Generation Of Electron Pulses Research Articles

Isolated attosecond electron and hard x-ray pulse generation by ultra-intense spatiotemporal vortex laser

The spatiotemporal optical vortex (STOV) laser pulse, characterized by containing a space-time dependent spiral phase structure and intrinsic transverse orbital angular momentum (OAM), has been of much recent research interest for basic physics as well as potential applications in optical communication and manipulation, as well as the attosecond sciences. Here we consider electron acceleration and generation of attosecond hard x-rays by irradiating a thin foil with intense STOV laser pulse. It is found that the affected foil electrons can be trapped and acquire transverse OAM, or synchrotron-like motion, from the STOV, as well as accelerated forward by the transverse and enhanced axial laser electric fields to form a tiny energetic bunch at the front of the laser pulse and emit short ( ∼400 attosecond) high-flux ( >109 photons per pulse) hard (100–1000 keV) x rays.

Capturing the non-equilibrium state in light–matter–free-electron interactions through ultrafast transmission electron microscopy

Ultrafast transmission electron microscope (UTEM) with the multimodality of time-resolved diffraction, imaging, and spectroscopy provides a unique platform to reveal the fundamental features associated with the interaction between free electrons and matter. In this review, we summarize the principles, instrumentation, and recent developments of the UTEM and its applications in capturing dynamic processes and non-equilibrium transient states. The combination of the transmission electron microscope with a femtosecond laser via the pump–probe method guarantees the high spatiotemporal resolution, allowing the investigation of the transient process in real, reciprocal and energy spaces. Ultrafast structural dynamics can be studied by diffraction and imaging methods, revealing the coherent acoustic phonon generation and photo-induced phase transition process. In the energy dimension, time-resolved electron energy-loss spectroscopy enables the examination of the intrinsic electronic dynamics of materials, while the photon-induced near-field electron microscopy extends the application of the UTEM to the imaging of optical near fields with high real-space resolution. It is noted that light–free-electron interactions have the ability to shape electron wave packets in both longitudinal and transverse directions, showing the potential application in the generation of attosecond electron pulses and vortex electron beams.

Water Flow Visualization And Velocity Measurement Using Hydrogen Bubble Generation Technique In Low Speed Open Channel

Visualization of water flow around different bluff bodies at different Reynolds number ranging (1505 - 2492) was realized by designing and building a test rig which contains an open channel capable to ensure water velocity range (4-8cm/s) in this channel. Hydrogen bubbles generated from the ionized water using DC power supply are visualized by a light source and photographed by a digital camera. Flow pattern around a circular disk of (3.6cm) diameter and (3mm) thickness, a sphere of (3.8cm) diameter and a cylinder of (3.2cm) diameter and (10cm) length are studied qualitatively. Parameters of the vortex ring generated in the wake region of the disk and the separation angle of water stream lines from the surface of the sphere are plotted versus Reynolds number. Proper empirical formulas are investigated to describe the behavior of vortex ring parameters and separation angle versus Reynolds number. Vortex growth history in the wake region of the cylinder is identified by analyzing the photographs extracted from the digital camera used for photography purposes. Water velocity measurement in the upstream region and near the edge of the disk is conducted at different Reynolds number by measuring the length of Hydrogen bubble pulse streaks generated in the upstream region of the disk using electronic pulse generator circuit. Special electronic circuit is designed and fabricated to cut off the applied DC voltage. The calibration of the designed pulse generator is conducted using the proper oscilloscope device. The pictures extracted from the digital camera are used for analyzing the generated Hydrogen pulses.

