Ken Soong

Bio: Ken Soong is an academic researcher from Stanford University. The author has contributed to research in topics: Photolithography & Particle accelerator. The author has an hindex of 4, co-authored 5 publications receiving 419 citations.

Demonstration of electron acceleration in a laser-driven dielectric microstructure

Abstract: Acceleration of relativistic electrons in a dielectric laser accelerator at high electric field gradients is reported, setting the stage for the development of future multi-staged accelerators of this type. Conventional particle accelerators, based on radio-frequency technology, are large-scale installations that are expensive to run. Micro-fabricated dielectric laser accelerators (DLAs) offer an attractive alternative, as they are able to support much larger accelerating fields than current accelerators, while being compact, economical and simple to manufacture using lithographic techniques. This paper presents the first experimental demonstration of a DLA capable of sustained, high-gradient (beyond 250 MeV m−1) acceleration of relativistic electrons. The results set the stage for the development of future multi-staged DLA devices composed of integrated on-chip systems, which would enable compact table-top MeV–GeV-scale accelerators. Applications include security scanners and medical therapy, X-ray light sources for biological and materials research, and portable medical imaging devices. The enormous size and cost of current state-of-the-art accelerators based on conventional radio-frequency technology has spawned great interest in the development of new acceleration concepts that are more compact and economical. Micro-fabricated dielectric laser accelerators (DLAs) are an attractive approach, because such dielectric microstructures can support accelerating fields one to two orders of magnitude higher than can radio-frequency cavity-based accelerators. DLAs use commercial lasers as a power source, which are smaller and less expensive than the radio-frequency klystrons that power today’s accelerators. In addition, DLAs are fabricated via low-cost, lithographic techniques that can be used for mass production. However, despite several DLA structures having been proposed recently1,2,3,4, no successful demonstration of acceleration in these structures has so far been shown. Here we report high-gradient (beyond 250 MeV m−1) acceleration of electrons in a DLA. Relativistic (60-MeV) electrons are energy-modulated over 563 ± 104 optical periods of a fused silica grating structure, powered by a 800-nm-wavelength mode-locked Ti:sapphire laser. The observed results are in agreement with analytical models and electrodynamic simulations. By comparison, conventional modern linear accelerators operate at gradients of 10–30 MeV m−1, and the first linear radio-frequency cavity accelerator was ten radio-frequency periods (one metre) long with a gradient of approximately 1.6 MeV m−1 (ref. 5). Our results set the stage for the development of future multi-staged DLA devices composed of integrated on-chip systems. This would enable compact table-top accelerators on the MeV–GeV (106–109 eV) scale for security scanners and medical therapy, university-scale X-ray light sources for biological and materials research, and portable medical imaging devices, and would substantially reduce the size and cost of a future collider on the multi-TeV (1012 eV) scale.

Design of a subnanometer resolution beam position monitor for dielectric laser accelerators.

Abstract: We present a new concept for a beam position monitor with the unique ability to map particle beam position to a measurable wavelength. Coupled with an optical spectrograph, this beam position monitor is capable of subnanometer resolution. We describe one possible design, and through finite-element frequency-domain simulations, we show a resolution of 0.7 nm. Because of its high precision and ultracompact form factor, this device is ideal for future x-ray sources and laser-driven particle accelerators “on a chip.”

Electron beam position monitor for a dielectric microaccelerator

Abstract: We report the fabrication and first demonstration of an electron beam position monitor for a dielectric microaccelerator. This device is fabricated on a fused silica substrate using standard optical lithography techniques and uses the radiated optical wavelength to measure the electron beam position with a resolution of 10 μm, or 7% of the electron beam spot size. This device also measures the electron beam spot size in one dimension.

Design of a Subnanometer Resolution Beam Position Monitor for Dielectric Laser Accelerators

Abstract: We present a new concept for a beam position monitor with the unique ability to map particle beam position to a measurable wavelength. Coupled with an optical spectrograph, this beam position monitor is capable of subnanometer resolution. We describe one possible design, and through finite-element frequency-domain simulations, we show a resolution of 0.7 nm. Because of its high precision and ultracompact form factor, this device is ideal for future

Fabrication and Characterization of Woodpile Structures for Direct Laser Acceleration

Abstract: Eight and nine layer three dimensional photonic crystals with a defect designed specifically for accelerator applications have been fabricated. The structures were fabricated using a combination of nanofabrication techniques, including low pressure chemical vapor deposition, optical lithography, and chemical mechanical polishing. Limits imposed by the optical lithography set the minimum feature size to 400 nm, corresponding to a structure with a bandgap centered at 4.26 μm. Reflection spectroscopy reveal a peak in reflectivity about the predicted region, and good agreement with simulation is shown. The eight and nine layer structures will be aligned and bonded together to form the complete seventeen layer woodpile accelerator structure.

