Photofield Emission Research Articles

Nanometric probing with a femtosecond, intra-cavity standing wave

Abstract Optical standing waves are intrinsically nanometric, spatially fixed interference-field patterns. At a commensurate scale, metallic nanotips serve as coherent, atomic-sized electron sources. Here, we explore the localized photofield emission from a tungsten nanotip with a transient standing wave. It is generated within an optical cavity with counter-propagating femtosecond pulses from a near-infrared, 100-MHz frequency comb. Shifting the phase of the standing wave at the tip reveals its nodes and anti-nodes through a strong periodic modulation of the emission current. We find the emission angles to be distinct from those of a traveling wave, and attribute this to the ensuing localization of emission from different crystallographic planes. Supported by a simulation, we find that the angle of maximum field enhancement is controlled by the phase of the standing wave. Intra-cavity nanotip interaction not only provides higher intensities than in free-space propagation, but also allows for structuring the light field even in the transverse direction by selection of high-order cavity modes.

Evaluating LaB6 (310) nanotip as an ultrafast electron emitter

The crystallographic dependence of electron emission properties from LaB6 single-crystal tips and its importance has been highlighted in several experimental and theoretical studies. Here, we report on the cold field electron emission from LaB6 nanotips in the ⟨310⟩ orientation under static (DC) and laser-assisted conditions for possible use as an ultrafast electron emitter. By changing the voltage and laser intensity, we observe different emission regimes such as the photo-field emission and the multiphoton photoemission. The field electron microscopy patterns change under ultrafast laser illumination and correspond to the region of a high laser field. The emission properties of LaB6 nanotips in the ⟨310⟩ orientation are compared with those of the ⟨100⟩ orientation.

Evidence for Non-existence of Oscillations in Photofield Emission Current in Gallium Arsenide

Evidence for Non-existence of Oscillations in Photofield Emission Current in Gallium Arsenide

Photoemission of Plasmonic Gold Nanostars in Laser-Controlled Electron Current Devices for Technical and Biomedical Applications.

The main goal of this work was to modify the previously developed blade-type planar structure using plasmonic gold nanostars in order to stimulate photofield emission and provide efficient laser control of the electron current. Localization and enhancement of the field at the tips of gold nanostars provided a significant increase in the tunneling electron current in the experimental sample (both electrical field and photofield emission). Irradiation at a wavelength in the vicinity of the plasmon resonance (red laser) provided a gain in the photoresponse value of up to 5 times compared to irradiation far from the resonance (green laser). The prospects for transition to regimes of structure irradiation by femtosecond laser pulses at the wavelength of surface plasmon resonance, which lead to an increase in the local optical field, are discussed. The kinetics of the energy density of photoinduced hot and thermalized electrons is estimated. The proposed laser-controlled matrix current source is promising for use in X-ray computed tomography systems.

Photofield electron emission from an optical fiber nanotip

We demonstrate a nanotip electron source based on a graded index multimode silica optical fiber, tapered at one end to a radius of curvature r ∼50 nm and coated with a thin film of gold. We report observation of laser-induced electron photoemission at tip bias potentials below the onset of dark field emission. Single-photon photofield emission is identified as the emission mechanism that exhibits fast switching times with an upper limit on the order of 1 μs. The explored fiber optic nanotips are flexible back-illuminated emitters, which can be operated in continuous wave and pulsed modes using lasers with photon energies in the visible range or higher. The mechanical flexibility of the source can facilitate externally controlled positioning. Multiple, individually addressable, nanotips may be assembled into a bundle for applications such as computational electron ghost imaging.

An in situ characterization technique for electron emission behavior under a photo-electric-common-excitation field: study on the vertical few-layer graphene individuals

The in situ characterization on the individuals offers an effective way to explore the dynamic behaviors and underlying physics of materials at the nanoscale, and this is of benefit for actual applications. In the field of vacuum micro-nano electronics, the existing in situ techniques can obtain the material information such as structure, morphology and composition in the process of electron emission driven by a single source of excitation. However, the relevant process and mechanism become more complicated when two or more excitation sources are commonly acted on the emitters. In this paper, we present an in situ nano characterization technique to trigger and record the electron emission behavior under the photo-electric-common-excitation multiple physical fields. Specifically, we probed into the in situ electron emission from an individual vertical few-layer graphene (vFLG) emitter under a laser-plus-electrostatic driving field. Electrons were driven out from the vFLG’s emission edge, operated in situ under an external electrostatic field coupled with a 785 nm continuous-wave laser-triggered optical field. The incident light has been demonstrated to significantly improve the electron emission properties of graphene, which were recorded as an obvious decrease of the turn-on voltage, a higher emission current by factor of 35, as well as a photo-response on-off ratio as high as 5. More importantly, during their actual electron emission process, a series of in situ characterizations such as SEM observation and Raman spectra were used to study the structure, composition and even real-time Raman frequency changes of the emitters. These information can further reveal the key factors for the electron emission properties, such as field enhancement, work function and real-time surface temperature. Thereafter, the emission mechanism of vFLG in this study has been semi-quantitatively demonstrated to be the two concurrent processes of photon-assisted thermal enhanced field emission and photo field emission.

