Subcycle terahertz nanoscopy of ultrafast interlayer dynamics in van der Waals heterostructures

Tunneling is one of the most fundamental manifestations of quantum mechanics determining elementary physical processes, chemical reaction pathways and shaping life as we know it. Moreover, it is crucial for modern data storage and electronics, and is essential for highly efficient solar technology. In this work, we introduce a novel, non-invasive concept to resolve electron tunneling on the relevant length- and timescales that even works on insulating samples. The central idea is to monitor the evolution of the local polarizability of electron-hole pairs during the tunneling process with evanescent terahertz nearfields, which are detected with subcycle temporal resolution. In a proof of concept, we resolve the ultrafast interlayer dynamics in van der Waals heterobilayers utilizing our new technique of subcycle contact-free nanoscopy to access the full life cycle of photo-induced spatially separated interlayer electron-hole pairs. Our approach builds on the drastic change of the polarizability of the electron-hole pairs during interlayer tunneling as explained by ab initio density functional theory calculations. We confirm the temporal dynamics using a complementary terahertz emission experiment that is directly linked to the ultrafast charge separation. Moreover, we reveal pronounced variations of the local formation and annihilation of interlayer excitons on deeply subwavelength, nanometer lengthscales. Such contact-free nanoscopy of tunneling-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

Ultrafast Infrared Nanoscopy with Sub-Cycle Temporal Resolution

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Nanoscale Near-Field Tomography of Surface States on (Bi(0.5)b(0.5))(2)Te-3

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 customtailored (Bi0.5Sb0.5)(2)Te-3 structures with nanometer precision in all three spatial dimensions. Applying nanotomography and nanospectroscopy, we reveal a few-nanometer-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

Ultrafast Nanoscopy of High-Density Exciton Phases in WSe2

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 WSe2 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 × 1012 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 two-dimensional materials

Dataset of "Subcycle terahertz nanoscopy of ultrafast interlayer dynamics in van der Waals heterostructures"

These are the data for the dissertation ""Subcycle terahertz nanoscopy of ultrafast interlayer dynamics in van der Waals heterostructures" which will be published in September 2022. Tunneling is one of the most fundamental manifestations of quantum mechanics determining elementary physical processes, chemical reaction pathways and shaping life as we know it. Moreover, it is crucial for modern data storage and electronics, and is essential for highly efficient solar technology. In this work, we introduce a novel, non-invasive concept to resolve electron tunneling on the relevant length- and timescales that even works on insulating samples. The central idea is to monitor the evolution of the local polarizability of electron-hole pairs during the tunneling process with evanescent terahertz nearfields, which are detected with subcycle temporal resolution. In a proof of concept, we resolve the ultrafast interlayer dynamics in van der Waals heterobilayers utilizing our new technique of subcycle contact-free nanoscopy to access the full life cycle of photo-induced spatially separated interlayer electron-hole pairs. Our approach builds on the drastic change of the polarizability of the electron-hole pairs during interlayer tunneling as explained by ab initio density functional theory calculations. We confirm the temporal dynamics using a complementary terahertz emission experiment that is directly linked to the ultrafast charge separation. Moreover, we reveal pronounced variations of the local formation and annihilation of interlayer excitons on deeply subwavelength, nanometer lengthscales. Such contact-free nanoscopy of tunneling-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

Quantifying nanoscale electromagnetic fields in near-field microscopy by Fourier demodulation analysis

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 nano-tomography 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

Quantitative terahertz emission nanoscopy with multiresonant near-field probes

By sampling terahertz waveforms emitted from InAs surfaces, we reveal how the entire, realistic geometry of typical near-field probes drastically impacts the broadband electromagnetic fields. In the time domain, these modifications manifest as a shift in the carrier-envelope phase and emergence of a replica pulse with a time delay dictated by the length of the cantilever. This interpretation is fully corroborated by quantitative simulations of terahertz emission nanoscopy based on the finite element method. Our approach provides a solid theoretical framework for quantitative nanospectroscopy and sets the stage for a reliable description of subcycle, near-field microscopy at terahertz frequencies

Ultrafast Mid-Infrared Nanoscopy of Strained Vanadium Dioxide Nanobeams

Long regarded as a model system for studying insulator-to-metal phase transitions, the correlated electron material vanadium dioxide (VO2) is now finding novel uses in device applications. Two of its most appealing aspects are its accessible transition temperature (similar to 341 K) and its rich phase diagram. Strain can be used to selectively stabilize different VO2 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 nano domains. Nanostructuring of strained VO2-could potentially yield unique components for future devices. However, the most spectacular property of VO2- 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 VO2 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 photo excitation. These results suggest that it may be possible to tailor the local photoresponse of VO2 using strain and thereby realize new types of ultrafast nano-optical devices

Ultrafast Nanoscopy of an Exciton Mott Transition in Twisted Bilayer WSe2

The density-driven transition of an exciton gas into a plasma of unbound electron-hole pairs has provided a compelling testbed for exploring many-body physics. Here, we use ultrafast polarization nanoscopy to trace a Mott transition of excitons in a twisted bilayer of WSe2. An initially monomolecular recombination of optically dark excitons continuously evolves into bimolecular recombination of unbound electrons and holes as the density is increased. Furthermore, we reveal directly how the Mott transition varies on nanometer length scales, demonstrating how the technique is indispensable in the study of intrinsically disordered van der Waals materials