Terahertz-driven linear electron acceleration

Abstract: The cost, size and availability of electron accelerators are dominated by the achievable accelerating gradient. Conventional high-brightness radio-frequency accelerating structures operate with 30-50 MeV m(-1) gradients. Electron accelerators driven with optical or infrared sources have demonstrated accelerating gradients orders of magnitude above that achievable with conventional radio-frequency structures. However, laser-driven wakefield accelerators require intense femtosecond sources and direct laser-driven accelerators suffer from low bunch charge, sub-micron tolerances and sub-femtosecond timing requirements due to the short wavelength of operation. Here we demonstrate linear acceleration of electrons with keV energy gain using optically generated terahertz pulses. Terahertz-driven accelerating structures enable high-gradient electron/proton accelerators with simple accelerating structures, high repetition rates and significant charge per bunch. These ultra-compact terahertz accelerators with extremely short electron bunches hold great potential to have a transformative impact for free electron lasers, linear colliders, ultrafast electron diffraction, X-ray science and medical therapy with X-rays and electron beams.

Quantum coherent optical phase modulation in an ultrafast transmission electron microscope

Abstract: Coherent manipulation of quantum systems with light is expected to be a cornerstone of future information and communication technology, including quantum computation and cryptography. The transfer of an optical phase onto a quantum wavefunction is a defining aspect of coherent interactions and forms the basis of quantum state preparation, synchronization and metrology. Light-phase-modulated electron states near atoms and molecules are essential for the techniques of attosecond science, including the generation of extreme-ultraviolet pulses and orbital tomography. In contrast, the quantum-coherent phase-modulation of energetic free-electron beams has not been demonstrated, although it promises direct access to ultrafast imaging and spectroscopy with tailored electron pulses on the attosecond scale. Here we demonstrate the coherent quantum state manipulation of free-electron populations in an electron microscope beam. We employ the interaction of ultrashort electron pulses with optical near-fields to induce Rabi oscillations in the populations of electron momentum states, observed as a function of the optical driving field. Excellent agreement with the scaling of an equal-Rabi multilevel quantum ladder is obtained, representing the observation of a light-driven 'quantum walk' coherently reshaping electron density in momentum space. We note that, after the interaction, the optically generated superposition of momentum states evolves into a train of attosecond electron pulses. Our results reveal the potential of quantum control for the precision structuring of electron densities, with possible applications ranging from ultrafast electron spectroscopy and microscopy to accelerator science and free-electron lasers.

Segmented Terahertz Electron Accelerator and Manipulator (STEAM).

Abstract: Acceleration and manipulation of electron bunches underlie most electron and X-ray devices used for ultrafast imaging and spectroscopy. New terahertz-driven concepts offer orders-of-magnitude improvements in field strengths, field gradients, laser synchronization and compactness relative to conventional radiofrequency devices, enabling shorter electron bunches and higher resolution with less infrastructure while maintaining high charge capacities (pC), repetition rates (kHz) and stability. We present a segmented terahertz electron accelerator and manipulator (STEAM) capable of performing multiple high-field operations on the six-dimensional phase space of ultrashort electron bunches. With this single device, powered by few-microjoule, single-cycle, 0.3 THz pulses, we demonstrate record terahertz acceleration of >30 keV, streaking with 2 kT m–1 strength, compression to ~100 fs as well as real-time switching between these modes of operation. The STEAM device demonstrates the feasibility of terahertz-based electron accelerators, manipulators and diagnostic tools, enabling science beyond current resolution frontiers with transformative impact.

Efficient generation of THz pulses with 0.4 mJ energy.

Abstract: Efficient generation of THz pulses with high energy was demonstrated by optical rectification of 785-fs laser pulses in lithium niobate using tilted-pulse-front pumping. The enhancement of conversion efficiency by a factor of 2.4 to 2.7 was demonstrated up to 186 μJ THz energy by cryogenic cooling of the generating crystal and using up to 18.5 mJ/cm2 pump fluence. Generation of THz pulses with more than 0.4 mJ energy and 0.77% efficiency was demonstrated even at room temperature by increasing the pump fluence to 186 mJ/cm2. The spectral peak is at about 0.2 THz, suitable for charged-particle manipulation.

Electron-beam spectroscopy for nanophotonics.

Abstract: Progress in electron-beam spectroscopies has recently enabled the study of optical excitations with combined space, energy and time resolution in the nanometre, millielectronvolt and femtosecond domain, thus providing unique access into nanophotonic structures and their detailed optical responses. These techniques rely on ~1–300 keV electron beams focused at the sample down to sub-nanometre spots, temporally compressed in wavepackets a few femtoseconds long, and in some cases controlled by ultrafast light pulses. The electrons undergo energy losses and gains (also giving rise to cathodoluminescence light emission), which are recorded to reveal the optical landscape along the beam path. This Review portraits these advances, with a focus on coherent excitations, emphasizing the increasing level of control over the electron wavefunctions and ensuing applications in the study and technological use of optically resonant modes and polaritons in nanoparticles, 2D materials and engineered nanostructures. Spatially resolved electron microscopy techniques, such as cathodoluminescence and electron energy-loss spectroscopy can provide high space, energy and time resolutions for the structural and optical characterization of materials; this Review discusses recent progress and future directions in the field of nanophotonics.