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Editorial: Lasers in Accelerator Science and Secondary Emission Light Source Technology
摘要： Unifying laser and accelerator physics holds great promise for the development of future particle accelerators, light sources, and other scienti?c instruments due to increasingly synergistic advances at the cross section between these two ?elds. Their combined action has recently ushered in advanced accelerator facilities around the world that have enabled unique scienti?c and technological breakthroughs: from advanced electron and ion sources for high-energy physics to the ultrabright x-ray pulses to study ultrafast phenomena at elemental spatio-temporal scales [1–4]. The progress of ultra-intense femtosecond lasers, now attaining multi PetaWatt peak power, has recently enabled the demonstration of GeV electron beams in centimeter scale plasma accelerating section [5, 6], with the recent world record reaching 8 GeV in 20 cm [7]. As for ultrafast x-ray science, in their relatively short time since their advent, x-ray free electron lasers [8, 9] (FEL) have demonstrated the capacity to answer grand fundamental questions in a diverse set of areas in physics, chemistry, and biology, such as revealing vibration coherence in molecules [10], molecular bond formation, charge migration, and dissociation dynamics [11, 12], or ultrafast isomerization in biomolecules [13, 14], among many others. Further advances in facilities—such as augmented brightness, attosecond duration, or seeded emission—are poised to creating new scienti?c frontiers in atomic-scale correlated systems and ultrahigh resolution inner shell spectroscopies.
关键词： free electron laser (FEL),ultrafast optics,X-ray emission,non-linear optics,laser accelerated particles,secondary emission and photoemission
更新于2025-09-16 10:30:52
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Electronic Structure of Sr <sub/>1?</sub><i> <sub/>y</sub></i> Ca <i> <sub/>y</sub></i> Fe <sub/>2</sub> (As <sub/>1?</sub><i> <sub/>x</sub></i> P <i> <sub/>x</sub></i> ) <sub/>2</sub> ( <i>x</i> = 0.25, <i>y</i> = 0.08) Revealed by Angle-Resolved Photoemission Spectroscopy
摘要： We have investigated the electronic structure of Sr1?yCayFe2(As1?xPx)2 (x = 0.25, y = 0.08) by means of angle-resolved photoemission spectroscopy. From the comparison with the results of BaFe2(As1?xPx)2, the effects of smaller structural anisotropy (c/a) on the Fermi surfaces (FSs) and the gap structures are discussed. The observed FSs have three dimensional shapes. One of the hole FSs is strongly warped between the Γ and Z points, and the innermost FS observed at the Z point disappears at the Γ point, which is similar to the FS features of SrFe2(As1?xPx)2 (x = 0.35). In the superconducting state, the node like gap-minimum is present for the dxy electron FS near the X point, while the gaps around the other high symmetry points are isotropic. Several theoretical models based on the spin and/or the orbital fluctuation are examined to explain all these experimental results.
关键词： spin fluctuation,orbital fluctuation,electronic structure,Fermi surfaces,angle-resolved photoemission spectroscopy,superconducting gap
更新于2025-09-16 10:30:52
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<i>Operando</i> observation of resistive switching in a resistive random-access memoryby laser-excited photoemission electron microscope
摘要： We developed a laser-excited photoemission electron microscope (Laser-PEEM) that enables us to perform a non-destructive operando observation for elucidating the changes in the physical properties of electronic devices. By utilizing the Laser-PEEM, the non-volatile resistance change in the resistive random-access memory (ReRAM) was clearly visualized, even though the resistance change occurred under the electrode of the ReRAM, thanks to the deep probing depth. The operando observation of the Laser-PEEM is very promising as an observation method for various kinds of devices because the observation simultaneously provides us with morphological and electrical properties in real time.