Photofield emission from SiGe nanoislands under green light illumination

Photofield emission from SiGe nanoislands formed by molecular beam epitaxy (MBE) have been investigated. Two types of nanoislands, namely the domes and pyramids with different heights, have been addressed. It was found that the arrays of SiGe nanoislands exhibited a low onset voltage for field emission. The increase of emission current and the decrease of the curve slope in Fowler-Nordheim coordinates under green light illumination have been revealed. Electron field emission and photoemission from SiGe nanoislands have been explained based on the energy band diagram of Si-Ge heterostructure and some energy barriers have been determined.

Characterization of GaN nanostructures by electron field and photo-field emission

The electron field and photo-field emission from GaN nanostructures has been analyzed in this review. In order to explain the obtained experimental results, a model was proposed taking into account the change in carrier concentration distribution in the main and the satellite valley during the emission process. The lowering of work function (due to the increased number of carriers in the satellite valley) can explain the decrease in the Fowler-Nordheim plot slope. It was shown that the energy difference between the main and satellite valley in GaN was decreased in the case of quantum confinement, thus increasing the probability of electron transition from Γ to X valley at same electric fields.

Calculation of photofield emission current by using the projection operator method of group theory

A calculation of photofield emission is discussed in which initial state wave function has been deduced by using the projection operator method of group theory. A spatial dependent vector field is used to evaluate the matrix element for calculating the photofield emission current density.

A theoretical study of Photofield emission in Gallium Arsenide using Kronig-Penney potential model

In this report, we have shown the variation of photofield emission current as a function of applied electric field, the photon energy and initial state energy of the electrons with reference to the Fermi level. The photofield emission current is calculated with the help of Kronig- Penney model potential. The origin of peak in photofield emission current in the valence band is explained with the help of result of density of state calculated.

Study of Photofield Emission in GaAs Using Kronig-Penney Model

Study of Photofield Emission in GaAs Using Kronig-Penney Model

Large Charge Extraction from Metallic MultifilamentaryNb3SnPhotocathode

The current density limit for photoemission from metals was measured in an rf photogun to be below 10(9) A/m2. We have achieved 1.6×10(11) A/m2 by photofield emission from a new type of photocathode made from a metallic-composite, multifilamentary Nb3Sn wire driven by a 266 nm picosecond laser pulse and a 2 ns, 50 kV accelerating voltage. This cathode has a micrometer arrayed structure with tens of thousands of Nb/Nb3Sn filaments embedded in a bronze matrix. Our measurements revealed the existence of a new electron emission regime at high laser fluence (100 mJ/cm2). We have extracted stably, and without any surface ablation, up to 4800 pC of charge. This corresponds to 0.9% quantum efficiency, 100 times larger than what is measured from conventional metallic photocathodes. The unexpected large and stable charge extraction cannot be explained by the 3-step model. Thanks to the small emitting area, the measured emittance (0.6 mm·mrad) is low in spite of the high current density and space charge effects. This cathode will be of benefit for many applications based on short and bright electron bunches.

Five picocoulomb electron bunch generation by ultrafast laser-induced field emission from metallic nano-tip arrays

Laser-induced field emission from metallic field emitter array cathodes excited by femtosecond near infrared laser pulses is explored. When 50 fs laser pulses irradiated a 1.2 × 105-tip emitter array under a DC field emission bias, electron bunches with bunch charge up to 5.2 pC were observed. The variation of the bunch charge at different laser intensities and polarizations indicated that electrons were produced from the field emitters by a photofield emission process. The result demonstrates the feasibility of metallic field emitter array cathodes for high-charge short-pulse electron source applications.

