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 (Barnett, 1989). 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 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 and characterized by its scattering parameters. These calculations have also been used to design the metallic post dimensions that define the photonic crystal. The resulting mode gives an electric field component parallel to the electron beam path thus allowing for better coupling between the electron beam and the circuit, compared to an interdigitated circuit. The circuits are being microfabricated with deep reactive ion etching, gold evaporation, and substrate bonding. Photonic crystal circuits fabricated at the University of Utah which also include horn antennas formed with anisotropic etching of the silicon substrate, designed for 2.5 THz operation was shown (Vuppala, 200- - 6)