关键词： ReRAM,Laser-PEEM,operando observation,photoemission electron microscope,resistive switching
更新于2025-09-16 10:30:52
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CMOS-Compatible Titanium Nitride for On-Chip Plasmonic Schottky Photodetectors
摘要： Here, we propose a waveguide-integrated plasmonic Schottky photodetector (PD) operating based on an internal photoemission process with a titanium nitride plasmonic material. The theoretically examined structure employs an asymmetric metal?semiconductor?metal waveguide configuration with one of the electrodes being gold and the second being either gold, titanium, or titanium nitride. For the first time, we measured a Schottky barrier height of 0.67 eV for titanium nitride on p-doped silicon, which is very close to the optimal value of 0.697 eV. This barrier height will enable photodetection with a high signal-to-noise ratio when operating at a wavelength of 1550 nm. In addition to the measured optical properties of high absorption losses and reasonably large real part of the permittivity that are desired for this type of PD, titanium nitride is also compatible with easy integration on existing complementary metal?oxide?semiconductor technology. The use of titanium nitride results in a shorter penetration depth of the optical mode into the metal when compared to Ti, which in turn enhances the probability for transmission of hot electrons to the adjacent semiconductor, giving rise to an enhancement in responsivity.
关键词： titanium nitride,internal photoemission,CMOS-compatible,waveguide-integrated plasmonic Schottky photodetector,signal-to-noise ratio
更新于2025-09-16 10:30:52
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[IEEE 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Munich, Germany (2019.6.23-2019.6.27)] 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Plasmon-Plasmon Coupling Probed by Ultrafast, Strong-Field Photoemission with <7 ? Sensitivity
摘要： The coupling of propagating surface plasmon waves and localized plasmon oscillations in nanostructures is an essential phenomenon determining electromagnetic field enhancement on the nanoscale. With our recently developed experimental method [1], we can measure the maximum plasmonic field enhancement at any nanostructured metal surface. Here we use our method to investigate the fundamental question of plasmon-plasmon coupling and its effect on large field enhancement factors. Coupling is studied on different nanostructured Ag thin films supporting not only propagating plasmons, but also localized plasmon oscillations due to the different surface nanostructures. Ultrashort laser pulses excite propagating plasmons in Kretschmann geometry (Fig. 1 (a)), while localized plasmons are excited on the surface nanostructures via the coupling of propagating and localized surface plasmons. Photoelectron spectra of the electrons photoemitted due to the plasmonic near fields are measured by a time-of-flight spectrometer [1,2]. The analysis of the cutoffs (highest electron energies, Fig. 1 (b)) of the electron spectra yields maximum plasmonic field enhancement values ×21, ×23 and ×31 for surfaces exhibiting 0.8, 1.6 and 4.5 nm average roughness values, respectively. The finite-difference time-domain (FDTD) simulation of the individual rough surfaces not only support the measured field enhancement values, but also reveal the contributions from propagating and localized plasmons (Fig. 1. (c) and (d)). The dependence of the field localization, i. e. the resulting field enhancement values on the grain size is also demonstrated. It is shown, that when resonance conditions are met, a significant portion of the field enhancement can be attributed to the generation of localized plasmons on the grainy surface nanostructures, acting as dipole sources resonantly driven by the propagating plasmon field [3].
关键词： plasmon-plasmon coupling,ultrafast photoemission,field enhancement,nanostructured Ag thin films
更新于2025-09-12 10:27:22
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Highly Dispersive Nearly Free Electron Bands at a 2D-Assembled C <sub/>60</sub> Monolayer
摘要： Superatomic molecular orbitals (SAMO), which are atom-like diffuse orbitals formed at a molecule, play an important role in controlling the electronic functionality at one- to three-dimensional (1D–3D) assemblies of organic nanomaterials because they form highly dispersive and nearly free electron (NFE) bands, such as a bulk metal, by mixing their wavefunctions with neighbors. Herein, we identify a series of delocalized SAMOs at a two-dimensional assembled C60 fullerene monolayer utilizing resonant angle-resolved two-photon photoemission spectroscopy and theoretical calculations. SAMOs exhibit distinct NFE band dispersion characteristics to be hybridized between the diffuse orbitals at 2D-assembly, unlike the well-known frontier unoccupied π* orbitals with localized wave functions at the carbon framework of C60. Density functional theory (DFT) calculations for the NFE bands quantitatively reproduce the experimental observations including their energies and effective electron masses. These NFE bands comprising SAMOs above the Fermi level dominate charge transfer or photofunctional properties at the 1D–3D molecular assemblies, particularly when the band energies are lowered through a doping strategy.
