Multibranch pulse synthesis and electro-optic detection of subcycle multi-terahertz electric fields

We present a robust, compact pulse synthesis scheme generating intense phase-locked subcycle multi-terahertz waveforms. The ultrabroadband laser fundamental is split into two parallel branches driving optical rectification in crystals of GaSe and LiGaS2, each operated at the group velocity matching point. The coherent combination of the resulting pulses yields a continuous multi-terahertz spectrum covering 1.5 optical octaves. The corresponding 0.8-cycle electric field waveform is directly mapped out by electro-optic sampling, revealing peak fields of 15 kV/cm at a repetition rate of 0.4 MHz. The multiplexable and power scalable scheme opens the door to strong-field custom-tailored waveforms driving nonlinear optics and light wave electronics. (C) 2019 Optical Society of Americ

Ultrafast atomic-scale scanning tunnelling spectroscopy of a single vacancy in a monolayer crystal

Defects in atomically thin semiconductors and their moiré heterostructures have emerged as a unique testbed for quantum science. Strong light–matter coupling, large spin–orbit interaction and enhanced Coulomb correlations facilitate a spin–photon interface for future qubit operations and efficient single-photon quantum emitters. Yet, directly observing the relevant interplay of the electronic structure of a single defect with other microscopic elementary excitations on their intrinsic length, time and energy scales remained a long-held dream. Here we directly resolve in space, time and energy how a spin–orbit-split energy level of an isolated selenium vacancy in a moiré-distorted WSe2 monolayer evolves under the controlled excitation of lattice vibrations, using lightwave scanning tunnelling microscopy and spectroscopy. By locally launching a phonon oscillation and taking ultrafast energy-resolved snapshots of the vacancy’s states faster than the vibration period, we directly measure the impact of electron–phonon coupling in an isolated single-atom defect. The combination of atomic spatial, sub-picosecond temporal and millielectronvolt energy resolution marks a disruptive development towards a comprehensive understanding of complex quantum materials, where the key microscopic elementary interactions can now be disentangled, one by one

Sub-cycle atomic-scale forces coherently control a single-molecule switch

Scanning probe techniques can leverage atomically precise forces to sculpt matter at surfaces, atom by atom. These forces have been applied quasi-statically to create surface structures(1-7)and influence chemical processes(8,9), but exploiting local dynamics(10-14)to realize coherent control on the atomic scale remains an intriguing prospect. Chemical reactions(15-17), conformational changes(18,19)and desorption(20)have been followed on ultrafast timescales, but directly exerting femtosecond forces on individual atoms to selectively induce molecular motion has yet to be realized. Here we show that the near field of a terahertz wave confined to an atomically sharp tip provides femtosecond atomic-scale forces that selectively induce coherent hindered rotation in the molecular frame of a bistable magnesium phthalocyanine molecule. Combining lightwave-driven scanning tunnelling microscopy(21-24)with ultrafast action spectroscopy(10,13), we find that the induced rotation modulates the probability of the molecule switching between its two stable adsorption geometries by up to 39 per cent. Mapping the response of the molecule in space and time confirms that the force acts on the atomic scale and within less than an optical cycle (that is, faster than an oscillation period of the carrier wave of light). We anticipate that our strategy might ultimately enable the coherent manipulation of individual atoms within single molecules or solids so that chemical reactions and ultrafast phase transitions can be manipulated on their intrinsic spatio-temporal scales. The near field of a terahertz wave confined to a scanning probe tip provides femtosecond atomic-scale forces that coherently modulate the switching probability of a molecule between two stable adsorption geometries

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Quantitative sampling of atomic-scale electromagnetic waveforms

Tailored nanostructures can confine electromagnetic waveforms in extremely sub-wavelength volumes, opening new avenues in lightwave sensing and control down to sub-molecular resolution. Atomic light--matter interaction depends critically on the absolute strength and the precise time evolution of the near field, which may be strongly influenced by quantum-mechanical effects. However, measuring atom-scale field transients has remained out of reach. Here we introduce quantitative atomic-scale waveform sampling in lightwave scanning tunnelling microscopy to resolve a tip-confined near-field transient. Our parameter-free calibration employs a single-molecule switch as an atomic-scale voltage standard. Although salient features of the far-to-near-field transfer follow classical electrodynamics, we develop a comprehensive understanding of the atomic-scale waveforms with time-dependent density functional theory. The simulations validate our calibration and confirm that single-electron tunnelling ensures minimal back-action of the measurement process on the electromagnetic fields. Our observations access an uncharted domain of nano-opto-electronics where local quantum dynamics determine femtosecond atomic near fields

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 confinement1, 2, 3, 4, 5 and tailored nanophotonics6, 7, 8. Graphene9, 10 and its heterostructures11, 12, 13, 14 have attracted particular attention because the absence of an energy gap allows plasmon polaritons to be tuned continuously. Here, we introduce black phosphorus15, 16, 17, 18, 19 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