Ultrafast switching of photoemission electron through quantum pathways interference in metallic nanostructure

Ultrafast Electron Tunneling Devices-From Electric-Field Driven to Optical-Field Driven

The search for ever higher frequency information processing has become an area of intense research activity within the micro, nano, and optoelectronics communities. Compared to conventional semiconductor-based diffusive transport electron devices, electron tunneling devices provide significantly faster response times due to near-instantaneous tunneling that occurs at sub-femtosecond timescales. As a result, the enhanced performance of electron tunneling devices is demonstrated, time and again, to reimagine a wide variety of traditional electronic devices with a variety of new "lightwave electronics" emerging, each capable of reducing the electron transport channel transit time down to attosecond timescales. In response to unprecedented rapid progress within this field, here the current state-of-the-art in electron tunneling devices is reviewed, current challenges and opportunities are highlighted, and possible future research directions are identified.

Continuous angular control over anisotropic photoemission from isotropic gold nanoshells

A variety of applications rely on the efficient generation of hot carriers within metal nanoparticles and charge transfer to surrounding molecules or materials. The optimization of such processes requires a detailed understanding of excited carrier spatial, temporal, and momentum distributions, which also leads to opportunities for active optical control over hot carrier dynamics on nanometer and femtosecond scales. Such capabilities are emerging in nanoplasmonic systems and typically rely on tuning optical polarization and/or frequency to selectively excite one or more discrete hot spots defined by the particle geometry. Here, we introduce a unique case in which hot electron excitation and emission distributions can instead be continuously controlled via linear laser polarization in the azimuthal plane of a gold nanoshell supported on a substrate. In this configuration, it is the laser field that breaks the azimuthal symmetry of the supported nanoshell and determines the plasmonic field distribution. Using angle-resolved photoelectron velocity map imaging, we find that the hot electrons are predominantly emitted orthogonal to the nanoshell dipolar surface plasmon resonance axis defined by the laser polarization. Furthermore, such anisotropic emission is only observed for nanoshells, while solid gold nanospheres are found to be isotropic emitters. We show that all of these effects are recapitulated via simulation of the plasmonic electric field distributions within the nanoparticle volume and ballistic Monte Carlo modeling of the hot electron dynamics. These results demonstrate a highly predictive level of understanding of the underlying physics and possibilities for ultrafast spatiotemporal control over hot carrier dynamics.

Ultrafast Photoemission Electron Microscopy: Imaging Plasmons in Space and Time

Plasmonics is a rapidly growing field spanning research and applications across chemistry, physics, optics, energy harvesting, and medicine. Ultrafast photoemission electron microscopy (PEEM) has demonstrated unprecedented power in the characterization of surface plasmons and other electronic excitations, as it uniquely combines the requisite spatial and temporal resolution, making it ideally suited for 3D space and time coherent imaging of the dynamical plasmonic phenomena on the nanofemto scale. The ability to visualize plasmonic fields evolving at the local speed of light on subwavelength scale with optical phase resolution illuminates old phenomena and opens new directions for growth of plasmonics research. In this review, we guide the reader thorough experimental description of PEEM as a characterization tool for both surface plasmon polaritons and localized plasmons and summarize the exciting progress it has opened by the ultrafast imaging of plasmonic phenomena on the nanofemto scale.

Distinct spatiotemporal imaging of femtosecond surface plasmon polaritons assisted with the opening of the two-color quantum pathway effect

Accurately capturing the spatiotemporal information of surface plasmon polaritons (SPPs) is the basis for expanding SPP applications. Here, we report spatio-temporal evolution imaging of femtosecond SPPs launched from a rectangular trench in silver film with a 400-nm light pulse assisted femtosecond laser interferometric time-resolved (ITR) photoemission electron microscopy. It is found that introducing the 400nm light pulse in the spatially separated near-infrared (NIR) laser pump-probe ITR scheme enables distinct spatiotemporal imaging of the femtosecond SPPs with a weak probe pulse in the ITR scheme, which is free from the risk of sample damage due to the required high monochromatic field for a clear photoelectron image as well as the entangled interference fringe (between the SPPs and probe pulse) in the usual spatially overlapped pump-probe ITR scheme. The demonstrated great improvement of the visibility of the SPPs spatiotemporal image with an additional 400nm light pulse scheme facilitates further analysis of the femtosecond SPPs, and carrier wavelength (785nm), group velocity (0.94C) and phase velocity (0.98C) of SPPs are extracted from the distinct spatio-temporal evolution images of SPPs. Furthermore, the modulation of photoemission induced by the quantum pathway interference effect in the 400nm-assisted scheme is proposed to play a major role in the distinct visualization for SPPs. The probabilities of electrons in different quantum pathways are obtained quantitatively through fitting the experimental results with the quantum pathway interference model. The probability that electrons emit through the quantum pathway allows us to quantitatively analyze the contribution to electron emission from the different quantum pathways. These findings pave a way for the spatiotemporal imaging of the near-infrared light-induced SPPs, such as the communication wave band using PEEM.

Ultrafast optical switching based on mutually enhanced resonance modes in gold nanowire gratings

We report an efficient ultrafast optical switching device consisting of periodically arranged gold nanowires, which were produced by the multistage deposition of colloidal gold nanoparticles into deep grooves, so that they are as high as 220 nm and continuous as long as 20 mm. Due to the large thickness of the gold nanowires, two resonance modes became efficient and mutually enhanced: the waveguide resonance mode and the Bragg microcavity resonance mode. These resonance modes are based on the same diffraction conditions and have a completely overlapped spectroscopic response. Thus, a sharp resonance mode with a large amplitude and a steep rising edge is observed in the optical extinction spectrum at normal incidence. Strong optical excitation induced a red shift of the resonance spectrum and resulted in an enhanced optical transmission spectrum with a narrow bandwidth and a high response speed. Such an optical switching device with new physics has potential applications in optical logic circuits and integrated optics.

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.