Enhanced photoelectron emission from dark plasmon mode in gold nanoring

By the advantage of light concentration in nanoscale and strong light absorption, LSPRs have presented benefits for ultrafast electron pulse generation, promoting high spatio-temporal electron microscopy and spectroscopy. However, previous studies generally pay more attention to the bright mode of LSPRs; the dark mode, which corresponds to longer dephasing time and less radiative decay that is potential to deliver a leap forward in plasmon-enhanced photoemission, is still neglected. Here, we report on the investigation of quadrupole plasmon mode (dark mode) induced photoemission electron from a gold nanoring with photoemission electron microscopy. The result shows that the photoemission electron yield is around 11,200 for the dark mode supported by nanoring under s-polarized light irradiation, which is higher than the one under p-polarized light irradiation (around 8600). The higher photoemission electron yield is attributed to stronger absorption of the s-pol light excited mode. In addition, the result shows that the dephasing time of s polarization excitation mode is longer than that of p polarization excitation. The experiment results are well reproduced by FDTD simulation. Our investigation may lay the foundation for dark plasmon mode enhanced photoemission and offers an additional benefit for the progress of plasmon-based femtosecond on-chip electronics, high spatial-temporal electron microscopy and spectroscopy.

Ultrafast Strong-Field Electron Emission and Collective Effects at a One-Dimensional Nanostructure

Enhanced near-fields at metallic nanostructures enable the generation of ultrafast nanometric electron pulses and the investigation of fundamental ultrafast dynamics in electron emission. Here we show strong-field induced photoemission from a nanometer-sharp tungsten-covered silicon nanoblade and report the systematic measurement of intensity-dependent electron energy spectra and yields. The observed plateau and cutoff features in the electron spectra indicate the presence of elastic electron rescattering in the enhanced near-fields at the surface of the one-dimensional nanostructure. For the first time, we can hence observe strong-field features from a one-dimensional object, as opposed to zero-dimensional needle tips employed so far. A comparison with results from classical and quantum simulations reveals that the extended geometry of the nanoblades and a cascaded near-field enhancement due to surface roughness leads to a broad energy distribution and high electron energies. A systematic analysis of the electron yield demonstrates nonlinear photoemission at moderate laser intensities and a clear transition to a regime with linear intensity dependence. This distinct feature is interpreted as the onset of space-charge trapping. The presented one-dimensional nanostructure enables us to generate above keV electrons without noticeable target damage and more than 13000 electrons per laser pulse, which is of utmost interest for novel classes of ultrafast photocathodes.

Sub-threshold ultrafast one-photon photoemission from a Cu(111) photocathode

Single-photon photoemission of electrons for incident photon energies below the surface work function is reported for a single-crystal Cu(111) photocathode. Spectral characterization of the quantum efficiency and mean transverse energy of the emitted electrons is shown to be consistent with emission from a thermalized hot electron distribution photoexcited on sub-picosecond time scales into an upper conduction band at the L-point of the Brillouin zone of copper. To our knowledge, this is the first time that such excited-state thermionic emission has been observed from a planar metal photocathode, and then from a commonly used photocathode material. The results, therefore, indicate the potential importance of such band structure dependent sub-threshold photoemission mechanisms on the performance of photocathodes employed for the generation of short electron pulses using sub-picosecond laser pulses. Consequently, the combined experimental and theoretical work presented in this paper contributes to solid-state photocathode-based research aimed at our understanding and selection (or discovery) of high brightness photo-electron sources required for many electron-based diffraction, imaging, and accelerator applications.

Water jet space charge spectroscopy: route to direct measurement of electron dynamics for organic systems in their natural environment

The toolbox for time-resolved direct measurements of electron dynamics covers a variety of methods. Since the experimental effort is increasing rapidly with achievable time resolution, there is an urge for simple and robust measurement techniques. Within this paper prove-of-concept experiments and numerical simulations are utilized to investigate the applicability of a new setup for the generation of ultrashort electron pulses in the energy range of 300 eV up to 1.6 keV. The experimental approach combines an in-vacuum liquid microjet and a few-cycle femtosecond laser system, while the threshold for electron impact ionization serves as a gate for the effective electron pulse duration. The experiments prove that electrons in the keV regime are accessible and that the electron spectrum can be easily tuned by laser intensity and focal position alignment with respect to the water jet. Numerical simulations show that a sub-picosecond temporal resolution is achievable.

Generation of sub-100fs electron pulses for time-resolved electron diffraction using a direct synchronization method.

