Femtosecond photo-switching of interface polaritons in black phosphorus heterostructures

Femtosecond photo-switching of interface polaritons
in black phosphorus heterostructures Markus A. Huber1, Fabian Mooshammer1, Markus Plankl1, Leonardo Viti2, Fabian Sandner1, Lukas Z. Kastner1, Tobias Frank1, Jaroslav Fabian1, Miriam S. Vitiello2*, Tyler L. Cocker1* and Rupert Huber1 1 Department of Physics, University of Regensburg, 93040 Regensburg, Germany 2 NEST, CNR – Istituto Nanoscienze and Scuola Normale Superiore, 56127 Pisa, Italy 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 confinement1-5 and tailored nanophotonics6-8. Graphene9,10 and its heterostructures11-14 have attracted particular attention because the absence of an energy gap allows for plasmon polaritons to be continuously tuned. Here, we introduce black phosphorus15-19 (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 SiO2/BP/SiO2 heterostructure in which the surface phonon modes of the SiO2 layers hybridize with surface plasmon modes in BP 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 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. *e-mail: miriam.vitiello@sns.it; tyler.cocker@physik.uni-regensburg.de

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The BP flakes studied here were transferred onto a Si/SiO2 substrate (oxide thickness: 300 nm) by mechanical exfoliation20. Since BP rapidly degrades under ambient conditions21 it must be encapsulated under a protective layer to prevent oxidation22,23. We deposited a 5 nm protective layer of SiO2 over the entire sample by sputtering. The protected BP flakes remained stable and clean for months. A schematic of a SiO2/BP/SiO2 heterostructure is shown in Fig. 1a, alongside a typical optical microscope image. We explore the system with ultrafast near-IR pump / mid-IR probe s-SNOM, which combines the strengths of near-field microscopy with femtosecond time resolution and has successfully accessed the ultrafast local photoconductivity of graphene9, semiconductors24, and insulator-metal phase change materials25, as well as transient surface plasmon modes on graphene10.
Our SiO2/BP/SiO2 heterostructure is globally illuminated with a near-IR pump pulse (centre wavelength 1560 nm, pulse duration 40 fs FWHM), which drives interband excitations in the BP flake (Fig. 1b) but does not affect the large-bandgap SiO2 layers. The photo-generated free carrier density defines a plasma frequency in the mid-IR. As a consequence, the scattered near-field intensity I4 (for further details on the demodulation technique see Methods) of our mid-IR probe pulses is strongly modified when the near-field tip is located over the BP heterostructure (Fig. 1c). The signal sets on rapidly, for example with a rise time of ~380 fs (from 20% to 80% of its maximum) for the heterostructure shown in Fig. 1, followed by a non-exponential decay for pump-probe delay times tpp < 5 ps and a persistent state of reduced scattering response that survives for >10 ps. These dynamics are related to the ultrafast evolution of the BP complex conductivity – and hence the dynamics of electron-hole pair generation and recombination – but the nonlinear scattering response makes this relation nontrivial24 (see Supplementary Section 1 for details). Interestingly, the pump introduces a sub-micron-scale spatial inhomogeneity into the probe scattering response. Figure 1d shows a topographic image of a selected region of the heterostructure recorded by atomic force microscopy. The surface is clean and flat (nominal flake thickness: 110 nm) aside from folds at the borders of the investigated area. The simultaneously recorded broadband mid-IR near-field intensity scattered off the flake is small relative to the SiO2 substrate and roughly uniform prior to near- IR photoexcitation (Fig. 1e, top). In contrast, near-IR pumping leads to a drastic increase in m

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