High-coherence picosecond electron bunches from cold atoms

The development of ultrafast gas electron diffraction with nonrelativistic electrons has enabled the determination of molecular structures with atomic spatial resolution. It has, however, been challenging to break the picosecond temporal resolution barrier and achieve the goal that has long been envisioned—making space- and-time resolved molecular movies of chemical reaction in the gas-phase. Recently, an ultrafast electron diffraction (UED) apparatus using mega-electron-volt (MeV) electrons was developed at the SLAC National Accelerator Laboratory for imaging ultrafast structural dynamics of molecules in the gas phase. The SLAC gas-phase MeV UED has achieved 65 fs root mean square temporal resolution, 0.63 Å spatial resolution, and 0.22 Å−1 reciprocal-space resolution. Such high spatial-temporal resolution has enabled the capturing of real-time molecular movies of fundamental photochemical mechanisms, such as chemical bond breaking, ring opening, and a nuclear wave packet crossing a conical intersection. In this paper, the design that enables the high spatial-temporal resolution of the SLAC gas phase MeV UED is presented. The compact design of the differential pump section of the SLAC gas phase MeV UED realized five orders-of-magnitude vacuum isolation between the electron source and gas sample chamber. The spatial resolution, temporal resolution, and long-term stability of the apparatus are systematically characterized.

Laser technology is finding applications in areas such as high resolution spectroscopy, radar-lidar, velocimetry, or atomic clock where highly coherent tunable high power light sources are required. The Vertical External Cavity Surface Emitting Laser (VECSEL) technology [1] has been identified for years as a good candidate to reach high power, high coherence and broad tunability while covering a wide emission wavelength range exploiting III-V semiconductor technologies. Offering such performances in the Near- and Middle-IR range, GaAs- and Sb-based VECSEL technologies seem to be a well suited path to meet the required specifications of demanding applications. Built up in this field, our expertise allows the realization of compact and low power consumption marketable products, with performances that do not exist on the market today in the 0.8-1.1 μm and 2-2.5 μm spectral range. Here we demonstrate highly coherent broadly tunable single frequency laser micro-chip, intracavity element free, based on a patented VECSEL technology, integrated into a compact module with driving electronics. VECSEL devices emitting in the Near and Middle-IR developed in the frame of this work [2] exhibit exciting features compared to diode-pumped solid-state lasers and DFB diode lasers; they combine high power (>100mW) high temporal coherence together with a low divergence diffraction limited TEM00 beam. They exhibit a class-A dynamics with a Relative Intensity Noise as low as -140dB/Hz and at shot noise level reached above 200MHz RF frequency (up to 160GHz), a free running narrow linewidth at sub MHz level (fundamental limit at Hz level) with high spectral purity (SMSR >55dB), a linear polarization (>50dB suppression ratio), and broadband continuous tunability greater than 400GHz ( Those devices surpass the state of the art commercial technologies thanks to a combination of power-coherence-wavelength tunability performances and integration.

Ultracold atom-based electron sources have recently been proposed as an alternative to the conventional photo-injectors or thermionic electron guns widely used in modern particle accelerators. The advantages of ultracold atom-based electron sources lie in the fact that the electrons extracted from the plasma (created from near threshold photo-ionization of ultracold atoms) have a very low temperature, i.e. down to tens of Kelvin. Extraction of these electrons has the potential for producing very low emittance electron bunches. These features are crucial for the next generation of particle accelerators, including free electron lasers, plasma-based accelerators and future linear colliders. The source also has many potential direct applications, including ultrafast electron diffraction (UED) and electron microscopy, due to its intrinsically high coherence. In this paper, the basic mechanism of ultracold electron beam production is discussed and our new research facility for an ultracold, low emittance electron source is introduced. This source is based on a novel alternating current Magneto-Optical Trap (the AC-MOT). Detailed simulations for a proposed extraction system have shown that for a 1 pC bunch charge, a beam emittance of 0.35 mm mrad is obtainable, with a bunch length of 3 mm and energy spread 1%.

Ultrafast electron-based coherent radiation sources, such as free-electron lasers (FELs), ultrafast electron diffraction (UED) and Thomson-scattering sources, are becoming more important sources in today’s ultrafast science. Photocathode laser is an indispensable common subsystem in these sources that generates ultrafast electron pulses. To fully exploit the potentials of these sources, especially for pump-probe experiments, it is important to achieve high-precision synchronization between the photocathode laser and radio-frequency (RF) sources that manipulate electron pulses. So far, most of precision laser-RF synchronization has been achieved by using specially designed low-noise Er-fibre lasers at telecommunication wavelength. Here we show a modular method that achieves long-term (>1 day) stable 10-fs-level synchronization between a commercial 79.33-MHz Ti:sapphire laser oscillator and an S-band (2.856-GHz) RF oscillator. This is an important first step toward a photocathode laser-based femtosecond RF timing and synchronization system that is suitable for various small- to mid-scale ultrafast X-ray and electron sources.

