Observation of few-cycle, strong-field phenomena in surface plasmon fields

Ultrafast Surface Plasmon Probing of Interband and Intraband Hot Electron Excitations

Upon the interaction of light with metals, nonthermal electrons are generated with intriguing transient behavior. Here, we present femtosecond hot electron probing in a noveloptical pump/plasmon probe scheme. With this, we probed ultrafast interband and intraband dynamics with 15 nm interface selectivity, observing a two-component-decay of hot electron populations. Results are in good agreement with a three-temperature model of the metal; thus, we could attribute the fast (∼100 fs) decay to the thermalization of hot electrons and the slow (picosecond) decay to electron-lattice thermalization. Moreover, we could modulate the transmission of our plasmonic channel with ∼40% depth, hinting at the possibility of ultrafast information processing applications with plasmonic signals.

Few-Cycle Surface Plasmon Polaritons

Surface plasmon polaritons (SPPs) can confine and guide light in nanometer volumes and are ideal tools for achieving electric field enhancement and the construction of nanophotonic circuitry. The realization of the highest field strengths and fastest switching requires confinement also in the temporal domain. Here, we demonstrate a tapered plasmonic waveguide with an optimized grating structure that supports few-cycle surface plasmon polaritons with >70 THz bandwidth while achieving >50% light-field-to-plasmon coupling efficiency. This enables us to observe the─to our knowledge─shortest reported SPP wavepackets. Using time-resolved photoelectron microscopy with suboptical-wavelength spatial and sub-10 fs temporal resolution, we provide full spatiotemporal imaging of co- and counter-propagating few-cycle SPP wavepackets along tapered plasmonic waveguides. By comparing their propagation, we track the evolution of the laser-plasmon phase, which can be controlled via the coupling conditions.

Subcycle surface electron emission driven by strong-field terahertz waveforms

The advent of intense terahertz (THz) sources opened a new era when the demonstration of the acceleration and manipulation of free electrons by THz pulses became within reach. THz-field-driven electron emission was predicted to be confined to a single burst due to the single-cycle waveform. Here we demonstrate the confinement of single-cycle THz-waveform-driven electron emission to one of the two half cycles from a solid surface emitter. Either the leading or the trailing half cycle was active, controlled by reversing the field polarity. THz-driven single-burst surface electron emission sources, which do not rely on field-enhancement structures, will impact the development of THz-powered electron acceleration and manipulation devices, all-THz compact electron sources, THz waveguides and telecommunication, THz-field-based measurement techniques and solid-state devices.

Femtosecond Thermal and Nonthermal Hot Electron Tunneling Inside a Photoexcited Tunnel Junction

Efficient operation of electronic nanodevices at ultrafast speeds requires understanding and control of the currents generated by femtosecond bursts of light. Ultrafast laser-induced currents in metallic nanojunctions can originate from photoassisted hot electron tunneling or lightwave-induced tunneling. Both processes can drive localized photocurrents inside a scanning tunneling microscope (STM) on femto- to attosecond time scales, enabling ultrafast STM with atomic spatial resolution. Femtosecond laser excitation of a metallic nanojunction, however, also leads to the formation of a transient thermalized electron distribution, but the tunneling of thermalized hot electrons on time scales faster than electron-lattice equilibration is not well understood. Here, we investigate ultrafast electronic heating and transient thermionic tunneling inside a metallic photoexcited tunnel junction and its role in the generation of ultrafast photocurrents in STM. Phase-resolved sampling of broadband terahertz (THz) pulses via the THz-field-induced modulation of ultrafast photocurrents allows us to probe the electronic temperature evolution inside the STM tip and to observe the competition between instantaneous and delayed tunneling due to nonthermal and thermal hot electron distributions in real time. Our results reveal the pronounced nonthermal character of photoinduced hot electron tunneling and provide a detailed microscopic understanding of hot electron dynamics inside a laser-excited tunnel junction.

Ultrafast plasmonic photoemission in the single-cycle and few-cycle regimes

Due to the highly increased interest in the development of state-of-the-art applications of photoemission in ultrafast electron microscopy, development of photocathodes and many more applications, a correct theoretical understanding of the underlying phenomena is needed. Within the framework of the single active electron approximation the most accurate results can be obtained by the direct solution of the time-dependent Schrödinger equation (TDSE). In this work, after a brief presentation of a numerically improved version of a mixed 1D-TDSE method, we investigated the characteristics of electron spectra obtained from the surface of metal nanoparticles irradiated with ultrashort laser pulses. During our investigation different decay lengths of the plasmonic-enhanced incident field in the vicinity of the metal were considered. Using the simulated spectra we managed to identify the behavior of the cutoff energy as a function of decay length in the strong-field, multiphoton and transition regimes.

