Markus Plankl

Ultrafast multi-terahertz nano-spectroscopy with sub-cycle temporal resolution

The authors demonstrate ultrabroadband time-resolved THz spectroscopy on a single InAs nanowire with 10 nm spatial resolution and sub-100 fs time resolution. Phase-locked ultrashort pulses in the rich terahertz spectral range1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18 have provided key insights into phenomena as diverse as quantum confinement7, first-order phase transitions8,12, high-temperature superconductivity11 and carrier transport in nanomaterials1,6,13,14,15. Ultrabroadband electro-optic sampling of few-cycle field transients1 can even reveal novel dynamics that occur faster than a single oscillation cycle of light4,8,10. However, conventional terahertz 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 terahertz spectroscopy 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)16,17,18,19,20,21,22,23,24,25,26. 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.

Femtosecond photo-switching of interface polaritons in black phosphorus heterostructures

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 SiO2/black phosphorus/SiO2 heterostructure in which the surface phonon modes of the SiO2 layers hybridize with surface plasmon modes in black phosphorus 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 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.

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 (∼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 nanodomains. 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 photoexcitation. 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 photo-activation of interface polaritons in black phosphorus heterostructures

Van der Waals layered materials, such as graphene, hexagonal boron nitride, and transition metal dichalcogenides, have redefined the perspectives of future ultra-compact electronics and optics on the nanoscale. Specifically, the possibility of hybridizing collective electronic motion with mid-infrared photons in so-called surface polaritons has allowed for extreme light confinement and represents a key ingredient for tailored nanophotonics [1].

Terahertz subcycle control: From high-harmonic generation to molecular snapshots

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.

Terahertz imaging with ultimate resolution

Summary form only given. Field-resolved detection of ultrafast pulses in the THz (0.1-10 THz) and multi-THz (10-100 THz) spectral range has provided key insights into the dynamics of low-energy collective excitations in condensed matter systems. However, the spatial resolution of these far-field studies is intrinsically limited to the scale of the probing wavelength by diffraction. Thus, the measured optical response is an average over sub-wavelength structures such as nanoparticles, nanocrystals, nanodomains, and microscopic defects. Apertureless scattering-type near-field scanning optical microscopy (s-NSOM) bypasses this fundamental limit by utilizing the strong confinement of the optical near-field at the apex of a sharp metal tip. We have combined ultra-sensitive, field-resolved multi-THz spectroscopy with s-NSOM to access dynamic complex conductivities on the surfaces of nanostructures with 10 nm spatial resolution. Electro-optic sampling of the scattered near-field pulses enables sub-cycle detection (10 fs temporal resolution) of waveforms consisting of less than one coherent photon per pulse. We have applied our versatile microscope to two nanostructures of similar shape but vastly different composition. First, carrier dynamics were studied in indium arsenide nanowires with sub-cycle temporal resolution, revealing the ultrafast (<; 50 fs) formation of a carrier depletion layer at the nanowire surface. Second, we studied heterogeneous local dynamics in vanadium dioxide nanowires. Vanadium dioxide is a model system for insulator-to-metal phase transitions and is promising for technological applications. In our study, we found that substrate-induced strain drives a periodic modulation of the ultrafast photoconductivity along the nanowire. Finally, we have explored the ultimate limits of THz imaging resolution using a new technique called THz scanning tunnelling microscopy, where THz pulses drive femtosecond local currents across an atomic-scale tunnel junction.

Ultrafast field-resolved multi-THz spectroscopy on the sub-nanoparticle scale

Terahertz spectroscopy plays a key role in understanding ultrafast carrier dynamics in nanomaterials. Diffraction, however, limits time-resolved terahertz spectroscopy to ensemble measurements. By combining time-resolved terahertz spectroscopy in the multi-terahertz range with scattering-type near-field scanning optical microscopy, we show that we can directly trace ultrafast local carrier dynamics in single nanoparticles with sub-cycle temporal resolution (10 fs). Our microscope provides both 10 nm lateral resolution and tomographic sensitivity, allowing us to observe the ultrafast build-up of a local surface depletion layer in an InAs nanowire.

Ultrafast Infrared Nanoscopy with Sub-Cycle Temporal Resolution

Ultrafast microscopy of surfaces with simultaneous nanometer spatial resolution and femtosecond temporal resolution can be achieved by coupling ultrafast optical pulses to sharp metal tips [1-10]. In the case of scattering-type near-field scanning optical microscopy (s-NSOM), evanescent fields scattered from the tip apex provide a window into a drastically subwavelength world. Moreover, since these scattered fields can be measured using established far-field technologies, ultrafast infrared pulses emerging from a nanoscale volume can be detected with the best possible time resolution: faster than a single optical oscillation cycle [1].