Terahertz Backward-wave Oscillators with Photonic Crystal Waveguides

We are developing backward wave oscillators (BWOs) for use above 1 THz. The devices employ photonic crystal waveguide (PCW) circuits for the slow-wave circuits, micromachined into silicon substrates and coated with gold to form the metallic photonic crystals. These circuits are being tested with an electron beam gun in a BWO tube previously used with an interdigitated circuit that tuned over an octave, up to 270 GHz, and gave approximately 0.5 mW over this range (Fig. 1) [1]. The PCW circuits with E-plane bends allow for a much greater coupling of the electron beam to the circuit, enabling higher frequencies and powers. The main design ideas include using a serpentine PCW formed with a series of photonic waveguide bends illustrated in Fig. 3 that have been designed for 1.5 THz operation. Straight waveguide models were matched to waveguide bends with three dimentional calculations. The entire circuit was simulated with a two dimentional approximation. The photonic crystals allow a gap to be opened between the circuit halves, which allows for an electron sheet beam that couples well with the circuit. Three-dimensional waveguide simulations with a finite element software package have indicated that a gap on the order of half the free-space wavelength may be accommodated, with a calculation of the electric field intensity given in Fig. 2 and characterized by its scattering parameters. These calculations have also been used to design the metallic post dimensions that define the photonic crystal.