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As conventional electronic materials approach their physical limits, the application of ultrafast optical fields to access transient states of matter cap- tures imagination. The inversion symmetry governs the optical parity selection rule, differentiating between accessible and inaccessible states of matter. To circumvent parity-forbidden transitions, the common practice is to break the inversion symmetry by material design or external fields. Here we report how the application of femtosecond ultraviolet pulses can energize a parity-forbidden dark exciton state in black phosphorus while maintaining its intrinsic material symmetry. Unlike its conventional bandgap absorption in visible-to-infrared, femtosecond ultraviolet excitation turns on efficient Coulomb scattering, promoting carrier multiplication and electronic heating to ~3000 K, and consequently populating its parity-forbidden states. Interfero- metric time- and angle-resolved two-photon photoemission spectroscopy reveals dark exciton dynamics of black phosphorus on ~100 fs time scale and its anisotropic wavefunctions in energy-momentum space, illuminating its potential applications in optoelectronics and photochemistry under ultraviolet optical excitation. 

Molecular materials offer a boundless design palette for light absorption and charge transport in both natural photosynthesis and engineered photovoltaics. They function in combination as chromophores, donors, conductors, and acceptors, enabling the excitation and charge carrier transport through space and wire-like intramolecular pathways. Although quantum coher- ence is believed to enhance photoexcitation and photoinduced charge transfer, fluctuating and inhomogeneous environments accelerate decoherence. Here, we assemble a nanoporous medium consisting of a templated bipyridyl ethylene (BPE) molecule array on a Ag(111) surface that functions as an exceptional intermolecular nonnuclear quantum well conductor of coherent electron waves spanning over 20 Å length. Time-periodic driving of the Ag/BPE interface by femtosecond pulses promotes electrons into a ladder of Floquet quasi-energy donor states, where intermolecular quantum well states act as a resonant doorway for coherent electron transport into BPE/vacuum image potential acceptor states. The bifurcation of electron passage between the Floquet donor ladder and the charge transfer acceptor channel is recorded by projecting the active electrons into the photoemission continuum in an interferometric time- and angle-resolved multiphoton photoemission experiment. We find that exceptional decoupling of electrons from the metal substrate by the molecule- dressed vacuum preserves the coherence on the ∼150 fs time scale. This offers a new paradigm for quantum state design where a molecule-dressed vacuum mediates coherent electron transport in nanoporous molecular architectures.