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.
- bismuth telluride
- infrared nanoscopy
- near-field optical microscopy
- surface plasmon