To investigate photoinduced phenomena in various materials and molecules, ultrashort pulsed x-ray and electron sources with high brightness and high repetition rates are required. The x-ray and electron's typical and de Broglie wavelengths are shorter than lattice constants of materials and molecules. Therefore, photoinduced structural dynamics on the femtosecond to picosecond timescales can be directly observed in a diffraction manner by using these pulses. This research created a tabletop ultrashort pulsed electron diffraction setup that used a femtosecond laser and electron pulse compression cavity that was directly synchronized to the microwave master oscillator (∼3GHz). A compressed electron pulse with a 1kHz repetition rate contained 228 000 electrons. The electron pulse duration was estimated to be less than 100fs at the sample position by using photoinduced immediate lattice changes in an ultrathin silicon film (50nm). The newly developed time-resolved electron diffraction setup has a pulse duration that is comparable to femtosecond laser pulse widths (35-100fs). The pulse duration, in particular, fits within the timescale of photoinduced phenomena in quantum materials. Our developed ultrafast time-resolved electron diffraction setup with a sub-100fs temporal resolution would be a powerful tool in material science with a combination of optical pump-probe, time-resolved photoemission spectroscopic, and pulsed x-ray measurements.

Sub-optical-cycle electron pulse trains from metal nanotips

The coherent modulation of swift electron beams with strong laser fields has enabled the generation of attosecond electron pulses, opening up new research avenues in ultrafast science. Here we study a comparatively simple alternative, the production of electron pulse trains directly at the source. In our theory work, we show that sub-optical-cycle electron bursts induced by tunneling photoemission from a metal nanotip can retain the temporal fingerprint of their emission dynamics in a typical low-energy point-projection microscope setup. We find that strong acceleration by a static field, a short propagation distance and a sufficiently large optical cycle duration mitigate temporal smearing due to matter-wave dispersion. Our approach enables studies of coherent interactions of slow electrons with matter on sub-femtosecond and nanometer scales, a regime which has hitherto remained inaccessible.

Control of the Longitudinal Compression and Transverse Focus of Ultrafast Electron Beam for Detecting the Transient Evolution of Materials.

Ultrafast detection is an effective method to reveal the transient evolution mechanism of materials. Compared with ultra-fast X-ray diffraction (XRD), the ultra-fast electron beam is increasingly adopted because the larger scattering cross-section is less harmful to the sample. The keV single-shot ultra-fast electron imaging system has been widely used with its compact structure and easy integration. To achieve both the single pulse imaging and the ultra-high temporal resolution, magnetic lenses are typically used for transverse focus to increase signal strength, while radio frequency (RF) cavities are generally utilized for longitudinal compression to improve temporal resolution. However, the detection signal is relatively weak due to the Coulomb force between electrons. Moreover, the effect of RF compression on the transverse focus is usually ignored. We established a particle tracking model to simulate the electron pulse propagation based on the 1-D fluid equation and the 2-D mean-field equation. Under considering the relativity effect and Coulomb force, the impact of RF compression on the transverse focus was studied by solving the fifth-order Rung–Kutta equation. The results show that the RF cavity is not only a key component of longitudinal compression but also affects the transverse focusing. While the effect of transverse focus on longitudinal duration is negligible. By adjusting the position and compression strength of the RF cavity, the beam spot radius can be reduced from 100 m to 30 m under the simulation conditions in this paper. When the number of single pulse electrons remains constant, the electrons density incident on the sample could be increased from to , which is 11 times the original. The larger the electron density incident on the sample, the greater the signal intensity, which is more conducive to detecting the transient evolution of the material.

Selectivity in electron emission induced by ultrashort circularly polarized laser pulses

