Coherent control of a surface structural phase transition.

Active optical control over matter is desirable in many scientific disciplines, with prominent examples in all-optical magnetic switching1,2, light-induced metastable or exotic phases of solids3-8 and the coherent control of chemical reactions9,10. Typically, these approaches dynamically steer a system towards states or reaction products far from equilibrium. In solids, metal-to-insulator transitions are an important target for optical manipulation, offering ultrafast changes of the electronic4 and lattice11-16 properties. The impact of coherences on the efficiencies and thresholds of such transitions, however, remains a largely open subject. Here, we demonstrate coherent control over a metal-insulator structural phase transition in a quasi-one-dimensional solid-state surface system. A femtosecond double-pulse excitation scheme17-20 is used to switch the system from the insulating to a metastable metallic state, and the corresponding structural changes are monitored by ultrafast low-energy electron diffraction21,22. To govern the transition, we harness vibrational coherence in key structural modes connecting both phases, and observe delay-dependent oscillations in the double-pulse switching efficiency. Mode-selective coherent control of solids and surfaces could open new routes to switching chemical and physical functionalities, enabled by metastable and non-equilibrium states.

Non-thermal phonon dynamics and a quenched exciton condensate probed by surface-sensitive electron diffraction.

Interactions among and between electrons and phonons steer the energy flow in photo-excited materials and govern the emergence of correlated phases. The strength of electron-phonon interactions, decay channels of strongly coupled modes and the evolution of three-dimensional order are revealed by electron or X-ray pulses tracking non-equilibrium structural dynamics. Despite such capabilities, the growing relevance of inherently anisotropic two-dimensional materials and functional heterostructures still calls for techniques with monolayer sensitivity and, specifically, access to out-of-plane phonon polarizations. Here, we resolve non-equilibrium phonon dynamics and quantify the excitonic contribution to the structural order parameter in 1T-TiSe2. To this end, we introduce ultrafast low-energy electron diffuse scattering and trace strongly momentum- and fluence-dependent phonon populations. Mediated by phonon-phonon scattering, a few-picosecond build-up near the zone boundary precedes a far slower generation of zone-centre acoustic modes. These weakly coupled phonons are shown to substantially delay overall equilibration in layered materials. Moreover, we record the surface structural response to a quench of the material's widely investigated exciton condensate, identifying an approximate 30:70 ratio of excitonic versus Peierls contributions to the total lattice distortion in the charge density wave phase. The surface-sensitive approach complements the ultrafast structural toolbox and may further elucidate the impact of phonon scattering in numerous other phenomena within two-dimensional materials, such as the formation of interlayer excitons in twisted bilayers.

Near-Field Manipulation in a Scanning Tunneling Microscope Junction with Plasmonic Fabry-Pérot Tips.

Near-field manipulation in plasmonic nanocavities can provide various applications in nanoscale science and technology. In particular, a gap plasmon in a scanning tunneling microscope (STM) junction is of key interest to nanoscale imaging and spectroscopy. Here we show that spectral features of a plasmonic STM junction can be manipulated by nanofabrication of Au tips using focused ion beam. An exemplary Fabry-Pérot type resonator of surface plasmons is demonstrated by producing the tip with a single groove on its shaft. Scanning tunneling luminescence spectra of the Fabry-Pérot tips exhibit spectral modulation resulting from interference between localized and propagating surface plasmon modes. In addition, the quality factor of the plasmonic Fabry-Pérot interference can be improved by optimizing the overall tip shape. Our approach paves the way for near-field imaging and spectroscopy with a high degree of accuracy.

Near-Field Enhanced Photochemistry of Single Molecules in a Scanning Tunneling Microscope Junction.

Optical near-field excitation of metallic nanostructures can be used to enhance photochemical reactions. The enhancement under visible light illumination is of particular interest because it can facilitate the use of sunlight to promote photocatalytic chemical and energy conversion. However, few studies have yet addressed optical near-field induced chemistry, in particular at the single-molecule level. In this Letter, we report the near-field enhanced tautomerization of porphycene on a Cu(111) surface in a scanning tunneling microscope (STM) junction. The light-induced tautomerization is mediated by photogenerated carriers in the Cu substrate. It is revealed that the reaction cross section is significantly enhanced in the presence of a Au tip compared to the far-field induced process. The strong enhancement occurs in the red and near-infrared spectral range for Au tips, whereas a W tip shows a much weaker enhancement, suggesting that excitation of the localized plasmon resonance contributes to the process. Additionally, using the precise tip-surface distance control of the STM, the near-field enhanced tautomerization is examined in and out of the tunneling regime. Our results suggest that the enhancement is attributed to the increased carrier generation rate via decay of the excited near-field in the STM junction. Additionally, optically excited tunneling electrons also contribute to the process in the tunneling regime.

Graphene-enhanced infrared near-field microscopy.

Graphene is a promising two-dimensional platform for widespread nanophotonic applications. Recent theories have predicted that graphene can also enhance evanescent fields for subdiffraction-limited imaging. Here, for the first time we experimentally demonstrate that monolayer graphene offers a 7-fold enhancement of evanescent information, improving conventional infrared near-field microscopy to resolve buried structures at a 500 nm depth with λ/11-resolution.

Mode-selective ballistic pathway to a metastable electronic phase.

Exploiting vibrational excitation for the dynamic control of material properties is an attractive goal with wide-ranging technological potential. Most metal-to-insulator transitions are mediated by few structural modes and are, thus, ideal candidates for selective driving toward a desired electronic phase. Such targeted navigation within a generally multi-dimensional potential energy landscape requires microscopic insight into the non-equilibrium pathway. However, the exact role of coherent inertial motion across the transition state has remained elusive. Here, we demonstrate mode-selective control over the metal-to-insulator phase transition of atomic indium wires on the Si(111) surface, monitored by ultrafast low-energy electron diffraction. We use tailored pulse sequences to individually enhance or suppress key phonon modes and thereby steer the collective atomic motion within the potential energy surface underlying the structural transformation. Ab initio molecular dynamics simulations demonstrate the ballistic character of the structural transition along the deformation vectors of the Peierls amplitude modes. Our work illustrates that coherent excitation of collective modes via exciton-phonon interactions evades entropic barriers and enables the dynamic control of materials functionality.

Near-Field Spectral Response of Optically Excited Scanning Tunneling Microscope Junctions Probed by Single-Molecule Action Spectroscopy.

The near-field spectral response of metallic nanocavities is a key characteristic in plasmon-assisted photophysical and photochemical processes. Here, we show that the near-field spectral response of an optically excited plasmonic scanning tunneling microscope (STM) junction can be probed by single-molecule reactions that serve as a nanoscale sensor detecting the local field intensity. Near-field action spectroscopy for the cis ↔ cis tautomerization of porphycene on a Cu(110) surface reveals that the field enhancement in the STM junction largely depends on microscopic structures not only on the tip apex, but also on its shaft. Using nanofabrication of Au tips with focused ion beam, we show that the spectral response is strongly modulated through the interference between the localized surface plasmon in the junction and propagating surface plasmon polariton generated on the shaft. Furthermore, it is demonstrated that the near-field spectral response can be manipulated by precisely shaping the tip shaft.