关键词： Superatomic molecular orbitals,Two-photon photoemission,Fullerenes,2D-materials,Organic devices
更新于2025-09-12 10:27:22
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Interferometric time- and energy-resolved photoemission electron microscopy for few-femtosecond nanoplasmonic dynamics
摘要： 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.
关键词： photoemission electron microscopy,nanoplasmonic dynamics,few-femtosecond,gold nanospheres,time-resolved
更新于2025-09-12 10:27:22
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Development of a Multialkali Photocathode Dc Gun for a Smith-Purcell Terahertz Free-Electron Laser
摘要： We have developed a photocathode dc gun for a compact Smith-Purcell free-electron laser in the terahertz wavelength region. The gun system consists of an alkali antimonide photocathode preparation chamber, a dc gun with a 250 kV-50 mA Cockcroft-Walton high-voltage power supply, and a downstream beamline with a water-cooled beam dump to accommodate a beam power of 5 kW. We fabricated a Cs3Sb photocathode with quantum efficiency of 5.8% at a wavelength of 532 nm and generated a 150 keV beam with current of up to 4.3 mA with a 500 mW laser. A vacuum chamber for the Smith-Purcell free-electron laser has been installed in the downstream beamline. We describe the present status of our work.
关键词： photoemission dc gun,free-electron laser,Smith-Purcell radiation
更新于2025-09-12 10:27:22
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Performance of photoelectron spin polarimeters with continuous and pulsed sources: from storage rings to free electron lasers
摘要： In this work the experimental uncertainties concerning electron spin polarization (SP) under various realistic measurement conditions are theoretically derived. The accuracy of the evaluation of the SP of the photoelectron current is analysed as a function of the detector parameters and speci?cations, as well as of the characteristics of the photoexcitation sources. In particular, the different behaviour of single counter or twin counter detectors when the intensity ?uctuations of the source are considered have been addressed, leading to a new de?nition of the SP detector performance. The widely used parameter called the ?gure of merit is shown to be inadequate for describing the ef?ciency of SP polarimeters, especially when they are operated with time-structured excitation sources such as free-electron lasers. Numerical simulations have been performed and yield strong implications in the choice of the detecting instruments in spin-polarization experiments, that are constrained in a limited measurement time. Our results are therefore applied to the characteristics of a wide set of state-of-the-art spectroscopy facilities all over the world, and an ef?ciency diagram for SP experiments is derived. These results also de?ne new mathematical instruments for handling the correct statistics of SP measurements in the presence of source intensity ?uctuations.
关键词： continuous source,intensity fluctuations,spin-polarimetry statistics,pulsed source,spin-resolved photoemission
更新于2025-09-11 14:15:04
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Growth of InGaN films on hardness-controlled bulk GaN substrates
摘要： We carried out an evaluation of the crystalline quality of bulk GaN substrates and the properties of InGaN ?lms grown on them. The Urbach energy estimated by photothermal de?ection spectroscopy and the tail states near the valence band maximum determined by hard x-ray photoemission spectroscopy were larger for hardness-controlled bulk GaN (hard GaN) than those for conventional bulk GaN (conventional GaN). However, InGaN on hard GaN grows in a step-?ow-like mode, while InGaN grown on conventional GaN exhibits spiral-like growth. The photoluminescence decay at room temperature for InGaN grown on the hard GaN was 470 ps, compared with 50 ps for that grown on the conventional GaN. This can be attributed to the suppression of spiral-like growth due to the resistance to deformation of the hard GaN. These results indicate that substrate hardness is one of the most important factors for III–V nitride growth on the bulk GaN substrate.
关键词： hard x-ray photoemission spectroscopy,photoluminescence,bulk GaN substrates,InGaN films,photothermal de?ection spectroscopy,crystalline quality
更新于2025-09-11 14:15:04