We theoretically investigate the ionization of neonlike atoms by an intense ultrashort circularly polarized laser pulse as a function of wavelength, spanning the regime from single-photon ionization to multiphoton and tunneling ionization. In order to determine the dependence of the ionization probabilities on the magnetic quantum number of the initial state, we perform ab initio numerical calculations of the corresponding time-dependent Schr\"odinger equation in the single-active-electron approximation. Specific emphasis of our analysis is given to the intermediate-wavelength regime of about 50 nm to 300 nm, in which the trend in the ionization probability of counterrotating electrons strongly differs from those for electrons in the other two initial magnetic sublevels. Theoretical analysis and numerical results indicate that the enhanced emission of counterrotating electrons occurs via photon absorption channels, which are only accessible for electrons rotating opposite to the rotation direction of the external electric field. Physical mechanisms behind the enhancement are identified as threshold effects, in which the emission into continuum states with low angular momentum quantum numbers is favored, and resonant enhanced ionization involving transitions via specific excited states in the atom. Numerical results showing the strong population in the relevant excited states, specifically those with low angular momentum quantum number, for the ionization of counterrotating electrons, and the overall similar trends in ionization and excitation for electrons with the other two initial magnetic quantum numbers support our interpretation. Overall, these effects lead to a change in the ionization ratio of co- to counterrotating electrons by two orders of magnitude from 10:1 to 1:10. This strong selectivity in the emission of electrons may lead to new opportunities in the generation of ultrashort spin-polarized electron pulses.

Jitter-free 40-fs 375-keV electron pulses directly accelerated by an intense laser beam and their application to direct observation of laser pulse propagation in a vacuum

We report the generation of ultrashort bright electron pulses directly driven by irradiating a solid target with intense femtosecond laser pulses. The duration of electron pulses after compression by a phase rotator composed of permanent magnets was measured as 89 fs via the ponderomotive scattering of electron and laser pulses, which were almost at the compression limit due to the dispersion of the electron optics. The electron pulse compression system consisting of permanent magnets enabled extremely high timing stability between the laser pulse and electron pulse. The long-term RMS arrival time drift was below 14 fs in 4 h, which was limited by the resolution of the current setup. Because there was no time-varying field to generate jitter, the timing jitter was essentially reduced to zero. To demonstrate the capability of the ultrafast electron pulses, we used them to directly visualize laser pulse propagation in a vacuum and perform 2D mapping of the electric fields generated by low-density plasma in real time.

Power Electronic Pulse Generators for Water Treatment Application: A Review

Water treatment is one of the most important issues for all walks of life around the world. Different from the conventional water treatment technology, the advanced power electronic pulse technology has unique features and advantages for the modern water treatment. Unfortunately, there is no literature reported to describe them in a comprehensive and systematic way. To fill this gap, an overview of modern power electronic pulse generators (PPGs) for water treatment is presented. This article, for the first time, classifies the different types of PPGs from the viewpoint of pulse formation. Each of them is discussed, and the advantages and disadvantages are compared and summarized. Aside from that, this article presents the development and trend of the pulse generators suitable for the specified water treatment processes such as electrolysis, sterilization, and discharge degradation. Finally, a list of more than 100 relevant technical papers is also appended for a quick reference.

Design of ultrabright 270 keV DC photoelectron gun for ultrafast electron diffraction

Compact DC photoelectron guns of high-voltage are highly desired to output ultrabright and ultrashort electron pulses for accessing irreversible processes by using the ultrafast electron diffraction (UED) technique. The high-voltage breakdown, however, is a major technical barrier to providing an intense electric field strength in a condensed space between the photocathode and the anode when the voltage is over 120 kV. In this work, by adopting the concept of voltage division, we propose a novel design of ultrabright near-relativistic DC photoelectron guns for UED. The electric field breakdown mainly caused by micro-particle collision could be avoided such that an optimized three-level acceleration DC gun can work with an electron energy of up to 270 keV and an electric field strength of up to 15 MV/m. N-particle simulations of the electron pulse propagation show that, with such a DC electron gun, it is possible to have ultrabright and ultrashort electron probe pulses with no jitter issue.

Intense microsecond electron pulses from a Schottky emitter

Thanks to their high brightness, field emitters are the electron sources of choice in most high-end electron microscopes. Under typical operating conditions, the available emission current from these emitters is largely limited by practical considerations, and extracting significantly larger currents is usually not possible without reducing the lifetime of the emitter or even damaging it. Such limitations may, however, not apply if the emitter is only briefly subjected to extreme operating conditions so that damage can be outrun. Here, we demonstrate that it is possible to temporarily operate a Schottky emitter far outside its stable operating regime and significantly increase its emission current. We do so by locally heating the tip of the emitter with a microsecond laser pulse, which boosts the emission current by a factor of 3.7 to nearly 450 μA. We believe that the generation of intense microsecond electron pulses from a field emitter will particularly benefit the atomic-resolution imaging of fast processes that occur on the microsecond timescale.

