11 research outputs found

    Ultrafast Microscopy: Imaging Light with Photoelectrons on the Nano鈥揊emto Scale

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    Experimental methods for ultrafast microscopy are advancing rapidly. Promising methods combine ultrafast laser excitation with electron-based imaging or rely on super-resolution optical techniques to enable probing of matter on the nano鈥揻emto scale. Among several actively developed methods, ultrafast time-resolved photoemission electron microscopy provides several advantages, among which the foremost are that time resolution is limited only by the laser source and it is immediately capable of probing of coherent phenomena in solid-state materials and surfaces. Here we present recent progress in interference imaging of plasmonic phenomena in metal nanostructures enabled by combining a broadly tunable femtosecond laser excitation source with a low-energy electron microscope

    Self-Catalyzed Carbon Dioxide Adsorption by Metal鈥揙rganic Chains on Gold Surfaces

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    Efficient capture of CO<sub>2</sub> by chemical means requires a microscopic understanding of the interactions of the molecule鈥搒ubstrate bonding and adsorption-induced collective phenomena. By molecule-resolved imaging with scanning tunneling microscopy (STM), we investigate self-catalyzed CO<sub>2</sub> adsorption on one-dimensional (1D) substrates composed of self-assembled metal鈥搊rganic chains (MOCs) supported on gold surfaces. CO<sub>2</sub> adsorption turns on attractive interchain interactions, which induce pronounced surface structural changes; the initially uniformly dispersed chains gather into close packed bundles, which are held together by highly ordered, single molecule wide CO<sub>2</sub> ranks. CO<sub>2</sub> molecules create more favorable adsorption sites for further CO<sub>2</sub> adsorption by mediating the interchain attraction, thereby self-catalyzing their capture. The release of CO<sub>2</sub> molecules by thermal desorption returns the MOCs to their original structure, indicating that the CO<sub>2</sub> capture and release are reversible processes. The real space microscopic characterization of the self-catalyzed CO<sub>2</sub> adsorption on 1D substrates could be exploited as platform for design of molecular materials for CO<sub>2</sub> capture and reduction

    Ultrafast Microscopy of Spin-Momentum-Locked Surface Plasmon Polaritons

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    Using two-photon photoemission electron microscopy (2P-PEEM) we image the polarization dependence of coupling and propagation of surface plasmon polaritons (SPPs) launched from edges of a triangular, micrometer size, single-crystalline Ag crystal by linearly or circularly polarized light. 2P-PEEM records interferences between the optical excitation field and SPPs it creates with nanofemto space-time resolution. Both the linearly and circularly polarized femtosecond light pulses excite spatially asymmetric 2PP yield distributions, which are imaged. We attribute the asymmetry for linearly polarized light to the relative alignments of the laser polarization and triangle edges, which affect the efficiency of excitation of the longitudinal component of the SPP field. For circular polarization, the asymmetry is caused by matching of the spin angular momenta (SAM) of light and the transverse SAM of SPPs. Moreover, we show that the interference patterns recorded in the 2P-PEEM images are cast by phase shifts and amplitudes for coupling of light into the longitudinal and transverse components of SPP fields. While the interference patterns depend on the excitation polarization, nanofemto movies show that the phase and group velocities of SPPs are independent of SAM of light in time-reversal invariant media. Simulations of the wave interference reproduce the polarization and spin-dependent coupling of optical pulses into SPPs

    Ultrafast Microscopy of Spin-Momentum-Locked Surface Plasmon Polaritons

    No full text
    Using two-photon photoemission electron microscopy (2P-PEEM) we image the polarization dependence of coupling and propagation of surface plasmon polaritons (SPPs) launched from edges of a triangular, micrometer size, single-crystalline Ag crystal by linearly or circularly polarized light. 2P-PEEM records interferences between the optical excitation field and SPPs it creates with nanofemto space-time resolution. Both the linearly and circularly polarized femtosecond light pulses excite spatially asymmetric 2PP yield distributions, which are imaged. We attribute the asymmetry for linearly polarized light to the relative alignments of the laser polarization and triangle edges, which affect the efficiency of excitation of the longitudinal component of the SPP field. For circular polarization, the asymmetry is caused by matching of the spin angular momenta (SAM) of light and the transverse SAM of SPPs. Moreover, we show that the interference patterns recorded in the 2P-PEEM images are cast by phase shifts and amplitudes for coupling of light into the longitudinal and transverse components of SPP fields. While the interference patterns depend on the excitation polarization, nanofemto movies show that the phase and group velocities of SPPs are independent of SAM of light in time-reversal invariant media. Simulations of the wave interference reproduce the polarization and spin-dependent coupling of optical pulses into SPPs

