Lightwave valleytronics in a monolayer of tungsten diselenide.

As conventional electronics approaches its limits 1 , nanoscience has urgently sought methods of fast control of electrons at the fundamental quantum level 2 . Lightwave electronics 3 -the foundation of attosecond science 4 -uses the oscillating carrier wave of intense light pulses to control the translational motion of the electron's charge faster than a single cycle of light5-15. Despite being particularly promising information carriers, the internal quantum attributes of spin 16 and valley pseudospin17-21 have not been switchable on the subcycle scale. Here we demonstrate lightwave-driven changes of the valley pseudospin and introduce distinct signatures in the optical readout. Photogenerated electron-hole pairs in a monolayer of tungsten diselenide are accelerated and collided by a strong lightwave. The emergence of high-odd-order sidebands and anomalous changes in their polarization direction directly attest to the ultrafast pseudospin dynamics. Quantitative computations combining density functional theory with a non-perturbative quantum many-body approach assign the polarization of the sidebands to a lightwave-induced change of the valley pseudospin and confirm that the process is coherent and adiabatic. Our work opens the door to systematic valleytronic logic at optical clock rates.

Lightwave-driven quasiparticle collisions on a subcycle timescale.

Ever since Ernest Rutherford scattered α-particles from gold foils, collision experiments have revealed insights into atoms, nuclei and elementary particles. In solids, many-body correlations lead to characteristic resonances--called quasiparticles--such as excitons, dropletons, polarons and Cooper pairs. The structure and dynamics of quasiparticles are important because they define macroscopic phenomena such as Mott insulating states, spontaneous spin- and charge-order, and high-temperature superconductivity. However, the extremely short lifetimes of these entities make practical implementations of a suitable collider challenging. Here we exploit lightwave-driven charge transport, the foundation of attosecond science, to explore ultrafast quasiparticle collisions directly in the time domain: a femtosecond optical pulse creates excitonic electron-hole pairs in the layered dichalcogenide tungsten diselenide while a strong terahertz field accelerates and collides the electrons with the holes. The underlying dynamics of the wave packets, including collision, pair annihilation, quantum interference and dephasing, are detected as light emission in high-order spectral sidebands of the optical excitation. A full quantum theory explains our observations microscopically. This approach enables collision experiments with various complex quasiparticles and suggests a promising new way of generating sub-femtosecond pulses.

Extraction of many-body configurations from nonlinear absorption in semiconductor quantum wells.

Detailed electronic many-body configurations are extracted from quantitatively measured time-resolved nonlinear absorption spectra of resonantly excited GaAs quantum wells. The microscopic theory assigns the observed spectral changes to a unique mixture of electron-hole plasma, exciton, and polarization effects. Strong transient gain is observed only under cocircular pump-probe conditions and is attributed to the transfer of pump-induced coherences to the probe.

Super-resolution lightwave tomography of electronic bands in quantum materials.

Searching for quantum functionalities requires access to the electronic structure, constituting the foundation of exquisite spin-valley-electronic, topological, and many-body effects. All-optical band-structure reconstruction could directly connect electronic structure with the coveted quantum phenomena if strong lightwaves transported localized electrons within preselected bands. Here, we demonstrate that harmonic sideband (HSB) generation in monolayer tungsten diselenide creates distinct electronic interference combs in momentum space. Locating these momentum combs in spectroscopy enables super-resolution tomography of key band-structure details in situ. We experimentally tuned the optical-driver frequency by a full octave and show that the predicted super-resolution manifests in a critical intensity and frequency dependence of HSBs. Our concept offers a practical, all-optical, fully three-dimensional tomography of electronic structure even in microscopically small quantum materials, band by band.

Terahertz coherent control of optically dark paraexcitons in Cu2O.

Intense multiterahertz fields of order megavolts per centimeter are used to coherently promote optically dark and dense paraexcitons in Cu2O from the 1s into the 2p state. The nonlinear field response of the intraexcitonic degrees of freedom is directly monitored in the time domain via ultrabroadband electro-optic sampling. The experimental results are analyzed with a microscopic many-body theory, identifying up to two internal Rabi cycles. The effects of population inversion and ponderomotive contributions are disentangled.

Interaction of strong single-cycle terahertz pulses with semiconductor quantum wells.

An experiment-theory comparison is presented to demonstrate terahertz-induced extreme-nonlinear transients in a GaAs/AlGaAs quantum-well system. The terahertz-pump and optical-probe experiments show pronounced spectral modulations of the light- and heavy-hole excitonic resonances. Excellent agreement with the results of microscopic many-body calculations is obtained, identifying clear ponderomotive contributions and the generation of terahertz harmonics.

Diamine oxidase induces neurite outgrowth in chick dorsal root ganglia by a nonenzymatic mechanism.

The enzyme diamine oxidase (DAO) catalyzes the oxidative deamination of histamine, diamines, and polyamines. DAO has been localized to several tissues, including thymus, kidney, intestine, seminal vesicles, placenta, and pregnancy plasma. DAO is not constitutively expressed in the mammalian brain, but it becomes detectable following focal injury. Although the physiologic role of DAO remains unknown, the observation that it is present at the interface between rapidly dividing and quiescent cells in several tissues suggests that it might be involved in regulating cell division or differentiation at tissue boundaries. In addition, the observation that DAO is expressed in the brain following injury suggests that the protein might play a role in the CNS response to focal neuronal damage. To test that hypothesis, we assessed the ability of purified DAO to alter the pattern of neuronal differentiation and nerve growth in vitro. In chick dorsal root ganglion explant cultures, purified porcine DAO induced neurite outgrowth in the low nanomolar range. Addition of aminoguanidine, which inhibits DAO enzyme activity, did not inhibit the protein's neurotrophic activity. These findings suggest that DAO can function as a neurotrophic ligand independent of its enzymatic activity.