title: Femtosecond photo-switching of interface polaritons in black phosphorus heterostructures.
abstract: The possibility of hybridizing collective electronic motion with mid-infrared light to form surface polaritons has made van der Waals layered materials a versatile platform for extreme light confinement and tailored nanophotonics. Graphene and its heterostructures have attracted particular attention because the absence of an energy gap allows plasmon polaritons to be tuned continuously. Here, we introduce black phosphorus as a promising new material in surface polaritonics that features key advantages for ultrafast switching. Unlike graphene, black phosphorus is a van der Waals bonded semiconductor, which enables high-contrast interband excitation of electron-hole pairs by ultrashort near-infrared pulses. Here, we design a SiO2/black phosphorus/SiO2heterostructure in which the surface phonon modes of the SiO2layers hybridize with surface plasmon modes in black phosphorus that can be activated by photo-induced interband excitation. Within the Reststrahlen band of SiO2, the hybrid interface polariton assumes surface-phonon-like properties, with a well-defined frequency and momentum and excellent coherence. During the lifetime of the photogenerated electron-hole plasma, coherent hybrid polariton waves can be launched by a broadband mid-infrared pulse coupled to the tip of a scattering-type scanning near-field optical microscopy set-up. The scattered radiation allows us to trace the new hybrid mode in time, energy and space. We find that the surface mode can be activated within ∼50 fs and disappears within 5 ps, as the electron-hole pairs in black phosphorus recombine. The excellent switching contrast and switching speed, the coherence properties and the constant wavelength of this transient mode make it a promising candidate for ultrafast nanophotonic devices.
authors.affiliation: Department of Physics, University of Regensburg, 93040 Regensburg, Germany.
authors.forename: Markus A
authors.initials: MA
authors.lastname: Huber
authors.affiliation: Department of Physics, University of Regensburg, 93040 Regensburg, Germany.
authors.forename: Fabian
authors.initials: F
authors.lastname: Mooshammer
authors.affiliation: Department of Physics, University of Regensburg, 93040 Regensburg, Germany.
authors.forename: Markus
authors.initials: M
authors.lastname: Plankl
authors.affiliation: NEST, CNR - Istituto Nanoscienze and Scuola Normale Superiore, 56127 Pisa, Italy.
authors.forename: Leonardo
authors.initials: L
authors.lastname: Viti
authors.affiliation: Department of Physics, University of Regensburg, 93040 Regensburg, Germany.
authors.forename: Fabian
authors.initials: F
authors.lastname: Sandner
authors.affiliation: Department of Physics, University of Regensburg, 93040 Regensburg, Germany.
authors.forename: Lukas Z
authors.initials: LZ
authors.lastname: Kastner
authors.affiliation: Department of Physics, University of Regensburg, 93040 Regensburg, Germany.
authors.forename: Tobias
authors.initials: T
authors.lastname: Frank
authors.affiliation: Department of Physics, University of Regensburg, 93040 Regensburg, Germany.
authors.forename: Jaroslav
authors.initials: J
authors.lastname: Fabian
authors.affiliation: NEST, CNR - Istituto Nanoscienze and Scuola Normale Superiore, 56127 Pisa, Italy.
authors.forename: Miriam S
authors.initials: MS
authors.lastname: Vitiello
authors.affiliation: Department of Physics, University of Regensburg, 93040 Regensburg, Germany.
authors.forename: Tyler L
authors.initials: TL
authors.lastname: Cocker
authors.affiliation: Department of Physics, University of Regensburg, 93040 Regensburg, Germany.
authors.forename: Rupert
authors.initials: R
authors.lastname: Huber