- 标题
- 摘要
- 关键词
- 实验方案
- 产品
-
Untangling the contributions of image charge and laser profile for optimal photoemission of high-brightness electron beams
摘要： Using our model for the simulation of photoemission of high brightness electron beams, we investigate the virtual cathode physics and the limits to spatio-temporal and spectroscopic resolution originating from the image charge on the surface and from the profile of the exciting laser pulse. By contrasting the effect of varying surface properties (leading to expanding or pinned image charge), laser profiles (Gaussian, uniform, and elliptical), and aspect ratios (pancake- and cigar-like) under different extraction field strengths and numbers of generated electrons, we quantify the effect of these experimental parameters on macroscopic pulse properties such as emittance, brightness (4D and 6D), coherence length, and energy spread. Based on our results, we outline optimal conditions of pulse generation for ultrafast electron microscope systems that take into account constraints on the number of generated electrons and on the required time resolution.
关键词： laser profile,coherence length,virtual cathode,electron beams,image charge,photoemission,emittance,energy spread,brightness
更新于2025-09-16 10:30:52
-
[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) - Modeling of the Ultra-Stable Operating Regime in Fourier Domain Mode Locked (FDML) Lasers
摘要： Fourier domain mode locked (FDML) ?ber lasers are broadband wavelength-swept ring systems with record sweep speeds. Lasing is achieved by synchronizing the roundtrip time of the optical ?eld in the ?ber delay cavity with the sweep period of a tunable Fabry-P′erot (FP) bandpass ?lter. Since their invention in 2006, FDML lasers have dramatically enhanced the capabilities of optical coherence tomography (OCT) and various sensing applications. However, the physical coherence limits, such as the maximum achievable coherence length, are yet unknown. An important breakthrough in reaching this limit is a recently experimentally demonstrated highly coherent operation mode over a bandwidth of more than 100 nm [1], referred to as the sweet spot. The sweet spot operation mode is characterized by nearly shot-noise limited ?uctuations in the intensity trace of the laser with signi?cantly enhanced coherence properties, whereas in conventional FDML laser systems the intensity trace is distorted by high frequency noise which negatively affects the coherence length. This ultra-low noise operating regime was generated by an almost perfect compensation of the ?ber dispersion with a manually ?ne tuned chirped ?ber Bragg grating and a highly synchronized sweep rate of the FP ?lter with an accuracy in the range of mHz. Polarization effects were controlled with a polarization maintaining semiconductor optical ampli?er (SOA) gain medium and a polarization controller.
关键词： Fourier domain mode locked (FDML) lasers,sweet spot operation mode,coherence length,semiconductor optical amplifier (SOA),polarization controller,optical coherence tomography (OCT)
更新于2025-09-11 14:15:04
-
Highly Efficient Indoor Organic Solar Cells by Voltage Losses Minimization through Fine-tuning of Polymer Structures
摘要： Herein we report a detailed study on the optoelectronic properties, photovoltaic performance, structural conformation, morphology variation, charge carrier mobility and recombination dynamics in bulk heterojunction (BHJ) solar cells comprising of a series of donor-acceptor (D-A) conjugated polymers as electron donors based on benzodithiophene (BDT) and 5,8-bis(5-bromothiophen-2-yl)-6,7-difluoro-2,3-bis(3-(octyloxy)phenyl)quinoxaline as a function of the BDT’s thienyl substitution (alkyl (WF3), alkylthio (WF3S) and fluoro (WF3F)). It is manifested the synergistic positive effects of the fluorine substituents on the minimization of the bimolecular recombination losses, the reduction of the series resistances (RS), the increment of the shunt resistances (RSh), the suppression of the trap-assisted recombination losses, the balanced charge transport, the finer nanoscale morphology and the deeper highest occupied molecular orbital (EHOMO) versus the alkyl- and alkylthio- substituents. According to these findings, WF3F:[6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) based-organic photovoltaic (OPV) device is a scarce example that feature a high power conversion efficiency (PCE) of 17.34% under 500 lx indoor LED light with a high open-circuit voltage (VOC) of 0.69 V, due to the suppression of the voltage losses and a PCE of 9.44% at 1-sun (100 mW/cm2) conditions, simultaneously.
关键词： Series and Shunt resistances,Coherence length,Conjugated polymers,Wide-angle X-ray scattering,Organic photovoltaic devices,Recombination losses,Indoor lighting conditions
更新于2025-09-11 14:15:04
-
Long spin coherence length and bulk-like spin–orbit torque in ferrimagnetic multilayers
摘要： Spintronics relies on magnetization switching through current-induced spin torques. However, because spin transfer torque for ferromagnets is a surface torque, a large switching current is required for a thick, thermally stable ferromagnetic cell, and this remains a fundamental obstacle for high-density non-volatile applications with ferromagnets. Here, we report a long spin coherence length and associated bulk-like torque characteristics in an antiferromagnetically coupled ferrimagnetic multilayer. We find that a transverse spin current can pass through >10-nm-thick ferrimagnetic Co/Tb multilayers, whereas it is entirely absorbed by a 1-nm-thick ferromagnetic Co/Ni multilayer. We also find that the switching efficiency of Co/Tb multilayers partially reflects a bulk-like torque characteristic, as it increases with ferrimagnet thickness up to 8 nm and then decreases, in clear contrast to the 1/thickness dependence of ferromagnetic Co/Ni multilayers. Our results on antiferromagnetically coupled systems will invigorate research towards the development of energy-efficient spintronics.
关键词： magnetization switching,Spintronics,ferrimagnetic multilayer,spin coherence length,spin torques,bulk-like torque
更新于2025-09-10 09:29:36
-
Experimental evidence of a very thin superconducting layer in epitaxial indium nitride
摘要： Indium nitride is one of the very few semiconductors which is known to have a superconducting phase at temperatures of Tc > 1 K. Superconductivity occurs in a window of carrier densities of approximately 1018–1020 cm?3. This is a very low density when compared to other superconductors (i.e. metals, alloys, high Tc oxides) and thus raises interesting fundamental questions as well as technological possibilities. In this paper we address one key question about the dimensionality of the superconducting state of InN by using angle dependent critical field measurements. Our samples were grown by two different growth techniques (chemical vapour deposition and plasma-assisted molecular beam epitaxy) on c-oriented sapphire, with and without a GaN buffer layer. In both cases we find that for film thicknesses much larger than the coherence length d ? ξ, the angle dependence of the critical field (down to T < 280 mK) with respect to the c-axis continues to be clearly two-dimensional, demonstrating a characteristic cusp when the angle crosses 90° with respect to the c-axis. This indicates that the superconducting electrons are most likely confined to a layer much thinner than the thickness of the InN film. Further we find the magnitude of the gap to be 2Δ(0)/kBTc = 3.6, very close to the BCS prediction.
关键词： indium nitride,penetration depth,angle dependent critical field,coherence length,critical current,superconductivity
更新于2025-09-09 09:28:46