Few-cycle localized plasmon oscillations

The generation of few-cycle laser pulses proved to be a key enabling technology in strong-field physics and ultrafast science. The question naturally arises whether one can induce few-cycle localized plasmon oscillations in optical near-fields. Here, we perform a comparative study of different plasmonic nanoresonators illuminated by few-cycle pulses. We analyze the number of cycles (NOC) of the plasmonic field, the near-field enhancement (NFE) as well as the figure of merit NFE/NOC. The pulse length dependence of these quantities is also investigated. Throughout the inspected pulse-length interval silica-gold and silica-silver core-shell monomers have the potential to preserve the NOC of the incoming pulse, silver bow-ties result in the highest NFE, whereas gold core-shell dimers have the highest NFE/NOC. Based on the analysis, silver bow-ties, gold core-shell and silver nanorod dimers proved to be the most suitable for few-cycle near-field amplification.

Sub-cycle dynamics in relativistic nanoplasma acceleration

The interaction of light with nanometer-sized solids provides the means of focusing optical radiation to sub-wavelength spatial scales with associated electric field enhancements offering new opportunities for multifaceted applications. We utilize collective effects in nanoplasmas with sub-two-cycle light pulses of extreme intensity to extend the waveform-dependent electron acceleration regime into the relativistic realm, by using 106 times higher intensity than previous works to date. Through irradiation of nanometric tungsten needles, we obtain multi-MeV energy electron bunches, whose energy and direction can be steered by the combined effect of the induced near-field and the laser field. We identified a two-step mechanism for the electron acceleration: (i) ejection within a sub-half-optical-cycle into the near-field from the target at >TVm-1 acceleration fields, and (ii) subsequent acceleration in vacuum by the intense laser field. Our observations raise the prospect of isolating and controlling relativistic attosecond electron bunches, and pave the way for next generation electron and photon sources.

Dynamics of electron-emission currents in plasmonic gaps induced by strong fields

The dynamics of ultrafast electron currents triggered by femtosecond laser pulse irradiation of narrow gaps in a plasmonic dimer is studied using quantum mechanical Time-Dependent Density Functional Theory (TDDFT). The electrons are injected into the gap due to the optical field emission from the surfaces of the metal nanoparticles across the junction. Further evolution of the electron currents in the gap is governed by the locally enhanced electric fields. The combination of TDDFT and classical modelling of the electron trajectories allows us to study the quiver motion of the electrons in the gap region as a function of the Carrier Envelope Phase (CEP) of the incident pulse. In particular, we demonstrate the role of the quiver motion in establishing the CEP-sensitive net electric transport between nanoparticles.

Plasmon-plasmon coupling probed by ultrafast, strong-field photoemission with <7 Å sensitivity

The coupling of propagating surface plasmon waves and localized plasmon oscillations in nanostructures is an essential phenomenon determining electromagnetic field enhancement on the nanoscale. Here, we use our recently developed ultrafast photoemission near-field probing technique to investigate the fundamental question of plasmon-plasmon coupling and its effect on large field enhancement factors. By measuring and analyzing plasmon field enhancement values at different nanostructured surfaces, we can separate the contributions from propagating and localized plasmons. When resonance conditions are met, a significant field enhancement factor can be attributed to the generation of localized plasmons on surface nanostructures, acting as dipole sources resonantly driven by the propagating plasmon field. Our plasmon-plasmon coupling results can contribute directly to applications in surface-enhanced Raman scattering (SERS) and the development of plasmonic sensors and nanostructured photocathodes.

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.

Generation of attosecond electron packets via conical surface plasmon electron acceleration

We present a method for the generation of high kinetic energy attosecond electron packets via magnetostatic and aperture filtering of conical surface plasmon (SP) accelerated electrons. The conical SP waves are excited by coupling an ultrafast radially polarized laser beam to a conical silica lens coated with an Ag film. Electromagnetic and particle tracking models are employed to characterize the ultrafast electron packets.

Optimization of a nanotip on a surface for the ultrafast probing of propagating surface plasmons

We theoretically analyze a method for characterizing propagating surface plasmon polaritons (SPPs) on a thin gold film. The SPPs are excited by few-cycle near-infrared pulses using Kretschmann coupling, and a nanotip is used as a local field sensor. This geometry removes the influence of the incident excitation laser from the near fields, and enhances the plasmon electric field strength. Using finite-difference-time-domain studies we show that the geometry can be used to measure SPP waveforms as a function of propagation distance. The effects of the nanotip shape and material on the field enhancement and plasmonic response are discussed.

