Light interacting with nanostructured metals excites the collective charge density fluctuations known as surface plasmons (SP). Through excitation of the localized SP eigenmodes incident light is trapped on the nanometer spatial and femtosecond temporal scales and its field is enhanced. Here we demonstrate the imaging and quantum control of SP dynamics in a nanostructured silver film. By inducing and imaging the nonlinear two-photon photoemission from the sample with a pair of identical 10-fs laser pulses while scanning the pulse delay, we record a movie of SP fields at a rate of 330-attoseconds/frame.

A movie of the dispersive and dissipative propagation of surface plasmon polariton (SPP) wave packets at a silver/vacuum interface is recorded by the interferometric time-resolved photoemission electron microscopy with 60 nm spatial resolution and 330 as frame interval. The evolution of SPP wave packets is imaged through a two-path interference created by a pair of 10 fs phase correlated pump-probe light pulses at 400 nm. The wave packet evolution is simulated using the complex dielectric function of silver.

Chemisorption of atoms and molecules controls many interfacial phenomena such as charge transport and catalysis. The question of how the intrinsic properties of the interacting materials define the electronic structure of their interface remains one of the most important, yet intractable problems in surface physics. Through two-photon photoemission spectroscopy we determine a common binding energy of ϳ1.8-2.0 eV with respect to the vacuum for the unoccupied resonance of the ns valence electron of alkali atoms ͑Li-Cs͒ chemisorbed at low coverage ͑less than 0.1 monolayer͒ on noble metal ͓Cu͑111͒ and Ag͑111͔͒ surfaces. We present a theoretical model based on the semiempirical potentials of the adsorbates and the substrates, their principal mode of interaction through the Coulomb interaction, and the ab initio adsorption structures. Our analysis reveals that atomic size and ionization potential independent interfacial electronic structure is a consequence of the Coulomb interaction among the ns electron, the alkali-atom ionic core, and the induced image charge in the substrate. We expect the same interactions to define the effective electronic potentials for a broad range of molecule/metal interfaces.

Localized surface plasmon resonance (LSPR) can be supported by metallic nanoparticles and engineered nanostructures. An understanding of the spatially resolved near-field properties and dynamics of LSPR is important, but remains experimentally challenging. We report experimental studies toward this aim using photoemission electron microscopy (PEEM) with high spatial resolution of sub-10 nm. Various engineered gold nanostructure arrays (such as rods, nanodisk-like particles and dimers) are investigated via PEEM using near-infrared (NIR) femtosecond laser pulses as the excitation source. When the LSPR wavelengths overlap the spectrum of the femtosecond pulses, the LSPR is efficiently excited and promotes multiphoton photoemission, which is correlated with the local intensity of the metallic nanoparticles in the near field. Thus, the local field distribution of the LSPR on different Au nanostructures can be directly explored and discussed using the PEEM images. In addition, the dynamics of the LSPR is studied by combining interferometric time-resolved pump-probe technique and PEEM. Detailed information on the oscillation and dephasing of the LSPR field can be obtained. The results identify PEEM as a powerful tool for accessing the near-field mapping and dynamic properties of plasmonic nanostructures. Keywords: femtosecond laser; local field enhancement; near-field imaging; photoemission electron microscopy; surface plasmon resonance INTRODUCTION Because of the rapid development of nanofabrication techniques, metallic nanostructures that can exhibit localized surface plasmon resonance (LSPR) can be fabricated using several methods. The resonance frequency and amplitude of LSPR on metallic nanostructures are known to depend on the metal materials, shapes, and surrounding media. [1][2][3][4] In addition, the LSPR can confine optical fields in nanoscale space, leading to the so-called local field enhancement effect. These unique properties promote the application of LSPR in many fields, such as surface-enhanced Raman scattering, 5-8 sensing, 1,9,10 plasmonassisted photochemical reactions 4,11,12 and photocurrent generation. [13][14][15] To further understand the LSPR mechanism and to optimize the design of the plasmonic nanostructures for most applications, the near-field properties of the LSPR fields (especially the near-field distribution of the plasmonic nanostructures) must be determined. To date, investigations of the optical properties of LSPR have largely relied on far-field spectroscopic techniques or numerical simulations. Several experimental approaches have been utilized to visualize the near field, including scanning photoionization microscopy, 16,17 scanning near-field optical microscopy, 18-21 nonlinear luminescence or fluorescent microscopy, 22,23 nonlinear photopolymerization 24,25 and near-field ablation of a substrate. [26][27][28][29] However, these approaches have

Nonlinear two-photon photoemission electron microscopy is used to image surface plasmon polariton (SPP) wave packets excited by an obliquely incident laser pulse (~10 fs) at a single slit fabricated in a thin silver film. We image the forward propagating polarization grating formed by the coherent superposition of the external excitation pulse and the SPP wave packet fields. By systematically varying the coupling slit width from sub-to multiple-wavelength scale, we observe a modulated increase of the grating intensity, which is phenomenologically accounted for by distinct contributions to the forward coupling efficiency from the incident to the SPP waves. Full-wave, vectorial finite-difference time-domain (FDTD) simulation of the experiments is in good agreement with the experimental observations and explains their origin. In particular, the FDTD simulation illustrates detailed spatial variation of the polarization grating as a function of the geometry of the slit under excitation by ultrafast laser pulses at an oblique incidence.2

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Dipole and quadrupole modes are the two lowest orders of localized surface plasmon resonance (LSPR) eigenmodes in metallic nanoparticles. Of these two modes, the quadrupole mode is forbidden for symmetric metallic nanoparticles excited by linearly polarized light at normal incidence. Here, we demonstrate excitation of the quadrupole mode in symmetrical gold (Au) nanoblocks shined with s-polarized light at oblique incidence. In particular, we probe the near-field LSPR in Au nanoblocks using photoemission electron microscopy (PEEM) and find that at oblique incidence, the dipole and quadrupole modes can be selectively excited, in terms of near-field enhancement, by manipulating the light polarization state. More importantly, by time-resolved PEEM measurements, we experimentally demonstrate that the quadrupole mode in symmetrical Au nanoblocks has longer dephasing time than that of the dipole mode.

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