Design of a Metamaterial Cavity for Harmonic Terahertz Gyrotrons

A resonant cavity is developed with a metallic photonic crystal (MPC) to reduce the magnetic field strength and suppress mode competition for a terahertz fourth-harmonic gyrotron. Due to the photonic band gap, electromagnetic waves radiated in certain directions at specified frequencies cannot propagate in the photonic crystal. The dispersion relations of MPC were numerically calculated using a finite-difference frequency-domain method. The gap map for a square lattice of metallic rods was derived from the dispersion relations as the radius of the rods varies. The map of the localized modes in the defects was plotted to illustrate the connection between the band structure and the TE modes. The MPC cavity was simulated and better performance was demonstrated in terms of mode competition compared with a circular waveguide cavity of equivalent cut-off radius at 1-THz.