Coherent light control of a metastable hidden state.

Metastable phases present a promising route to expand the functionality of complex materials. Of particular interest are light-induced metastable phases that are inaccessible under equilibrium conditions, as they often host new, emergent properties switchable on ultrafast timescales. However, the processes governing the trajectories to such hidden phases remain largely unexplored. Here, using time- and angle-resolved photoemission spectroscopy, we investigate the ultrafast dynamics of the formation of a hidden quantum state in the layered dichalcogenide 1T-TaS2 upon photoexcitation. Our results reveal the nonthermal character of the transition governed by a collective charge-density-wave excitation. Using a double-pulse excitation of the structural mode, we show vibrational coherent control of the phase-transition efficiency. Our demonstration of exceptional control, switching speed, and stability of the hidden state are key for device applications at the nexus of electronics and photonics.

Functionalization and passivation of ultrathin alumina films of defined sub-nanometer thickness with self-assembled monolayers.

Instability of ultrathin surface oxides on alloys under environmental conditions can limit the opportunities for applications of these systems when the thickness control of the insulating oxide film is crucial for device performance. A procedure is developed to directly deposit self-assembled monolayers (SAM) from solvent onto substrates prepared under ultra-high vacuum conditions without exposure to air. As an example, rhenium photosensitizers functionalized with carboxyl linker groups are attached to ultrathin alumina grown on NiAl(1 1 0). The thickness change of the oxide layer during the SAM deposition is quantified by x-ray photoelectron spectroscopy and can be drastically reduced to one atomic layer. The SAM acts as a capping layer, stabilizing the oxide thin film under environmental conditions. Ultraviolet photoelectron spectroscopy elucidates the band alignment in the resulting heterostructure. The method for molecule attachment presented in this manuscript can be extended to a broad class of molecules vulnerable to pyrolysis upon evaporation and presents an elegant method for attaching molecular layers on solid substrates that are sensitive to air.

Atomically Resolved Band Bending Effects in a p-n Heterojunction of Cu2O and a Cobalt Macrocycle.

We present a hetero junction based on macrocyclic hydrogen evolution catalysts (HEC) physisorbed on a single crystalline Cu2O(111) surface. Angle-resolved X-ray photoelectron spectroscopy (ARXPS) provides the spatial resolution of the band bending within the first nanometer of the subsurface region. Oxygen vacancies on the Cu2O(111) surface cause a downward band bending which is conserved upon adsorption of HEC layers of various thicknesses. This allows photoexcited electrons to be directed toward the surface where they can be made available for the reduction of protons by the HEC. Furthermore, Poisson's equation relates more subtle changes in the measured ARXPS spectra to the local charge density profile within the first 7 Å away from the surface and with atomic resolution. All observations are consistent with a polarization of the molecular layer in response to the electrical field at the oxide surface, which should be a general phenomenon at such organic-oxide heterointerfaces.

The impact of metalation on adsorption geometry, electronic level alignment and UV-stability of organic macrocycles on TiO2(110).

Metal complexes of the tetradentate bipyridine based macrocycle pyrphyrin (Pyr) have recently shown promise as water reduction catalysts in homogeneous photochemical water splitting reactions. In this study, the adsorption and metalation of pyrphyrin on stoichiometric TiO2(110) is investigated in ultrahigh vacuum by means of scanning tunneling microscopy, photoelectron spectroscopy, low-energy electron diffraction, and density functional theory. In a joint experimental and computational effort, the local adsorption geometry at low coverage, the long-range molecular ordering at higher coverage and the electronic structure have been determined for both the bare ligand and the cobalt-metalated Pyr molecule on TiO2. The energy level alignment of CoPyr/TiO2 supports electron injection into TiO2 upon photoexcitation of the CoPyr complex and thus renders it a potential sensitizer dye. Importantly, Co-incorporation is found to stabilize the Pyr molecule against photo-induced degradation, while the bare ligand is decomposed rapidly under continuous UV-irradiation. This interesting phenomenon is discussed in terms of additional de-excitation channels for electronically highly excited molecular states.

