  The possibility of hybridizing collective electronic motion with mid-infrared
(mid-IR) 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 for plasmon polaritons to
be continuously tuned. Here, we introduce black phosphorus (BP) as a promising
new material in surface polaritonics that features key advantages for ultrafast
switching. Unlike graphene, BP is a van der Waals bonded semiconductor, which
enables high-contrast interband excitation of electron-hole pairs by ultrashort
near-infrared (near-IR) pulses. We design a SiO$_2$/BP/SiO$_2$ heterostructure
in which the surface phonon modes of the SiO$_2$ layers hybridize with surface
plasmon modes in BP that can be activated by photo-induced interband
excitation. Within the Reststrahlen band of SiO$_2$, 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 polariton waves can be launched by a broadband
mid-IR pulse coupled to the tip of a scattering-type scanning near-field
optical microscopy (s-SNOM) setup. 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 BP 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.
