Field emission of electrons generated by the near field of strongly coupled plasmons

Polarization-Resolved Electron Energy Gain Nanospectroscopy With Phase-Structured Electron Beams

Free-electron-based measurements in scanning transmission electron microscopes (STEMs) reveal valuable information on the broadband spectral responses of nanoscale systems with deeply subdiffraction limited spatial resolution. Leveraging recent advances in manipulating the spatial phase profile of the transverse electron wavefront, we theoretically describe interactions between the electron probe and optically stimulated nanophotonic targets in which the probe gains energy while simultaneously transitioning between transverse states with distinct phase profiles. Exploiting the selection rules governing such transitions, we propose phase-shaped electron energy gain nanospectroscopy for probing the 3D polarization-resolved response field of an optically excited target with nanoscale spatial resolution. Considering ongoing instrumental developments, polarized generalizations of STEM electron energy loss and gain measurements hold the potential to become powerful tools for fundamental studies of quantum materials and their interaction with nearby nanostructures supporting localized surface plasmon or phonon polaritons as well as for noninvasive imaging and nanoscale 3D field tomography.

Suppression of the vacuum space-charge effect in fs-photoemission by a retarding electrostatic front lens

The performance of time-resolved photoemission experiments at fs-pulsed photon sources is ultimately limited by the e-e Coulomb interaction, downgrading energy and momentum resolution. Here, we present an approach to effectively suppress space-charge artifacts in momentum microscopes and photoemission microscopes. A retarding electrostatic field generated by a special objective lens repels slow electrons, retaining the k-image of the fast photoelectrons. The suppression of space-charge effects scales with the ratio of the photoelectron velocities of fast and slow electrons. Fields in the range from -20 to -1100 V/mm for E_kin = 100 eV to 4 keV direct secondaries and pump-induced slow electrons back to the sample surface. Ray tracing simulations reveal that this happens within the first 40 to 3 μm above the sample surface for E_kin = 100 eV to 4 keV. An optimized front-lens design allows switching between the conventional accelerating and the new retarding mode. Time-resolved experiments at E_kin = 107 eV using fs extreme ultraviolet probe pulses from the free-electron laser FLASH reveal that the width of the Fermi edge increases by just 30 meV at an incident pump fluence of 22 mJ/cm² (retarding field -21 V/mm). For an accelerating field of +2 kV/mm and a pump fluence of only 5 mJ/cm², it increases by 0.5 eV (pump wavelength 1030 nm). At the given conditions, the suppression mode permits increasing the slow-electron yield by three to four orders of magnitude. The feasibility of the method at high energies is demonstrated without a pump beam at E_kin = 3830 eV using hard x rays from the storage ring PETRA III. The approach opens up a previously inaccessible regime of pump fluences for photoemission experiments.

Plasmonic nanostar photocathodes for optically-controlled directional currents

Plasmonic nanocathodes offer unique opportunities for optically driving, switching, and steering femtosecond photocurrents in nanoelectronic devices and pulsed electron sources. However, angular photocurrent distributions in nanoplasmonic systems remain poorly understood and are therefore difficult to anticipate and control. Here, we provide a direct momentum-space characterization of multiphoton photoemission from plasmonic gold nanostars and demonstrate all-optical control over these currents. Versatile angular control is achieved by selectively exciting different tips on single nanostars via laser frequency or linear polarization, thereby rotating the tip-aligned directional photoemission as observed with angle-resolved 2D velocity mapping and 3D reconstruction. Classical plasmonic field simulations combined with quantum photoemission theory elucidate the role of surface-mediated nonlinear excitation for plasmonic field enhancements highly concentrated at the sharp tips (Rtip = 3.4 nm). We thus establish a simple mechanism for femtosecond spatiotemporal current control in designer nanosystems.

Electron number density conservation model combined with a self-absorption correction methodology for analysis of nanostructure plasma using laser-induced breakdown spectroscopy

We studied laser ablation and plasma property evolution for a nickel (Ni) doped tin (Sn) oxide nanostructures target using laser-induced breakdown spectroscopy (LIBS). The transition metal Ni doped tin oxide nanostructures were synthesized by co-precipitation and hydrothermal methodologies. The size of prepared nanoparticles was verified by X-ray diffraction and transmission electron microscopy techniques. A frequency-doubled pulsed Nd:YAG laser with a wavelength of 532 nm was used to produce ablated plasma nanostructures. Ablation of doped and undoped nanostructures revealed salient-enhanced spectral emissions compared with their bulky counterparts. The emission lines of the constituent elements of doped material were used to find plasma parameters. The plasma temperature was estimated from a Boltzmann plot, and the electron number density was determined from the Saha-Boltzmann equation. The self-absorption effect has been observed in tiny plasma of nanostructures. The affected profiles of spectral lines of Ni and Sn nanoparticles due to self-absorption in LIBS spectra were corrected by the internal reference self-absorption correction (IRSAC) methodology. After correction of emitted line intensities by IRSAC, the electron number density (END) conservation approach was applied for quantitative analysis of doped nanostructures. In the END conservation approach, quantitative analysis of samples was carried out using electron number densities. Quantitative results derived from the END conservation approach at high and low concentrations exhibited good correlation when these were compared and validated with results from a conventional calibration free approach and the standard recognized energy dispersive X-ray technique.

