Femtosecond switch-on of hybrid polaritons in black phosphorus heterostructures

Two-dimensional materials, such as graphene or hexagonal boron nitride, provide a versatile platform for tunable nanophotonics [1,2]. The hybridization of collective electronic motion with mid-infrared photons opens new paths towards plasmon-based nano-circuitry. In contrast to the semimetal graphene, semiconducting compounds hold promise for ultrafast control of polaritons with high contrast via interband excitation of carriers. Here, we demonstrate an ultrafast plasmonic switch based on the tunable bandgap semiconductor black phosphorus (BP) for femtosecond control of surface polaritons in the mid-infrared spectral range [3]. In our custom tailored SiO2/BP/SiO2 heterostructure, a hybrid surface phonon-plasmon mode is created upon photo-activation featuring ultrafast switching times (~50 fs) along with constant energy and momentum throughout its lifetime of ~5 ps. We trace the polariton's creation, evolution and decay using scattering-type near-field optical microscopy which allows us to map out the unique properties of the mode in time, energy and space.