Many of the fundamental optical and electronic properties of atomically thin transition metal dichalcogenides are dominated by strong Coulomb interactions between electrons and holes, forming tightly bound atom-like states called excitons. Here, we directly trace the ultrafast formation of excitons by monitoring the absolute densities of bound and unbound electron−hole pairs in single monolayers of WSe 2 on a diamond substrate following femtosecond nonresonant optical excitation. To this end, phaselocked mid-infrared probe pulses and field-sensitive electro-optic sampling are used to map out the full complex-valued optical conductivity of the nonequilibrium system and to discern the hallmark low-energy responses of bound and unbound pairs. While the spectral shape of the infrared response immediately after above-bandgap injection is dominated by free charge carriers, up to 60% of the electron−hole pairs are bound into excitons already on a subpicosecond time scale, evidencing extremely fast and efficient exciton formation. During the subsequent recombination phase, we still find a large density of free carriers in addition to excitons, indicating a nonequilibrium state of the photoexcited electron−hole system. KEYWORDS: Dichalcogenides, atomically thin 2D crystals, exciton formation, ultrafast dynamics A tomically thin transition metal dichalcogenides (TMDCs) have attracted tremendous attention due to their direct bandgaps in the visible spectral range, 1,2 strong interband optical absorption, 3,4 intriguing spin-valley physics, 5−7 and applications as optoelectronic devices. 8−11 The physics of twodimensional (2D) TMDCs are governed by strong Coulomb interactions owing to the strict quantum confinement in the out-of-plane direction and the weak dielectric screening of the environment. 12,13 Electrons and holes in these materials can form excitons with unusually large binding energies of many hundreds of millielectronvolts, 14−19 making these quasiparticles stable even at elevated temperatures and high carrier densities. 20,21 The properties of excitons in 2D TMDCs are a topic of intense research, investigating, for example, rapid exciton−exciton scattering, 22 interlayer excitons, 23 charged excitons and excitonic molecules, 24,25 ultrafast recombination dynamics, 19,26−28 or efficient coupling to light and lattice vibrations. 4,19,29,30 In many experiments, excitons are created indirectly through nonresonant optical excitation or electronic injection, which may prepare unbound charge carriers with energies far above the exciton resonance. 8,18 Subsequently, the electrons and holes are expected to relax toward their respective band minima and form excitons in the vicinity of the fundamental energy gap. In principle, strong Coulomb attraction in 2D TMDCs should foster rapid exciton formation. Recent optical pump−probe studies relying on interband transitions have reported characteristic formation times on subpicosecond time-scales. 31 The relaxation of large excess energies, however, requires many sca...

