Magnon-phonon Fermi resonance in antiferromagnetic CoF.

Understanding spin-lattice interactions in antiferromagnets is a critical element of the fields of antiferromagnetic spintronics and magnonics. Recently, coherent nonlinear phonon dynamics mediated by a magnon state were discovered in an antiferromagnet. Here, we suggest that a strongly coupled two-magnon-one phonon state in this prototypical system opens a novel pathway to coherently control magnon-phonon dynamics.

Right On Time: Ultrafast Charge Separation Before Hybrid Exciton Formation.

Organic/inorganic hybrid systems offer great potential for novel solar cell design combining the tunability of organic chromophore absorption properties with high charge carrier mobilities of inorganic semiconductors. However, often such material combinations do not show the expected performance: while ZnO, for example, basically exhibits all necessary properties for a successful application in light-harvesting, it was clearly outpaced by TiO in terms of charge separation efficiency. The origin of this deficiency has long been debated.

Tracing the dynamics of superconducting order via transient terahertz third-harmonic generation.

Ultrafast optical control of quantum systems is an emerging field of physics. In particular, the possibility of light-driven superconductivity has attracted much of attention. To identify nonequilibrium superconductivity, it is necessary to measure fingerprints of superconductivity on ultrafast timescales.

Efficient ultrafast field-driven spin current generation for spintronic terahertz frequency conversion.

Efficient generation and control of spin currents launched by terahertz (THz) radiation with subsequent ultrafast spin-to-charge conversion is the current challenge for the next generation of high-speed communication and data processing units. Here, we demonstrate that THz light can efficiently drive coherent angular momentum transfer in nanometer-thick ferromagnet/heavy-metal heterostructures. This process is non-resonant and does neither require external magnetic fields nor cryogenics.

A universal route to efficient non-linear response via Thomson scattering in linear solids.

Non-linear materials are cornerstones of modern optics and electronics. Strong dependence on the intrinsic properties of particular materials, however, inhibits the at-will extension of demanding non-linear effects, especially those second-order ones, to widely adopted centrosymmetric materials (for example, silicon) and technologically important burgeoning spectral domains (for example, terahertz frequencies). Here we introduce a universal route to efficient non-linear responses enabled by exciting non-linear Thomson scattering, a fundamental process in electrodynamics that was known to occur only in relativistic electrons in metamaterial composed of linear materials.

Ultrafast Tunable Terahertz-to-Visible Light Conversion through Thermal Radiation from Graphene Metamaterials.

Several technologies, including photodetection, imaging, and data communication, could greatly benefit from the availability of fast and controllable conversion of terahertz (THz) light to visible light. Here, we demonstrate that the exceptional properties and dynamics of electronic heat in graphene allow for a THz-to-visible conversion, which is switchable at a sub-nanosecond time scale. We show a tunable on/off ratio of more than 30 for the emitted visible light, achieved through electrical gating using a gate voltage on the order of 1 V.

Fano interference between collective modes in cuprate high-T superconductors.

Cuprate high-T superconductors are known for their intertwined interactions and the coexistence of competing orders. Uncovering experimental signatures of these interactions is often the first step in understanding their complex relations. A typical spectroscopic signature of the interaction between a discrete mode and a continuum of excitations is the Fano resonance/interference, characterized by the asymmetric light-scattering amplitude of the discrete mode as a function of the electromagnetic driving frequency.

Milliwatt terahertz harmonic generation from topological insulator metamaterials.

Achieving efficient, high-power harmonic generation in the terahertz spectral domain has technological applications, for example, in sixth generation (6G) communication networks. Massless Dirac fermions possess extremely large terahertz nonlinear susceptibilities and harmonic conversion efficiencies. However, the observed maximum generated harmonic power is limited, because of saturation effects at increasing incident powers, as shown recently for graphene.

Electrical tunability of terahertz nonlinearity in graphene.

Graphene is conceivably the most nonlinear optoelectronic material we know. Its nonlinear optical coefficients in the terahertz frequency range surpass those of other materials by many orders of magnitude. Here, we show that the terahertz nonlinearity of graphene, both for ultrashort single-cycle and quasi-monochromatic multicycle input terahertz signals, can be efficiently controlled using electrical gating, with gating voltages as low as a few volts.

Grating-Graphene Metamaterial as a Platform for Terahertz Nonlinear Photonics.

Nonlinear optics is an increasingly important field for scientific and technological applications, owing to its relevance and potential for optical and optoelectronic technologies. Currently, there is an active search for suitable nonlinear material systems with efficient conversion and a small material footprint. Ideally, the material system should allow for chip integration and room-temperature operation.

Non-perturbative terahertz high-harmonic generation in the three-dimensional Dirac semimetal CdAs.

Harmonic generation is a general characteristic of driven nonlinear systems, and serves as an efficient tool for investigating the fundamental principles that govern the ultrafast nonlinear dynamics. Here, we report on terahertz-field driven high-harmonic generation in the three-dimensional Dirac semimetal CdAs at room temperature. Excited by linearly-polarized multi-cycle terahertz pulses, the third-, fifth-, and seventh-order harmonic generation is very efficient and detected via time-resolved spectroscopic techniques.

Phase-resolved Higgs response in superconducting cuprates.

In high-energy physics, the Higgs field couples to gauge bosons and fermions and gives mass to their elementary excitations. Experimentally, such couplings can be inferred from the decay product of the Higgs boson, i.e.

Photoexcited organic molecules en route to highly efficient autoionization.

The conversion of optical and electrical energies in novel materials is key to modern optoelectronic and light-harvesting applications. Here, we investigate the equilibration dynamics of photoexcited 2,7-bis(biphenyl-4-yl)-2',7'-ditertbutyl-9,9'-spirobifluorene (SP6) molecules adsorbed on ZnO(10-10) using femtosecond time-resolved two-photon photoelectron and optical spectroscopies. We find that, after initial ultrafast relaxation on femtosecond and picosecond time scales, an optically dark state is populated, likely the SP6 triplet (T) state, that undergoes Dexter-type energy transfer (r = 1.

Extremely efficient terahertz high-harmonic generation in graphene by hot Dirac fermions.

Multiple optical harmonic generation-the multiplication of photon energy as a result of nonlinear interaction between light and matter-is a key technology in modern electronics and optoelectronics, because it allows the conversion of optical or electronic signals into signals with much higher frequency, and the generation of frequency combs. Owing to the unique electronic band structure of graphene, which features massless Dirac fermions, it has been repeatedly predicted that optical harmonic generation in graphene should be particularly efficient at the technologically important terahertz frequencies. However, these predictions have yet to be confirmed experimentally under technologically relevant operation conditions.

Real-time measurement of the vertical binding energy during the birth of a solvated electron.

Using femtosecond time-resolved two-photon photoelectron spectroscopy, we determine (i) the vertical binding energy (VBE = 0.8 eV) of electrons in the conduction band in supported amorphous solid water (ASW) layers, (ii) the time scale of ultrafast trapping at pre-existing sites (22 fs), and (iii) the initial VBE (1.4 eV) of solvated electrons before significant molecular reorganization sets in.

Large work function reduction by adsorption of a molecule with a negative electron affinity: pyridine on ZnO(1010).

Using thermal desorption and photoelectron spectroscopy to study the adsorption of pyridine on ZnO(1010), we find that the work function is significantly reduced from 4.5 eV for the bare ZnO surface to 1.6 eV for one monolayer of adsorbed pyridine.