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We retrieve the local dielectric function of a few-nm-thick surface layer on a threedimensional topological insulator by mid-infrared nano-tomography and find the coexistence of a massive electron gas and the topologically protected surface states.

Surface polaritons, which emerge from the coupling between collective electronic motion with mid-infrared photons, have made two-dimensional materials a versatile platform for nanophotonics [1,2]. Apart from the well-studied plasmonic material graphene, semiconducting layered compounds and heterostructures hold new promise for high-contrast, ultrafast control of polaritons. Here, we report on tailored structures based on black phosphorus and SiO2 with favorable properties for ultrafast nanophotonics [3]. We use femtosecond mid-infrared near-field microscopy to photo-activate a hybrid surface phonon-plasmon polariton that is nonexistent in the unexcited sample and trace its mode structure in time, energy and space. We find that the mode shows excellent coherence and is strongly confined in energy and momentum throughout its lifetime of ~5 ps. These properties combined with the superior switching speed of ~50 fs render it a promising candidate for ultrafast nano-optical devices. © OSA 2017.

Atomically strong THz fields accelerate electrons in bulk semiconductors, new 2D materials, and atomically sharp tunneling junctions. By tracking this lightwave-driven charge transport with subcycle resolution, we explore dynamical Bloch oscillations as well as quasiparticle collisions and record the first single-molecule femtosecond movie.

We combine ultrafast field-resolved terahertz spectroscopy with scattering-type near-field scanning optical microscopy to access sub-optical-cycle dynamics on the nanoscale. We trace the oscillating electric near field from a (10 nm)3 volume on the surface of an indium arsenide nanowire with 10 fs temporal resolution, revealing the formation of a depletion layer on the nanowire surface [1]. Furthermore, we apply ultrafast terahertz nanoscopy to strained vanadium dioxide nanobeams and observe a previously unseen correlation between the local steady-state phase-switching susceptibility and the ultrafast response to below threshold photoexcitation [2]. Our findings point towards a nanoscale periodic strain profile that is frozen into the VO 2 insulating state.