Generation and attosecond shaping of high coherence free-electron beams for ultrafast TEM

We demonstrate the generation and optical control of ultrashort high-coherence electron pulses. The free-electron quantum state is phase-modulated in the longitudinal and transverse dimensions, and the formation of attosecond electron pulse trains is quantitatively probed.

Simulation and Implementation of a SPWM Inverter Pulse Generator Circuit for Educational Purposes

This paper aims to develop and implement an educational kit for a Sinusoidal Pulse Width Modulation (SPWM) inverter pulse generator circuit, which can be used to educate Electronics Engineering undergraduate students the structure and behavior of a SPWM’s inverter pulse generator. The developed electronic circuit is simulated and implemented using low cost and reliable electronic parts. The concept is to offer under/postgraduate students the opportunity to deeply understand how a SPWM pulse generator works, by virtually and practically experimenting with the pulse generator itself creating the necessary models in the popular platform of MULTISIM (Simulation Tool of National Instruments) and designing/constructing the respective PCB circuits in the also popular platform of ULTIBOARD (Circuit Design Tool of National Instruments). This work is also useful for engineers who deal with operation and maintenance (O&M) of inverters, because it provides a deeper knowledge and understanding of all operational characteristics of every stage of the SPWM electronic pulse generator of an inverter.

Ultrafast Strong-Field Tunneling Emission in Graphene Nanogaps

We demonstrate subcycle electron pulse generation in a nanogap of graphene when irradiated by a femtosecond laser pulse in the near-infrared region (800 nm). A strong photoinduced emission was prod...

On the frequency response of a Wenglor particle-counting system for aeolian transport measurements

Abstract A commonly deployed particle-counting system for aeolian saltation flux, consisting of a Wenglor fork sensor and an Onset Hobo Pulse Input Adapter linked to an Onset Hobo Energy Logger Pro data logger, was tested for frequency response. The Wenglor fork sensor is an optical gate device that has very fast switching capacity that can accommodate the time of flight of saltating sand particles through the sensing volume with the exception of very fine sand or silt and very quickly moving particles. The Pulse Input Adapter, in contrast, imposes limitations on the frequency response of the system. The manufacturer of the pulse adapter specifies an upper limit of 120 Hz, although bench tests with an electronic pulse generator indicate that the frequency response of the Pulse Input Adapter, in isolation, is excellent up to 3000 Hz, with only small error (less than 1.6%) due to under-counting during data transfer intervals. A mechanical test of the integrated system (fork sensor, pulse input adapter, and data logger) demonstrates excellent performance up to about 700 Hz (less than 2% error), but significant under-counting above 1000 Hz for unknown reasons. This specific particle-counting system therefore has a frequency response that is well suited for investigation of the dynamics of aeolian saltation as typically encountered in most field conditions on coastal beaches with the exception of extreme wind events and very small particle sizes.

Nonadiabatic ponderomotive effects in photoemission from nanotips in intense midinfrared laser fields

Transient near fields around metallic nanotips drive many applications, including the generation of ultrafast electron pulses and their use in electron microscopy. We have investigated the electron emission from a gold nanotip driven by midinfrared few-cycle laser pulses. We identify a low-energy peak in the kinetic energy spectrum and study its shift to higher energies with increasing laser intensities from 1.7 to $8.9\ifmmode\times\else\texttimes\fi{}{10}^{11}\phantom{\rule{0.28em}{0ex}}\mathrm{W}/{\mathrm{cm}}^{2}$. The experimental observation of the upshift of the low-energy peak is compared to a simple model and numerical simulations, which show that the decay of the near field on a nanometer scale results in nonadiabatic transfer of the ponderomotive potential to the kinetic energy of emitted electrons and in turn to a shift of the peak. We derive an analytic expression for the nonadiabatic ponderomotive shift, which, after the previously found quenching of the quiver motion, completes the understanding of the role of inhomogeneous fields in strong-field photoemission from nanostructures.