A Traveling-Wave Forward Coupler Design for a New Accelerating Mode in a Silicon Woodpile Accelerator

Silicon woodpile photonic crystals provide a base structure that can be used to build a 3-D dielectric waveguide system for high-gradient laser-driven acceleration. A new woodpile waveguide design that hosts a phase synchronous centrally confined accelerating mode is proposed. Comparing with previously discovered silicon woodpile accelerating modes, this mode shows advantages in terms of better electron beam loading and higher achievable acceleration gradient. Several traveling-wave coupler design schemes developed for multicell RF cavity accelerators are adapted to the woodpile power coupler design for this new accelerating mode. Design of a forward coupled highly efficient silicon woodpile accelerator is achieved. Simulation shows high efficiency of over 75% of the drive laser power coupled to this fundamental accelerating mode, with less than 15% backward wave scattering. The estimated acceleration gradient, when the coupler structure is driven at the damage threshold fluence of silicon at its operating 1.506-μm wavelength, can reach 185 MV/m. A 17-layer woodpile waveguide structure was successfully fabricated, and the measured bandgap is in excellent agreement with simulation.