    Ultrafast Microscopy of Spin-Momentum-Locked Surface Plasmon Polaritons

    No full text
    Using two-photon photoemission electron microscopy (2P-PEEM) we image the polarization dependence of coupling and propagation of surface plasmon polaritons (SPPs) launched from edges of a triangular, micrometer size, single-crystalline Ag crystal by linearly or circularly polarized light. 2P-PEEM records interferences between the optical excitation field and SPPs it creates with nanofemto space-time resolution. Both the linearly and circularly polarized femtosecond light pulses excite spatially asymmetric 2PP yield distributions, which are imaged. We attribute the asymmetry for linearly polarized light to the relative alignments of the laser polarization and triangle edges, which affect the efficiency of excitation of the longitudinal component of the SPP field. For circular polarization, the asymmetry is caused by matching of the spin angular momenta (SAM) of light and the transverse SAM of SPPs. Moreover, we show that the interference patterns recorded in the 2P-PEEM images are cast by phase shifts and amplitudes for coupling of light into the longitudinal and transverse components of SPP fields. While the interference patterns depend on the excitation polarization, nanofemto movies show that the phase and group velocities of SPPs are independent of SAM of light in time-reversal invariant media. Simulations of the wave interference reproduce the polarization and spin-dependent coupling of optical pulses into SPPs

    Quasiparticle Interfacial Level Alignment of Highly Hybridized Frontier Levels: H<sub>2</sub>O on TiO<sub>2</sub>(110)

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    Knowledge of the frontier levels鈥 alignment prior to photoirradiation is necessary to achieve a complete quantitative description of H<sub>2</sub>O photocatalysis on TiO<sub>2</sub>(110). Although H<sub>2</sub>O on rutile TiO<sub>2</sub>(110) has been thoroughly studied both experimentally and theoretically, a quantitative value for the energy of the highest H<sub>2</sub>O occupied levels is still lacking. For experiment, this is due to the H<sub>2</sub>O levels being obscured by hybridization with TiO<sub>2</sub>(110) levels in the difference spectra obtained via ultraviolet photoemission spectroscopy (UPS). For theory, this is due to inherent difficulties in properly describing many-body effects at the H<sub>2</sub>O鈥揟iO<sub>2</sub>(110) interface. Using the projected density of states (DOS) from state-of-the-art quasiparticle (QP) <i>G</i><sub>0</sub><i>W</i><sub>0</sub>, we disentangle the adsorbate and surface contributions to the complex UPS spectra of H<sub>2</sub>O on TiO<sub>2</sub>(110). We perform this separation as a function of H<sub>2</sub>O coverage and dissociation on stoichiometric and reduced surfaces. Due to hybridization with the TiO<sub>2</sub>(110) surface, the H<sub>2</sub>O 3a<sub>1</sub> and 1b<sub>1</sub> levels are broadened into several peaks between 5 and 1 eV below the TiO<sub>2</sub>(110) valence band maximum (VBM). These peaks have both intermolecular and interfacial bonding and antibonding character. We find the highest occupied levels of H<sub>2</sub>O adsorbed intact and dissociated on stoichiometric TiO<sub>2</sub>(110) are 1.1 and 0.9 eV below the VBM. We also find a similar energy of 1.1 eV for the highest occupied levels of H<sub>2</sub>O when adsorbed dissociatively on a bridging O vacancy of the reduced surface. In both cases, these energies are significantly higher (by 0.6 to 2.6 eV) than those estimated from UPS difference spectra, which are inconclusive in this energy region. Finally, we apply self-consistent QP<i>GW</i> (scQP<i>GW</i>1) to obtain the ionization potential of the H<sub>2</sub>O鈥揟iO<sub>2</sub>(110) interface

