The Study Of Photonic Crystal Waveguide Slow Wave Structure At THz Band

The unique properties of terahertz(THz)waves enable abundant applications,such as imaging,detecting and communication.The output power of present solid-state THz source is relatively low and conventional high-power THz source have large volume,which are all difficult to meet the needs of miniaturized and practical demands.Traveling wave tube(TWT),as a kind of vacuum electron device with high power and wide bandwidth,is a suitable amplifier for low power solid-state terahertz sources.The core part of TWT is slow wave structure(SWS),which determines the performance of TWT.For now,folded waveguide(FWG)is the most common SWS at THz band,which have shown great properties of output power and bandwidth.However,FWG TWT requires a high operation voltage,demanding strict requirements on the power module.Besides,the significant self-oscillation phenomena in the FWG TWT will greatly affect the stability of TWT,which need to be suppressed by attenuator.Achieving stable output with high power and wide bandwidth is the focus of research on THz band TWT devices.This thesis proposes a SWS based on defect photonic crystal waveguide(DPCW),which effectively slow down the phase velocity of electromagnetic waves at THz band.This structure achieves low operation voltage,high power and wide bandwidth signal amplification at THz band.Firstly,a two-dimensional photonic crystal(PC)slab is proposed,and the photonic bandgap(PBG)properties of the structure are analyzed using the plane wave expansion method.By constructing specific defect in the PC array,a transverse magnetic(TM)polarized mode passband is induced into the PBG.The TM mode has a large axial electric field component,which can perform significant energy exchange with the electron beam.The geometrical parameters are optimized for flat dispersion curve and high pierce impedance in the operation band based on the dispersion and pierce impedance properties.Further,based on the mode field characteristics of the DPCW and WR-4 standard waveguide,a tapered input-output coupler is designed to achieve low-loss and low-reflection transmission properties.Finally,a TWT model based on DPCW is established,and the Particle-in-Cell(PIC)simulations are performed to verify the performance of the model.The time domain properties,bandwidth and spectrum properties are analyzed.Simulation results show that the DPCW TWT can achieve high output power and wide bandwidth with a low operation voltage.Due to the PBG effect,DPCW SWS suppresses the generation of higher order harmonic wave and backward wave oscillations around the operation band,which improve the stability of TWT.DPCW provides a better solution for miniaturizing THz devices.