THz BWO based on photonic crystal corrugated waveguide

Summary form only given. Vacuum electronics has been demonstrated as a promising solution to realize a portable, high power THz sources¹. However, due to the micrometer-scale structures required, the performance of THz vacuum electron devices can be improved with the use of wide sheet beams to increase the beam current and therefore achieve a more effective field-beam interaction. The corrugated rectangular waveguide has emerged as suitable Slow Wave Structure (SWS) for the use of sheet beam, however precise assembling and high quality vacuum need to be ensured to achieve good performance and they are difficult to realize in small sized, closed structures.In order to alleviate these issues, the flexible technology of photonic crystals (PhCs) is newly employed here for the design of a THz sheet-beam Backward Wave Oscillator (BWO)2. The BWO is based on a novel PhC-corrugated waveguide combined to a PhC-coupler. The latter can ease the coupling with very large sheet-beam, solving for the problem of beam tunnel cut-off in conventional waveguiding. The PhC-corrugated waveguide is firstly applied to a 0.65 THz BWO with an electron beam having beam voltage of 11kV. The bandgap property of PhCs is exploited to replace the lateral walls of the waveguide enclosure, providing a suitable confinement of the propagating field and, at the same time, increase the empty region within the waveguide to ease the vacuum pumping. A PhC - 90o bend waveguide is designed to form the novel BWO coupler. Tailored defects are introduced in the lattice to achieve return losses less than -20dB at each port of the waveguide bend and to embed a beam tunnel with -35dB of isolation. The PhC-coupler is suitable for effective coupling of very large sheet beams. Particle-in-cell simulations of the PhC-BWO have been performed considering 50 periods of SWS and a beam current of 6mA. A frequency tunability of 40GHz around 0.65 THz and an output power up to 70mW are obtained for a beam voltage i- the range 9-12kV.