    Ultrafast Microscopy of Spin-Momentum-Locked Surface Plasmon Polaritons

    No full text
    Using two-photon photoemission electron microscopy (2P-PEEM) we image the polarization dependence of coupling and propagation of surface plasmon polaritons (SPPs) launched from edges of a triangular, micrometer size, single-crystalline Ag crystal by linearly or circularly polarized light. 2P-PEEM records interferences between the optical excitation field and SPPs it creates with nanofemto space-time resolution. Both the linearly and circularly polarized femtosecond light pulses excite spatially asymmetric 2PP yield distributions, which are imaged. We attribute the asymmetry for linearly polarized light to the relative alignments of the laser polarization and triangle edges, which affect the efficiency of excitation of the longitudinal component of the SPP field. For circular polarization, the asymmetry is caused by matching of the spin angular momenta (SAM) of light and the transverse SAM of SPPs. Moreover, we show that the interference patterns recorded in the 2P-PEEM images are cast by phase shifts and amplitudes for coupling of light into the longitudinal and transverse components of SPP fields. While the interference patterns depend on the excitation polarization, nanofemto movies show that the phase and group velocities of SPPs are independent of SAM of light in time-reversal invariant media. Simulations of the wave interference reproduce the polarization and spin-dependent coupling of optical pulses into SPPs

    Universal Aspects of Ultrafast Optical Pulse Scattering by a Nanoscale Asperity

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    We combine photoemission electron microscopy and electromagnetic simulations to describe the surface plasmon polariton dynamics following interaction of an ultrafast optical pulse with a slit coupling structure in a silver film. Through analysis of interference phenomena that lead to photoelectron emission from the silver film, we establish the universal contributions of a nanoscale asperity to the scattered surface field. Our results reveal the important role of surface cylindrical waves within the slit in the excitation of surface plasmon

    Ultrafast Plasmon-Enhanced Hot Electron Generation at Ag Nanocluster/Graphite Heterojunctions

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    Hot electron processes at metallic heterojunctions are central to optical-to-chemical or electrical energy transduction. Ultrafast nonlinear photoexcitation of graphite (Gr) has been shown to create hot thermalized electrons at temperatures corresponding to the solar photo颅sphere in less than 25 fs. Plasmonic resonances in metallic nanoparticles are also known to efficiently generate hot electrons. Here we deposit Ag nanoclusters (NC) on Gr to study the ultrafast hot electron generation and dynamics in their plasmonic heterojunctions by means of time-resolved two-photon photoemission (2PP) spectroscopy. By tuning the wavelength of p-polarized femtosecond excitation pulses, we find an enhancement of 2PP yields by 2 orders of magnitude, which we attribute to excitation of a surface-normal Mie plasmon mode of Ag/Gr heterojunctions at 3.6 eV. The 2PP spectra include contributions from (<i>i</i>) coherent two-photon absorption of an occupied interface state (IFS) 0.2 eV below the Fermi level, which electronic structure calculations assign to chemisorption-induced charge transfer, and (<i>ii</i>) hot electrons in the 蟺*-band of Gr, which are excited through the coherent screening response of the substrate. Ultrafast pump鈥損robe measurements show that the IFS photoemission occurs via virtual intermediate states, whereas the characteristic lifetimes attribute the hot electrons to population of the 蟺*-band of Gr via the plasmon dephasing. Our study directly probes the mechanisms for enhanced hot electron generation and decay in a model plasmonic heterojunction

    Level Alignment of a Prototypical Photocatalytic System: Methanol on TiO<sub>2</sub>(110)

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    Photocatalytic activity depends on the optimal alignment of electronic levels at the molecule鈥搒emiconductor interface. Establishing the level alignment experimentally is complicated by the uncertain chemical identity of the surface species. We address the assignment of the occupied and empty electronic levels for the prototypical photocatalytic system consisting of methanol on a rutile TiO<sub>2</sub>(110) surface. Using many-body quasiparticle (QP) techniques, we show that the frontier levels measured in UV photoelectron and two-photon photoemission spectroscopy experiments can be assigned to molecularly chemisorbed methanol rather than its dissociated product, the methoxy species. We find that the highest occupied molecular orbital of the methoxy species is much closer to the valence band maximum, suggesting why it is more photocatalytically active than the methanol molecule. We develop a general semiquantitative model for predicting many-body QP energies based on the electronic screening within the bulk, molecular, or vacuum regions of the wave functions at molecule鈥搒emiconductor interfaces
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