Nanoscale Imaging of Local Few-Femtosecond Near-Field Dynamics within a Single Plasmonic Nanoantenna

The local enhancement of few-cycle laser pulses by plasmonic nanostructures opens up for spatiotemporal control of optical interactions on a nanometer and few-femtosecond scale. However, spatially resolved characterization of few-cycle plasmon dynamics poses a major challenge due to the extreme length and time scales involved. In this Letter, we experimentally demonstrate local variations in the dynamics during the few strongest cycles of plasmon-enhanced fields within individual rice-shaped silver nanoparticles. This was done using 5.5 fs laser pulses in an interferometric time-resolved photoemission electron microscopy setup. The experiments are supported by finite-difference time-domain simulations of similar silver structures. The observed differences in the field dynamics across a single particle do not reflect differences in plasmon resonance frequency or dephasing time. They instead arise from a combination of retardation effects and the coherent superposition between multiple plasmon modes of the particle, inherent to a few-cycle pulse excitation. The ability to detect and predict local variations in the few-femtosecond time evolution of multimode coherent plasmon excitations in rationally synthesized nanoparticles can be used in the tailoring of nanostructures for ultrafast and nonlinear plasmonics.

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.

High-density Au nanorod optical field-emitter arrays

We demonstrate the design, fabrication, characterization, and operation of high-density arrays of Au nanorod electron emitters, fabricated by high-resolution electron beam lithography, and excited by ultrafast femtosecond near-infrared radiation. Electron emission characteristic of multiphoton absorption has been observed at low laser fluence, as indicated by the power-law scaling of emission current with applied optical power. The onset of space-charge-limited current and strong optical field emission has been investigated so as to determine the mechanism of electron emission at high incident laser fluence. Laser-induced structural damage has been observed at applied optical fields above 5 GV m(-1), and energy spectra of emitted electrons have been measured using an electron time-of-flight spectrometer.

Electron acceleration and kinetic energy tailoring via ultrafast terahertz fields

We propose a mechanism for tuning the kinetic energy of surface plasmon generated electron pulses through control of the time delay between a pair of externally applied terahertz pulses. Varying the time delay results in translation, compression, and broadening of the kinetic energy spectrum of the generated electron pulse. We also observe that the electrons' kinetic energy dependence on the carrier envelope phase of the surface plasmon is preserved under the influence of a terahertz electric field.

Ponderomotive electron acceleration in a silicon-based nanoplasmonic waveguide

Ponderomotive electron acceleration is demonstrated in a semiconductor-loaded nanoplasmonic waveguide. Photogenerated free carriers are accelerated by the tightly confined nanoplasmonic fields and reach energies exceeding the threshold for impact ionization. Broadband (375 nm ≤ λ ≤ 650  nm) white light emission is observed from the nanoplasmonic waveguides. Exponential growth of visible light emission confirms the exponential growth of the electron population, demonstrating the presence of an optical-field-driven electron avalanche. Electron sweeping dynamics are visualized using pump-probe measurements, and a sweeping time of 1.98 ± 0.40 ps is measured. These findings offer a means to harness the potential of the emerging field of ultrafast nonlinear nanoplasmonics.

Ultrafast strong-field photoemission from plasmonic nanoparticles

We demonstrate the ultrafast generation of electrons from tailored metallic nanoparticles and unravel the role of plasmonic field enhancement in this process by comparing resonant and off-resonant particles, as well as different particle geometries. We find that electrons become strongly accelerated within the evanescent fields of the plasmonic nanoparticles and escape along straight trajectories with orientations governed by the particle geometry. These results establish plasmonic nanoparticles as versatile ultrafast, nanoscopic sources of electrons.

Strong-field above-threshold photoemission from sharp metal tips

We present energy-resolved measurements of electron emission from sharp metal tips driven with low energy pulses from a few-cycle laser oscillator. We observe above-threshold photoemission with a photon order of up to 9. At a laser intensity of ∼ 2 × 10(11)  W/cm2 the suppression of the lowest order peak occurs, indicating the onset of strong-field effects. We also observe peak shifting linearly with intensity, with a slope of around -1.0 eV/(10(12)  W/cm2). We attribute the magnitude of the laser field effects to field enhancement taking place at the tip's surface.