Ultrafast Field-Emission Electron Sources Based on Nanomaterials

The search for electron sources with simultaneous optimal spatial and temporal resolution has become an area of intense activity for a wide variety of applications in the emerging fields of lightwave electronics and attosecond science. Most recently, increasing efforts are focused on the investigation of ultrafast field-emission phenomena of nanomaterials, which not only are fascinating from a fundamental scientific point of view, but also are of interest for a range of potential applications. Here, the current state-of-the-art in ultrafast field-emission, particularly sub-optical-cycle field emission, based on various nanostructures (e.g., metallic nanotips, carbon nanotubes) is reviewed. A number of promising nanomaterials and possible future research directions are also established.

Interferometric time- and energy-resolved photoemission electron microscopy for few-femtosecond nanoplasmonic dynamics

We report a time-resolved normal-incidence photoemission electron microscope with an imaging time-of-flight detector using ∼7-fs near-infrared laser pulses and a phase-stabilized interferometer for studying ultrafast nanoplasmonic dynamics via nonlinear photoemission from metallic nanostructures. The interferometer's stability (35 ± 6 as root-mean-square from 0.2 Hz to 40 kHz) as well as on-line characterization of the driving laser field, which is a requirement for nanoplasmonic near-field reconstruction, is discussed in detail. We observed strong field enhancement and few-femtosecond localized surface plasmon lifetimes at a monolayer of self-assembled gold nanospheres with ∼40 nm diameter and ∼2 nm interparticle distance. A wide range of plasmon resonance frequencies could be simultaneously detected in the time domain at different nanospheres, which are distinguishable already within the first optical cycle or as close as about ±1 fs around time-zero. Energy-resolved imaging (microspectroscopy) additionally revealed spectral broadening due to strong-field or space charge effects. These results provide a clear path toward visualizing optically excited nanoplasmonic near-fields at ultimate spatiotemporal resolution.

A Quantum Chemistry Approach Based on the Analogy with π-System in Polymers for a Rapid Estimation of the Resonance Wavelength of Nanoparticle Systems

In this paper, the Variational Method based on the Hückel Theory is applied to NPs chain and aggregate systems in order to estimate the energy of the plasmon and, in turn, the resonance wavelength shift, which is caused by the interaction of adjacent NPs. This method is based on the analogies of NPs dipole interactions and the π-system in molecules. Differently from the Hartree-Fock method that is a self-consistent model, in this approach, the input data that this method requires is the dimer energy shift with respect to single NPs. This enables us to acquire a simultaneous estimation of the wavefunctions of the NPs system as well as the expectation energy value of every kind of NPs system. The main advantage of this approach is the rapid response and ease of application to every kind of geometries and spacing from the linear chain to clusters, without the necessity of a time-consuming calculation. The results obtained with this model are closely aligned to related literature and open the way to further development of this methodology for investigating other properties of NPs systems.

Spatial- and energy-resolved photoemission electron from plasmonic nanoparticles in multiphoton regime

Spatial-resolved photoelectron spectra have been observed from plasmonic metallic nanostructure and flat metal surface by a combination of time-of-flight photoemission electron microscope and femtosecond laser oscillator. The photoemission's main contribution is at localized 'hot spots,' where the plasmonic effect dominates and multiphoton photoemission is confirmed as the responsible mechanism for emission in both samples. Photoelectron spectra from hot spots exponentially decay in high energy regimes, smearing out the Fermi edge in Au flat surface. This phenomenon is explained by the emergence of above threshold photoemission that is induced by plasmonic effect; other competing mechanisms are ruled out. It is the first time that we have observed the emergence of high kinetic energy photoelectron in weak field region around 'hot spot.' We attribute the emergence of high kinetic energy photoelectron to the drifting of the liberated electron from plasmonic hot spot and driven by the gradient of plasmonic field.

Spatial Control of Multiphoton Electron Excitations in InAs Nanowires by Varying Crystal Phase and Light Polarization

We demonstrate the control of multiphoton electron excitations in InAs nanowires (NWs) by altering the crystal structure and the light polarization. Using few-cycle, near-infrared laser pulses from an optical parametric chirped-pulse amplification system, we induce multiphoton electron excitations in InAs nanowires with controlled wurtzite (WZ) and zincblende (ZB) segments. With a photoemission electron microscope, we show that we can selectively induce multiphoton electron emission from WZ or ZB segments of the same wire by varying the light polarization. Developing ab initio GW calculations of first to third order multiphoton excitations and using finite-difference time-domain simulations, we explain the experimental findings: While the electric-field enhancement due to the semiconductor/vacuum interface has a similar effect for all NW segments, the second and third order multiphoton transitions in the band structure of WZ InAs are highly anisotropic in contrast to ZB InAs. As the crystal phase of NWs can be precisely and reliably tailored, our findings open up for new semiconductor optoelectronics with controllable nanoscale emission of electrons through vacuum or dielectric barriers.