Thermal emission from blackbody is typically incoherent and broadband. Achieving highly coherent thermal source while eliminating the rainbow effect has been remaining a challenging task. In our study, we utilize the isolated nature of bound states in the continuum (BICs) at the Γ point to achieve thermal emission with high temporal and spatial coherence. Under the framework of temporal coupled mode theory (TCMT), we can significantly reduce the Q-factors of modes outside the Γ point by employing far-field coupling of modes in different polarization channels within momentum space, thereby suppressing the rainbow effect. Our design, experimentally validated through ternary grating structures, demonstrates thermal emission centered at 6.5 μm with a 23 nm bandwidth, confined within a 2° angular range. This advancement holds significant implications for the miniaturization and integration of thermal radiation devices, with potential applications in infrared imaging, sensing, and energy harvesting.

We demonstrate seamless channel multiplexing and high bitrate superchannel transmission of coherent optical orthogonal frequency division multiplexing (CO-OFDM) data signals utilizing a dissipative Kerr soliton (DKS) frequency comb generated in an on-chip microcavity. Aided by comb line multiplication through Nyquist pulse modulation, the high stability and mutual coherence among mode-locked Kerr comb lines are exploited for the first time, to the best of our knowledge, to eliminate the guard intervals between communication channels and achieve full spectral density bandwidth utilization. Spectral efficiency as high as 2.625bit/Hz/s is obtained for 180 CO-OFDM bands encoded with 12.75Gbaud 8-QAM data, adding to the total bitrate of 6.885Tb/s within a 2.295THz frequency comb bandwidth. This Letter confirms that high coherence is the key superiority of Kerr soliton frequency combs over independent laser diodes, as a multi-spectral coherent laser source for high-bandwidth high-spectral-density transmission networks.

Historically, high probe current in small probes has been the principal virtue of the field emission TEM. Analytical performance is greatly enhanced[1] and fine probe mapping an important analytical tool for trace element segregation and very fine scale high resolution chemical imaging. Less appreciated is the enhanced high resolution capability due to the small energy spread of the electron source and higher coherency due to reduced primary and virtual source size. The envelope function of the phase contrast transfer function (PCTF) is drastically improved reducing image blurring due to both chromatic aberration and specimen illumination angle. This is especially true for the high spatial frequencies beyond the first zero. Intensities sufficient for image formation can be obtained from these higher frequency regions. Since the PCTF does not change its form, photographing images at the Scherzer condition will yield higher resolution with higher coherency, however, the transfer of the specimen potential corresponding to contrast enhanced regions is in the oscillations form beyond the first zero. Changes in the defocus will lead to changes in transferred frequencies beyond the first zero and possible extinctions of otherwise transferred information since the oscillations will pass through zero contrast. Interpretation of images under these conditions will, therefore, require accurate measurement of the optical parameters of the microscope and especially accurate measurements of the defocus conditions. It is also important to calculate images under these conditions for comparison with measured images which give the insurance of defocus conditions supposed to be presented.

Charged particle sources based on photoionisation of laser cooled atoms can provide unique properties, in particular high spatial coherence and the ability to create complex three-dimensional spatial density distributions, allowing detailed measurement of the internal charged particle interactions. Cold electrons extracted from laser cooled atoms promise the spatial coherence and high current required for picosecond molecular scale imaging. Similarly, sources of cold ions provide the opportunity of ion microscopy and ion beam milling with unprecedented resolution. We use arbitrary and real-time control of the electron and ion bunch shapes to demonstrate and measure the high spatial coherence of the cold atom electron and ion source.

Surface effects and quantum confinement render nanomaterials' optoelectronic properties more susceptible to nonradiative processes than their bulk counterparts. These nonradiative processes usually contain a series of interwoven and competing subprocesses, which are challenging to disentangle. Here, we investigate the structural origin of Auger recombination in ZnO nanoparticles using transient absorption spectroscopy and ultrafast electron diffraction. The photogenerated hot holes are captured by oxygen vacancies via an excitonic Auger process, inducing significant local polaronic distortions around the oxygen vacancy and its neighboring zinc tetrahedron on a subpicosecond time scale. The recombination of trapped holes accelerates lattice thermalization and stabilizes the formed small hole polarons. Subsequently, the distorted lattice captures additional electrons in a conduction band, forming a long-lived (>6 ns) exciton-polaron complex that may account for the visible luminescence. Our findings are potentially applicable to other transition metal oxide nanomaterials, bringing insights for optimizing their functional properties.