Invited Article: High resolution angle resolved photoemission with tabletop 11 eV laser.

We developed a table-top vacuum ultraviolet (VUV) laser with 113.778 nm wavelength (10.897 eV) and demonstrated its viability as a photon source for high resolution angle-resolved photoemission spectroscopy (ARPES). This sub-nanosecond pulsed VUV laser operates at a repetition rate of 10 MHz, provides a flux of 2 × 10(12) photons/s, and enables photoemission with energy and momentum resolutions better than 2 meV and 0.012 Å(-1), respectively. Space-charge induced energy shifts and spectral broadenings can be reduced below 2 meV. The setup reaches electron momenta up to 1.2 Å(-1), granting full access to the first Brillouin zone of most materials. Control over the linear polarization, repetition rate, and photon flux of the VUV source facilitates ARPES investigations of a broad range of quantum materials, bridging the application gap between contemporary low energy laser-based ARPES and synchrotron-based ARPES. We describe the principles and operational characteristics of this source and showcase its performance for rare earth metal tritellurides, high temperature cuprate superconductors, and iron-based superconductors.

Thickness-Dependent Coherent Phonon Frequency in Ultrathin FeSe/SrTiO3 Films.

Ultrathin FeSe films grown on SrTiO3 substrates are a recent milestone in atomic material engineering due to their important role in understanding unconventional superconductivity in Fe-based materials. By using femtosecond time- and angle-resolved photoelectron spectroscopy, we study phonon frequencies in ultrathin FeSe/SrTiO3 films grown by molecular beam epitaxy. After optical excitation, we observe periodic modulations of the photoelectron spectrum as a function of pump-probe delay for 1-unit-cell, 3-unit-cell, and 60-unit-cell thick FeSe films. The frequencies of the coherent intensity oscillations increase from 5.00 ± 0.02 to 5.25 ± 0.02 THz with increasing film thickness. By comparing with previous works, we attribute this mode to the Se A1g phonon. The dominant mechanism for the phonon softening in 1-unit-cell thick FeSe films is a substrate-induced lattice strain. Our results demonstrate an abrupt phonon renormalization due to a lattice mismatch between the ultrathin film and the substrate.

Energy distribution curves of ultrafast laser-induced field emission and their implications for electron dynamics.

Energy distribution curves of laser-induced electron pulses from a tungsten tip have been measured as a function of tip voltage and laser power. Electron emission via tunneling through and/or excitation over the surface barrier from photoexcited nonequilibrium electron distributions are clearly observed. The spectral shapes largely vary with the emission processes and are strongly affected by electron dynamics. Simulations successfully reproduce the spectra, thus allowing direct insight into the involved electron dynamics and revealing the temporal tunability of electron emission via the two experimental parameters. These results should be useful to optimize the pulse characteristics for many applications based on ultrafast laser-induced electron emission.

Time-resolved photoelectron spectroscopy to probe ultrafast charge transfer and electron dynamics in solid surface systems and at metal-molecule interfaces.

Photoelectron spectroscopy (PES) is a versatile tool, which provides insight into electronic structure and dynamics in condensed matter, surfaces, interfaces and molecules. The history of PES is briefly outlined and illustrated by current developments in the field of time-resolved PES. Our group's research is mostly aimed at studying ultrafast processes and associated lifetimes related to electronic excitation at solid surfaces.

Optical control of field-emission sites by femtosecond laser pulses.

We have investigated field-emission patterns from a clean tungsten tip apex induced by femtosecond laser pulses. Strongly asymmetric modulations of the field-emission intensity distributions are observed depending on the polarization of the light and the laser incidence direction relative to the azimuthal orientation of tip apex. In effect, we have realized an ultrafast pulsed field-emission source with site selectivity. Simulations of local fields on the tip apex and of electron emission patterns based on photoexcited nonequilibrium electron distributions explain our observations quantitatively.