Eric R. Colby

Demonstration of electron acceleration in a laser-driven dielectric microstructure

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 recently, 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.

Gamma radiation studies on optical materials

Results for the effects of /spl gamma/s on materials for a new laser-driven accelerator are presented. Various optical and laser materials are compared. While Si and fused c-SiO/sub 2/ appear ideal for subbandgap laser wavelengths, other interesting candidates include certain fluorides and compound semiconductors.

Waveguides in three-dimensional photonic bandgap materials for particle-accelerator on a chip architectures

A promising concept for making high-energy particle accelerators less expensive and more compact is to make use of axially polarized optical modes supported by line defect waveguides in three-dimensional photonic bandgap materials. Following a theoretical proposal by B. M. Cowan we here present the first experimental realization of corresponding pilot samples for three-dimensional photonic bandgap particle-accelerator segments. The samples have been fabricated using a combination of direct laser writing and an improved silicon-double-inversion procedure. Regarding optical characterization we have performed transmittance measurements providing unambiguous evidence of a waveguide mode with axial polarization. These results represent an important first step towards actually putting into practice future “particle-accelerator on a chip” architectures.

Laser damage threshold measurements of optical materials for direct laser accelerators

The laser-damage threshold is a fundamental limit for any dielectric laser-driven accelerator and is set by the material of the structure. In this paper, we present a theoretical model of the laser damage mechanism, in comparison with experimental data on the damage threshold of silicon. Additionally, we present damage threshold measurement data of various optical materials, most of which have not been previously characterized in the picosecond-regime.

Electron beam position monitor for a dielectric microaccelerator

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.

Accelerating electrons with lasers and photonic crystals

We present work done towards the successful fabrication of a four-layer woodpile photonic crystal with a bandgap of 0.876 µm centered at 4.55 µm. The goal of this fabrication was to develop a process flow that will be used to fabricate a 15-layer woodpile structure with a defect used for accelerating electrons. The structure dimensions were designed using simulations to maximize the bandgap, and scaled by the minimum features achievable in the optical lithography process. Infrared spectroscopy measurements were taken of the resulting four-layer structure, demonstrating a clear bandgap region centered at 4.55 µm, with a full-width at half maximum (FWHM) of 2.72 µm. Variations in the actual fabricated structure were measured via SEM images, and the deviations were incorporated into simulations. The center bandgap wavelength predicted in simulations agrees to within 1% of the measured value, and the FWHM agrees to within 15% of the measurement. These results provide validation for the quality of the photonic crystal fabricated.

Designing photonic bandgap fibers for particle acceleration

Photonic bandgap (PBG) fibers with hollow core defects have been suggested for use as laser driven accelerator structures. The modes of a photonic crystal fiber lie in a set of allowed bands. A fiber with a central vacuum defect can support so-called defect modes with frequencies in the bandgap and electromagnetic fields confined spatially near the defect. A defect mode suitable for relativistic particle acceleration must have a longitudinal electric field in the central defect and a phase velocity at the speed of light (SOL). We explore the design of the defect geometry to support well confined accelerating modes in such PBG fibers. The dispersion diagram of an accelerating mode must cross the SOL line, and such modes form a special class of defect modes known as surface modes, which are lattice modes of the original PBG crystal that have been perturbed into the bandgap. The details of the surface boundary separating the defect from the surrounding PBG matrix are found to be the critical ingredients for optimizing the accelerator mode properties.

Grating-based deflecting, focusing, and diagnostic dielectric laser accelerator structures

Recent technological advances has made possible the realization of the first laser-driven particle accelerator structure to be fabricated lithographically. However, a complete particle accelerator requires more than just accelerating elements. In this paper, we present a grating-based design for three other quintessential accelerator elements: the focusing structure, the deflecting structure, and the diagnostic structure.

Design, fabrication, and testing of a fused-silica dual-layer grating structure for direct laser acceleration of electrons

A proof of principle fused-silica grating structure has been designed and fabricated for the purpose of direct laser acceleration of electrons. The optimal structure geometry was determined via 2D-FDTD and 3D-FEFD simulations to maximize the available acceleration gradient. The structure was fabricated with standard nanofabrication techniques, including optical lithography, reactive ion etching, and wafer bonding. Beam tests have been performed with the 60MeV beam at the Next Linear Collider Test Accelerator at SLAC, with successful demonstration of electron transmission through the micron-scale apertures.

Fabrication and Characterization of Woodpile Structures for Direct Laser Acceleration

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.