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Delocalized Bloch electrons and the low-energy correlations between them determine key optical 1 , electronic 2 and entanglement 3 functionalities of solids, all the way through to phase transitions 4 , 5 . To directly capture how many-body correlations affect the actual motion of Bloch electrons, subfemtosecond (1 fs = 10 −15  s) temporal precision 6 – 15 is desirable. Yet, probing with attosecond (1 as = 10 −18  s) high-energy photons has not been energy-selective enough to resolve the relevant millielectronvolt-scale interactions of electrons 1 – 5 , 16 , 17 near the Fermi energy. Here, we use multi-terahertz light fields to force electron–hole pairs in crystalline semiconductors onto closed trajectories, and clock the delay between separation and recollision with 300 as precision, corresponding to 0.7% of the driving field’s oscillation period. We detect that strong Coulomb correlations emergent in atomically thin WSe 2 shift the optimal timing of recollisions by up to 1.2 ± 0.3 fs compared to the bulk material. A quantitative analysis with quantum-dynamic many-body computations in a Wigner-function representation yields a direct and intuitive view on how the Coulomb interaction, non-classical aspects, the strength of the driving field and the valley polarization influence the dynamics. The resulting attosecond chronoscopy of delocalized electrons could revolutionize the understanding of unexpected phase transitions and emergent quantum-dynamic phenomena for future electronic, optoelectronic and quantum-information technologies. By forcing electron–hole pairs onto closed trajectories attosecond clocking of delocalized Bloch electrons is achieved, enabling greater understanding of unexpected phase transitions and quantum-dynamic phenomena.

Tunnelling is one of the most fundamental manifestations of quantum mechanics. The recent advent of lightwave-driven scanning tunnelling microscopy has revolutionized ultrafast nanoscience by directly resolving electron tunnelling in electrically conducting samples on the relevant ultrashort length- and timescales. Here, we introduce a complementary approach based on terahertz near-field microscopy to perform ultrafast nano-videography of tunnelling processes even in insulators. The central idea is to probe the evolution of the local polarizability of electron–hole pairs with evanescent terahertz fields, which we detect with subcycle temporal resolution. In a proof of concept, we resolve femtosecond interlayer transport in van der Waals heterobilayers and reveal pronounced variations of the local formation and annihilation of interlayer excitons on deeply subwavelength, nanometre scales. Such contact-free nanoscopy of tunnelling-induced dynamics should be universally applicable to conducting and non-conducting samples and reveal how ultrafast transport processes shape functionalities in a wide range of condensed matter systems.Subcycle nano-videography of charge-transfer dynamics in WSe2/WS2 heterostructures is obtained by using a terahertz near-field microscopy. The central idea is to probe the local polarizability of electron–hole pairs with evanescent terahertz fields.

Designing next-generation light-harvesting devices requires a detailed understanding of the transport of photoexcited charge carriers. The record-breaking efficiencies of metal halide perovskite solar cells have been linked to effective charge-carrier diffusion, yet the exact nature of charge-carrier out-of-plane transport remains notoriously difficult to explain. The characteristic spatial inhomogeneity of perovskite films with nanograins and crystallographic disorder calls for the simultaneous and hitherto elusive in situ resolution of the chemical composition, the structural phase and the ultrafast dynamics of the local out-of-plane transport. Here we simultaneously probe the intrinsic out-of-plane charge-carrier diffusion and the nanoscale morphology by pushing depth-sensitive terahertz near-field nanospectroscopy to extreme subcycle timescales. In films of the organic–inorganic metal halide perovskite FA 0.83 Cs 0.17 Pb(I 1− x Cl x ) 3 (where FA is formamidinium), domains of the cubic α-phase are clearly distinguished from the trigonal δ-phase and PbI 2 nano-islands. By analysing deep-subcycle time shifts of the scattered terahertz waveform after photoexcitation, we access the vertical charge-carrier dynamics within single grains. At all of the measured locations, despite topographic irregularities, diffusion is surprisingly homogeneous on the 100 nm scale, although it varies between mesoscopic regions. Linking in situ carrier transport with nanoscale morphology and chemical composition could introduce a paradigm shift for the analysis and optimization of next-generation optoelectronics that are based on nanocrystalline materials. Transient visible-pump terahertz-probe near-field microscopy enables the simultaneous retrieval of the local chemical composition, crystallographic structure, topography and out-of-plane charge-carrier diffusion in perovskite films.

Bringing optical microscopy to the shortest possible length and time scales has been a long-sought goal, connecting nanoscopic elementary dynamics with the macroscopic functionalities of condensed matter. Super-resolution microscopy has circumvented the far-field diffraction limit by harnessing optical nonlinearities 1 . By exploiting linear interaction with tip-confined evanescent light fields 2 , near-field microscopy 3 , 4 has reached even higher resolution, prompting a vibrant research field by exploring the nanocosm in motion 5 – 19 . Yet the finite radius of the nanometre-sized tip apex has prevented access to atomic resolution 20 . Here we leverage extreme atomic nonlinearities within tip-confined evanescent fields to push all-optical microscopy to picometric spatial and femtosecond temporal resolution. On these scales, we discover an unprecedented and efficient non-classical near-field response, in phase with the vector potential of light and strictly confined to atomic dimensions. This ultrafast signal is characterized by an optical phase delay of approximately π/2 and facilitates direct monitoring of tunnelling dynamics. We showcase the power of our optical concept by imaging nanometre-sized defects hidden to atomic force microscopy and by subcycle sampling of current transients on a semiconducting van der Waals material. Our results facilitate access to quantum light–matter interaction and electronic dynamics at ultimately short spatio-temporal scales in both conductive and insulating quantum materials. All-optical subcycle microscopy is achieved on atomic length scales, with picometric spatial and femtosecond temporal resolution.

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 (\\(\\sim\\)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 quasi-periodic 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 photo-response of VO\\(_2\\) using strain and thereby realize new types of ultrafast nano-optical devices.

The Coffee, Sugar & Cocoa Exchange (CS&CE) and the New York Cotton Exchange, located in the World Trade Center in lower Manhattan, have until mid-October to decide whether to pull the plug and move their trading facilities across the Hudson River to the shores of New Jersey. Threats by the CS&CE and the Cotton Exchange to leave the Big Apple recall a time years ago when we, here at the Chicago Mercantile Exchange (CME), contemplated similar action because of a hostile local business environment that was bent on taxing our industry. Because the threatened levy (which never was imposed) would have rendered our trading products uncompetitive and ultimately destroyed our exchange altogether, in the mid-'70s we pondered seriously a move either to the western suburbs or, even more startling, to Texas or Arizona. All had made attractive offers.

Those who fear that the United States will be a loser under the North American Free Trade Agreement would do well to heed the words of John F. Kennedy and the experience of the Chicago Mercantile Exchange. Many times during his all-too-brief presidency, Kennedy spoke passionately about creating a bigger economic pie for more Americans in terms of the time-honored fishermen's adage: \"A rising tide lifts all of the boats.\" From practical experience, the Mercantile Exchange has since learned just how right Kennedy had been. We set out to expand the global financial risk-management pie a dozen years ago, when we began to spread our trading technology and firsthand knowledge of the risk-management/price-discovery value of financial futures and options around the globe.

CHICAGO--A mini-economic time bomb is ticking away in New York City.