Sodium overlayers on low-index tungsten surfaces: Field and photofield emission currents and surface electronic structures

The total energy distributions (TEDs) of the emission currents in field emission and surface photofield emission and the overlayer-induced modifications in the surface electronic structures from the technologically important W surfaces with the commensurate W(100)/Na c(2x2), W(110)/Na (2x2) and W(111)/Na (1x1) overlayers are calculated. The TEDs obtained by our recent numerical method that extends the full-potential linear augmented plane wave method for the electronic structures to the study of field and photofield emission are used to interpret the shifts of the peaks in the experimental TEDs in field emission and photofield emission from the W(100) and W(110) surfaces at sub-monolayer and monolayer Na coverage. Hybridization of the 3s Na states with the pairs of dz2-like surface states of the strong Swanson hump in clean W(100) and surface resonances in clean W(111) below the Fermi energy shifts these W states by about -1.2 eV and -1.0 eV, thus stabilizing these states, to yield new strong peaks in the TEDs in field emission and photofield emission from W(100)/Na c(2x2) and W(111)/Na (1x1) respectively. The effect of Na intralayer interactions are discussed and are shown to shift the strong s- and p-like peaks in the surface density of states of W(110) below and above the Fermi energy respectively to lower energy with increased Na coverage, in agreement with experiments.

REPRINT OF: Surface electronic structure of Ti-covered W(1 1 1) by photofield emission

Photofield emission (PFE) measurements are employed to examine modifications of the surface electronic structure of the tungsten (1 1 1) facet upon deposition of thin films (1-3 monolayers) of titanium. With the help of DFT simulations, the observed PFE features are interpreted as adsorbate-induced resonance states with energies just below the Fermi level, localized predominantly at the exposed surface atoms. Comparison between the computed surface DOS distributions and the measured PFE spectra is also used to verify various possible arrangements of the Ti adatoms, supporting the DFT-favored model of Ti growth in registry with the W(1 1 1) substrate until a full physical overlayer of the adsorbate is completed.

Synthesis of single crystalline CdS nanocombs and their application in photo-sensitive field emission switches

Single crystalline CdS nanocombs were synthesized by a thermal evaporation route. The photo-sensitive field emission current shows a reproducible switching behavior, with a rise in current level of nearly five times the initial preset value of ∼1 μA. An ultra low turn-on field, required to draw an emission current density of ∼0.1 μA cm(-2) (100 nA), is found to be ∼0.26 V μm(-1) (260 V), which is much lower than the reported values for various other CdS nanostructures. Upon illumination with visible light the CdS nanocombs act as a photo field emission switch. At an applied field of ∼0.65 V μm(-1) the current densities are observed to be ∼14.6 μA cm(-2) and ∼26.9 μA cm(-2), without and with light illumination, respectively. The average emission current is seen to be stable over the duration of measurement for two preset values. The high sensitivity and fast response in the visible range indicates that the CdS nanocombs can be used as a photo-sensitive field emitting switch in device applications, and also in pulsed electron beam technology.

A coherent photofield electron source for fast diffractive and point-projection imaging

Prospects for high-resolution imaging at femtosecond speeds using electron diffractive imaging are reviewed in the context of recent achievements using free-electron X-ray lasers. The conflict between Coulomb interactions and the spatial coherence of electron beams is identified as a limiting factor. Experimental results showing the performance of a milliwatt laser-driven fast photofield GaAs electron emitter are presented, including emission current and measured energy spread for various laser energies illuminating the electron emission tip. Band-bending below the Fermi level, due to penetration of the tip field into the emitter, is found to limit the emission energy spread by thermalizing the electrons. Because of the absence of beam crossovers and consequent Coulomb interactions, the point-projection photofield emission microscope with its high spatial coherence is suggested as a method for obtaining femtosecond images at high resolution from atomic processes, which may be triggered repetitively. The incorporation of a photofield emitter into a microwave pulse compression gun is discussed, as is the use of electron photofield emission from semiconductor donor states.

Size Effect in Photofield Emission

Size Effect in Photofield Emission

Surface electronic structure of Ti-covered W(1 1 1) by photofield emission

Photofield emission (PFE) measurements are employed to examine modifications of the surface electronic structure of the tungsten (1 1 1) facet upon deposition of thin films (1–3 monolayers) of titanium. With the help of DFT simulations, the observed PFE features are interpreted as adsorbate-induced resonance states with energies just below the Fermi level, localized predominantly at the exposed surface atoms. Comparison between the computed surface DOS distributions and the measured PFE spectra is also used to verify various possible arrangements of the Ti adatoms, supporting the DFT-favored model of Ti growth in registry with the W(1 1 1) substrate until a full physical overlayer of the adsorbate is completed.

Study of photofield emission in tungsten by using free electron model

We present here the results of the calculations of photofield emission current by using the free electron model in which the appropriate wavefunctions are used. The transmission probability had been calculated by solving the Airy’s equation. The model developed is used to calculate photofield emission current from tungsten.