorus Heterostructure

The presence of a direct band gap and high carrier mobility in few-layer black phosphorus (BP) offers opportunities for using this material for infrared (IR) light detection. However, the poor air stability of BP and its large contact resistance with metals pose significant challenges to the fabrication of highly efficient IR photodetectors with long lifetimes. In this work, we demonstrate a graphene–BP heterostructure photodetector with ultrahigh responsivity and long-term stability at IR wavelengths. In our device architecture, the top layer of graphene functions not only as an encapsulation layer but also as a highly efficient transport layer. Under illumination, photoexcited electron–hole pairs generated in BP are separated and injected into graphene, significantly reducing the Schottky barrier between BP and the metal electrodes and leading to efficient photocurrent extraction. The graphene–BP heterostructure phototransistor exhibits a long-term photoresponse at near-infrared wavelength (1550 nm) with an ultrahigh photoresponsivity (up to 3.3 × 10³ A W^–1), a photoconductive gain (up to 1.13 × 10⁹), and a rise time of about 4 ms. Considering the thickness-dependent band gap in BP, this material represents a powerful photodetection platform that is able to sustain high performance in the IR wavelength regime with potential applications in remote sensing, biological imaging, and environmental monitoring