Near-field detection of gate-tunable anisotropic plasmon polaritons in black phosphorus at terahertz frequencies

• Published in: Nature communications Vol. 15; no. 1; p. 2373
• Main Authors: Pogna, Eva A. A., Pistore, Valentino, Viti, Leonardo, Li, Lianhe, Davies, A. Giles, Linfield, Edmund H., Vitiello, Miriam S.
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group UK 15.03.2024; Nature Publishing Group; Nature Portfolio
• Subjects: 639/301/357/1018; 639/925/357; Anisotropy; Carrier density; Confinement; Crystallography; Crystals; Directional control; Electromagnetic radiation; Field effect transistors; Graphene; Humanities and Social Sciences; multidisciplinary; Near fields; Nonlinear optics; Optics; Phosphorus; Photoelectric effect; Photoelectric emission; Plasmons; Polaritons; Science; Science (multidisciplinary); Semiconductor devices; Terahertz frequencies; Two dimensional materials; Wavelength
• ISSN: 2041-1723; 2041-1723
• DOI: 10.1038/s41467-024-45264-5

Polaritons in two-dimensional layered crystals offer an effective solution to confine, enhance and manipulate terahertz (THz) frequency electromagnetic waves at the nanoscale. Recently, strong THz field confinement has been achieved in a graphene-insulator-metal structure, exploiting THz plasmon polaritons (PPs) with strongly reduced wavelength ( λ p  ≈  λ 0 /66) compared to the photon wavelength λ 0 . However, graphene PPs propagate isotropically, complicating the directional control of the THz field, which, on the contrary, can be achieved exploiting anisotropic layered crystals, such as orthorhombic black-phosphorus. Here, we detect PPs, at THz frequencies, in hBN-encapsulated black phosphorus field effect transistors through THz near-field photocurrent nanoscopy. The real-space mapping of the thermoelectrical near-field photocurrents reveals deeply sub-wavelength THz PPs ( λ p  ≈  λ 0 /76), with dispersion tunable by electrostatic control of the carrier density. The in-plane anisotropy of the dielectric response results into anisotropic polariton propagation along the armchair and zigzag crystallographic axes of black-phosphorus. The achieved directional subwavelength light confinement makes this material system a versatile platform for sensing and quantum technology based on nonlinear optics. Polaritons in 2D materials offer the possibility to confine and manipulate light in the terahertz (THz) range. Here, the authors report the observation of THz elliptic plasmon polaritons in 2D black phosphorus, showing deep subwavelength light confinement and anisotropic polariton propagation.