Heterostructures of atomically thin van der Waals bonded monolayers have opened a unique platform to engineer Coulomb correlations, shaping excitonic 1-3 , Mott insulating 4 , or superconducting phases 5,6 . In transition metal dichalcogenide heterostructures 7 , electrons and holes residing in different monolayers can bind into spatially indirect excitons 1,3,8-11 with a strong potential for optoelectronics 11,12 , valleytronics 1,3,13 , Bose condensation 14 , superfluidity 14,15 , and moiré-induced nanodot lattices 16 . Yet these ideas require a microscopic understanding of the formation, dissociation, and thermalization dynamics of correlations including ultrafast phase transitions. Here we introduce a direct ultrafast access to Coulomb correlations between monolayers; phase-locked mid-infrared pulses allow us to measure the binding energy of interlayer excitons in WSe2/WS2 hetero-bilayers by revealing a novel 1s-2p resonance, explained by a fully quantum mechanical model. Furthermore, we trace, with subcycle time resolution, the transformation of an exciton gas photogenerated in the WSe2 layer directly into interlayer excitons. Depending on the stacking angle, intra-and interlayer species coexist on picosecond scales and the 1s-2p resonance becomes renormalized. Our work provides a direct measurement of the binding energy of interlayer excitons and opens the possibility to trace and control correlations in novel artificial materials.In monolayers of transition metal dichalcogenides (TMDs), the confinement of electronic motion into two dimensions and the suppression of dielectric screening facilitate unusually strong Coulomb interaction 17-20 . This gives rise to excitons with giant binding energies of several hundred meV 17 , small Bohr radii 18 and ultrashort radiative lifetimes 19 . When two monolayers are contacted with type-II band alignment, the conduction band minimum and the valence band maximum are located in two different layers 7 . Owing to their proximity, electron-hole (e-h) pairs in adjacent monolayers are still subject to strong mutual Coulomb attraction. Interband photoluminescence combined with theory has, indeed, provided evidence of interlayer excitons 8,21-23 . Because the composite electron and hole wavefunctions overlap only weakly in space, these excitons are long lived -a key asset for future applications 1,3,14-16,24 .Yet, the weak coupling to light renders these quasiparticles inaccessible to interband absorption spectroscopy. Hence the binding energies of interlayer excitons, which depend sensitively on the delocalization of the electronic wavefunctions over the heterostructure 23 , have not been measured.Signatures of the ultrafast interlayer charge transfer have been studied by interband spectroscopy 8,21 .These techniques, however, cannot measure Coulomb correlations or the formation of interlayer excitons, on the intrinsic ultrashort timescales.Meanwhile, phase-locked electromagnetic pulses in the terahertz (THz) and mid-infrared (MIR) range have directly accessed ultrafast low-...

Thiolutin is a disulfide-containing antibiotic and anti-angiogenic compound produced by Streptomyces. Its biological targets are not known. We show that reduced thiolutin is a zinc-chelator that inhibits the JAB1/MPN/Mov34 (JAMM) domain-containing metalloprotease Rpn11, a de-ubiquinating enzyme of the 19S proteasome. Thiolutin also inhibits the JAMM metalloproteases Csn5, the deneddylase of the COP9 signalosome, Associated-molecule-with-the-SH3-Domain-of-STAM (AMSH), which regulates ubiquitin-dependent sorting of cell-surface receptors, and Brcc36, a K63-specific deubiquitnase of BRCC36-containing isopeptidase complex (BRISC) and BRCA1-BRCA2-containing complex (BRCC). We provide evidence that other dithiolopyrrolones also function as inhibitors of JAMM metalloproteases.

The recent discovery of artificial phase transitions induced by stacking monolayer materials at magic twist angles represents a paradigm shift for solid state physics. Twist-induced changes of the single-particle band structure have been studied extensively, yet a precise understanding of the underlying Coulomb correlations has remained challenging. Here we reveal in experiment and theory, how the twist angle alone affects the Coulomb-induced internal structure and mutual interactions of excitons. In homobilayers of WSe 2 , we trace the internal 1s-2p resonance of excitons with phase-locked mid-infrared pulses as a function of the twist angle. Remarkably, the exciton binding energy is renormalized by up to a factor of two, their lifetime exhibits an enhancement by more than an order of magnitude, and the exciton-exciton interaction is widely tunable. Our work opens the possibility of tailoring quasiparticles in search of unexplored phases of matter in a broad range of van der Waals heterostructures.

Mammalian cells exhibit numerous strategies to recognize and contain viral infections. The best-characterized antiviral responses are those that are induced within the cytosol by receptors that activate interferon responses or shut down translation. Antiviral responses also occur in the nucleus, yet these intranuclear innate immune responses are poorly defined at the receptor-proximal level. In this study, we explored the ability of cells to restrict infection by assembling viral DNA into transcriptionally silent heterochromatin within the nucleus. We found that the IFI16 restriction factor forms filaments on DNA within infected cells. These filaments recruit antiviral restriction factors to prevent viral replication in various cell types. Mechanistically, IFI16 filaments inhibit the recruitment of RNA polymerase II to viral genes. We propose that IFI16 filaments with associated restriction factors constitute a “restrictosome” structure that can signal to other parts of the nucleus where foreign DNA is located that it should be silenced.