Momentum Distribution of Electrons Emitted from Resonantly Excited Individual Gold Nanorods

Electron emission by femtosecond laser pulses from individual Au nanorods is studied with a time-of-flight momentum resolving photoemission electron microscope (ToF k-PEEM). The Au nanorods adhere to a transparent indium-tin oxide substrate, allowing for illumination from the rear side at normal incidence. Localized plasmon polaritons are resonantly excited at 800 nm with 100 fs long pulses. The momentum distribution of emitted electrons reveals two distinct emission mechanisms: a coherent multiphoton photoemission process from the optically heated electron gas leads to an isotropic emission distribution. In contrast, an additional emission process resulting from the optical field enhancement at both ends of the nanorod leads to a strongly directional emission parallel to the nanorod's long axis. The relative intensity of both contributions can be controlled by the peak intensity of the incident light.

Imaging the Nonlinear Plasmoemission Dynamics of Electrons from Strong Plasmonic Fields

We use subcycle time-resolved photoemission microscopy to unambiguously distinguish optically triggered electron emission (photoemission) from effects caused purely by the plasmonic field (termed "plasmoemission"). We find from time-resolved imaging that nonlinear plasmoemission is dominated by the transverse plasmon field component by utilizing a transient standing wave from two counter-propagating plasmon pulses of opposite transverse spin. From plasmonic foci on flat metal surfaces, we observe highly nonlinear plasmoemission up to the fifth power of intensity and quantized energy transfer, which reflects the quantum-mechanical nature of surface plasmons. Our work constitutes the basis for novel plasmonic devices such as nanometer-confined ultrafast electron sources as well as applications in time-resolved electron microscopy.

Attosecond physics at the nanoscale

Recently two emerging areas of research, attosecond and nanoscale physics, have started to come together. Attosecond physics deals with phenomena occurring when ultrashort laser pulses, with duration on the femto- and sub-femtosecond time scales, interact with atoms, molecules or solids. The laser-induced electron dynamics occurs natively on a timescale down to a few hundred or even tens of attoseconds (1 attosecond  =  1 as  =  10^-18 s), which is comparable with the optical field. For comparison, the revolution of an electron on a 1s orbital of a hydrogen atom is  ∼152 as. On the other hand, the second branch involves the manipulation and engineering of mesoscopic systems, such as solids, metals and dielectrics, with nanometric precision. Although nano-engineering is a vast and well-established research field on its own, the merger with intense laser physics is relatively recent. In this report on progress we present a comprehensive experimental and theoretical overview of physics that takes place when short and intense laser pulses interact with nanosystems, such as metallic and dielectric nanostructures. In particular we elucidate how the spatially inhomogeneous laser induced fields at a nanometer scale modify the laser-driven electron dynamics. Consequently, this has important impact on pivotal processes such as above-threshold ionization and high-order harmonic generation. The deep understanding of the coupled dynamics between these spatially inhomogeneous fields and matter configures a promising way to new avenues of research and applications. Thanks to the maturity that attosecond physics has reached, together with the tremendous advance in material engineering and manipulation techniques, the age of atto-nanophysics has begun, but it is in the initial stage. We present thus some of the open questions, challenges and prospects for experimental confirmation of theoretical predictions, as well as experiments aimed at characterizing the induced fields and the unique electron dynamics initiated by them with high temporal and spatial resolution.

Strong-field plasmonic photoemission in the mid-IR at <1 GW/cm² intensity

We investigated nonlinear photoemission from plasmonic films with femtosecond, mid-infrared pulses at 3.1 μm wavelength. Transition between regimes of multi-photon-induced and tunneling emission is demonstrated at an unprecedentedly low intensity of <1 GW/cm(2). Thereby, strong-field nanophysics can be accessed at extremely low intensities by exploiting nanoscale plasmonic field confinement, enhancement and ponderomotive wavelength scaling at the same time. Results agree well with quantum mechanical modelling. Our scheme demonstrates an alternative paradigm and regime in strong-field physics.

Tunability of hybridized plasmonic waveguide mediated by surface plasmon polaritons

A hybridized plasmonic waveguide was proposed, which consisting of two kind of different metal films and a low-dielectric spacer layer inserted between. The spacer could be used to achieve the plasmonic resonance wavelength transfer from 450 nm to 600 nm, as well as the tunability of mode characteristics.