Phase-locked ultrashort pulses in the rich terahertz (THz) spectral range 1-18 have provided key insights into phenomena as diverse as quantum confinement 7 , first-order phase transitions 8,12 , high-temperature superconductivity 11 , and carrier transport in nanomaterials 1,6,13-15 . Ultrabroadband electro-optic sampling of few-cycle field transients 1 can even reveal novel dynamics that occur faster than a single oscillation cycle of light 4,8,10 . However, conventional THz spectroscopy is intrinsically restricted to ensemble measurements by the diffraction limit. As a result, it measures dielectric functions averaged over the size, structure, orientation and density of nanoparticles, nanocrystals or nanodomains. Here, we extend ultrabroadband time-resolved THz spectroscopy (20 -50 THz) to the sub-nanoparticle scale (10 nm) by combining sub-cycle, field-resolved detection (10 fs) with scattering-type near-field scanning optical microscopy (s-NSOM) 16-26 . We trace the time-dependent dielectric function at the surface of a single photoexcited InAs nanowire in all three spatial dimensions and reveal the ultrafast (<50 fs) formation of a local carrier depletion layer.

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 SiO/black phosphorus/SiO heterostructure in which the surface phonon modes of the SiO layers hybridize with surface plasmon modes in black phosphorus that can be activated by photo-induced interband excitation. Within the Reststrahlen band of SiO, 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.

Long regarded as a model system for studying insulator-to-metal phase transitions, the correlated electron material vanadium dioxide (VO 2 ) is now finding novel uses in device applications. Two of its most appealing aspects are its accessible transition temperature (∼341 K) and its rich phase diagram. Strain can be used to selectively stabilize different VO 2 insulating phases by tuning the competition between electron and lattice degrees of freedom. It can even break the mesoscopic spatial symmetry of the transition, leading to a quasiperiodic ordering of insulating and metallic nanodomains. Nanostructuring of strained VO 2 could potentially yield unique components for future devices. However, the most spectacular property of VO 2  its ultrafast transitionhas not yet been studied on the length scale of its phase heterogeneity. Here, we use ultrafast near-field microscopy in the mid-infrared to study individual, strained VO 2 nanobeams on the 10 nm scale. We reveal a previously unseen correlation between the local steady-state switching susceptibility and the local ultrafast response to below-threshold photoexcitation. These results suggest that it may be possible to tailor the local photoresponse of VO 2 using strain and thereby realize new types of ultrafast nano-optical devices. KEYWORDS: Near-field, femtosecond dynamics, VO 2 , NSOM, phase transition, correlated electrons T he insulator-to-metal phase transition in vanadium dioxide (VO 2 ) has been the subject of extensive investigation since its discovery in 1959 (ref 1). Interest has stemmed in part from its relatively simple, nonmagnetic structure 2 and its accessible transition temperature (T c ∼ 341 K), which makes it relevant for technological applications.3−6 Nevertheless, the enduring appeal of VO 2 can be traced to the complex interplay between electron and lattice degrees of freedom that produce its intricate free-energy landscape. 7−12 This fine balance between competing interactions can be tuned by strain, leading to a rich phase diagram.13,14 Below T c , unstrained VO 2 is an insulator characterized by both strong electron−electron correlations and lattice distortion, where the vanadium ions form chains of dimerized pairs (monoclinic structure, M1). These dimers are dissociated in the rutile (R), metallic phase (T > T c ) in a process reminiscent of a Peierls transition. However, cluster dynamical mean-field theory calculations have shown that both lattice distortion and strong on-site electron− electron Coulomb repulsion are necessary to accurately model the band gap.9 Moreover, tensile strain applied to the insulating state can produce new, stable lattice structures that are intermediates between M1 and R and, surprisingly, these states are also insulators. High tensile strain along the rutile c R axis (>2%, along the direction of the dimerized chains in the insulating state) induces the monoclinic insulating phase M2 in which only every second row of vanadium ions is dimerized. 15,16 Meanwhile, moderate tensile strain results in...

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Three-dimensional topological insulators (TIs) have attracted tremendous interest for their possibility to host massless Dirac fermions in topologically protected surface states (TSSs), which may enable new kinds of high-speed electronics. However, recent reports have outlined the importance of band bending effects within these materials, which results in an additional two-dimensional electron gas (2DEG) with finite mass at the surface. TI surfaces are also known to be highly inhomogeneous on the nanoscale, which is masked in conventional far-field studies. Here, we use near-field microscopy in the mid-infrared spectral range to probe the local surface properties of custom-tailored (Bi0.5Sb0.5)2Te3 structures with nanometer precision in all three spatial dimensions. Applying nano-tomography and nano-spectroscopy, we reveal a few-nm-thick layer of high surface conductivity and retrieve its local dielectric function, without assuming any model for the spectral response. This allows us to directly distinguish between different types of surface states. An intersubband transition within the massive 2DEG formed by quantum confinement in the bent conduction band manifests itself as a sharp surface-bound Lorentzian-shaped resonance. An additional broadband background in the imaginary part of the dielectric function may be caused by the TSS. Tracing the intersubband resonance with nanometer spatial precision, we observe changes of its frequency, likely originating from local variations of doping or/and the mixing ratio between Bi and Sb. Our results highlight the importance of studying the surfaces of these novel materials on the nanoscale to directly access the local optical and electronic properties via the dielectric function.

Confining light to sharp metal tips has become a versatile technique to study optical and electronic properties far below the diffraction limit. Particularly near-field microscopy in the mid-infrared spectral range has found a variety of applications in probing nanostructures and their dynamics. Yet, the ongoing quest for ultimately high spatial resolution down to the single-nanometer regime and quantitative three-dimensional nanotomography depends vitally on a precise knowledge of the spatial distribution of the near fields emerging from the probe. Here, we perform finite element simulations of a tip with realistic geometry oscillating above a dielectric sample. By introducing a novel Fourier demodulation analysis of the electric field at each point in space, we reliably quantify the distribution of the near fields above and within the sample. Besides inferring the lateral field extension, which can be smaller than the tip radius of curvature, we also quantify the probing volume within the sample. Finally, we visualize the scattering process into the far field at a given demodulation order, for the first time, and shed light onto the nanoscale distribution of the near fields and its evolution as the tip-sample distance is varied. Our work represents a crucial step in understanding and tailoring the spatial distribution of evanescent fields in optical nanoscopy.

The density-driven transition of an exciton gas into an electron−hole plasma remains a compelling question in condensed matter physics. In two-dimensional transition metal dichalcogenides, strongly bound excitons can undergo this phase change after transient injection of electron−hole pairs. Unfortunately, unavoidable nanoscale inhomogeneity in these materials has impeded quantitative investigation into this elusive transition. Here, we demonstrate how ultrafast polarization nanoscopy can capture the Mott transition through the density-dependent recombination dynamics of electron−hole pairs within a WSe 2 homobilayer. For increasing carrier density, an initial monomolecular recombination of optically dark excitons transitions continuously into a bimolecular recombination of an unbound electron−hole plasma above 7 × 10 12 cm −2 . We resolve how the Mott transition modulates over nanometer length scales, directly evidencing the strong inhomogeneity in stacked monolayers. Our results demonstrate how ultrafast polarization nanoscopy could unveil the interplay of strong electronic correlations and interlayer coupling within a diverse range of stacked and twisted twodimensional materials.

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