Quantitative phase imaging (QPI) technique is used to determine various biophysical parameters, such as refractive index, cell thickness, morphology, etc. On the other hand, fluorescence microscopy is used to acquire information regarding molecular specificity of the biological cells and tissues. Conventionally, a fully coherent light source such as laser is used in QPI technique to obtain the interference fringes with ease; however, its high coherence is also responsible for the generation of speckle and spurious fringes, which results in degraded image quality and affects the phase measurement results too. In this paper, we report a multi-modal system that can be effectively utilized to acquire time varied diverse information about the biological specimen with high spatial phase sensitivity. Herein, a single unit comprising of a fluorescence microscope and the Linnik based interferometer specially equipped with a partially spatially coherent light source illumination was developed. The integrated system is capable to procure molecular specificity and phase information of biological specimen, in a single shot, utilizing a single-chip color CCD camera. Here, we performed experiments on MG63 osteosarcoma cells, and the composite interferometric-fluorescence images were obtained and then digitally decomposed into red and green colors; and, the phase maps were reconstructed using the Fourier fringe analysis method. Furthermore, the cultured cells were monitored over a time-span to observe and investigate the time dependent morphological changes along with the quantification of cellular adhesion and spreading. Hence, the proposed system can be utilized to quantify time dependent changes in the cell’s morphology and in cell adhesion which can be an indicator for the detection of various range of diseases such as arthritis, cancer, osteoporosis and atherosclerosis.

The ultrafast photoinduced ring-opening of 1,3-cyclohexadiene constitutes a textbook example of electrocyclic reactions in organic chemistry and a model for photobiological reactions in vitamin D synthesis. Although the relaxation from the photoexcited electronic state during the ring-opening has been investigated in numerous studies, the accompanying changes in atomic distance have not been resolved. Here we present a direct and unambiguous observation of the ring-opening reaction path on the femtosecond timescale and subångström length scale using megaelectronvolt ultrafast electron diffraction. We followed the carbon-carbon bond dissociation and the structural opening of the 1,3-cyclohexadiene ring by the direct measurement of time-dependent changes in the distribution of interatomic distances. We observed a substantial acceleration of the ring-opening motion after internal conversion to the ground state due to a steepening of the electronic potential gradient towards the product minima. The ring-opening motion transforms into rotation of the terminal ethylene groups in the photoproduct 1,3,5-hexatriene on the subpicosecond timescale.

ABSTRACTWe report on the development of an ultrafast Transmission Electron Microscope based on a laser-driven cold-field emission source. We first describe the instrument before reporting on numerical simulations of the laser-driven electron emission. These simulations predict the temporal and spectral properties of the femtosecond electron pulses generated in our ultrafast electron source. We then discuss the effects that contribute to the spatial, temporal and spectral broadening of these electron pulses during their propagation from the electron source to the sample and finally to the detectors of the electron microscope. The spectro-temporal properties are then characterized in an electron/photon cross-correlation experiment based on the detection of electron energy gains. We finally illustrate the potential of this instrument for ultrafast electron holography and ultrafast electron diffraction.

A relativistic electron source is proposed, driven by the wakefield of an intense terahertz (THz) pulse in low-density gas plasma. In contrast to the optical and near-infrared regimes, the low (3.5 THz) frequency and the long (λT = 85.6 μm) wavelength of the THz pulse offers distinct advantages, such as the -scaling of the electron ponderomotive energy. Two-dimension-in-space and three-dimension-in-velocity particle-in-cell simulation results show that relativistic electrons of ∼1 MeV energy and high charge can be generated by an intense THz pulse at kilohertz repetition rate from a gas plasma target. These results may lead to a new regime of applications, such as ultrafast electron diffraction or high-repetition-rate gamma ray sources for materials characterization or medical radiography, which would benefit from lower energy (1–10 MeV) but higher repetition rate (∼1 kHz) sources of relativistic electrons.

Midinfraredspectroscopy is a universal way to identify chemicaland biological substances. Indeed, when interacting with a light beam,most molecules are responsible for absorption at specific wavelengthsin the mid-IR spectrum, allowing to detect and quantify small tracesof substances. On-chip broadband light sources in the mid-infraredare thus of significant interest for compact sensing devices. In thatregard, supercontinuum generation offers a mean to efficiently performcoherent light conversion over an ultrawide spectral range, in a singleand compact device. This work reports the experimental demonstrationof on-chip two-octave supercontinuum generation in the mid-infraredwavelength, ranging from 3 to 13 μm (that is larger than 2500cm–1) and covering almost the full transparencywindow of germanium. Such an ultrawide spectrum is achieved thanksto the unique features of Ge-rich graded SiGe waveguides, which allowsecond-order dispersion tailoring and low propagation losses overa wide wavelength range. The influence of the pump wavelength andpower on the supercontinuum spectra has been studied. A good agreementbetween the numerical simulations and the experimental results isreported. Furthermore, a very high coherence is predicted in the entirespectrum. These results pave the way for wideband, coherent, and compactmid-infrared light sources by using a single device and compatiblewith large-scale fabrication processes.

NMR spectroscopy is a powerful technique suitable for obtaining detailed structural and dynamic data at atomic resolution. Progress in NMR instrumentation has led the scientific community to produce novel techniques which provide valuable information to resolve demanding and crucial questions of molecular biology and rational drug design. This chapter outlines the progress of NMR spectroscopy in the rational drug design. In addition, it offers an example of a reaction in NMR tube for achieving rational drug design.