The Foundational Investigation of Photonic Band-gap Structure Applied on Micro-vacuum Electronic Devices

Micro-vacuum electronic devices have been playing an important role in national defend and civil economy. Many countries have been convinced that a sound research of micro-vacuum electronic devices can bring significance effect for global communication and national defend affair. Thus, micro-vacuum electronic devices have acquired rapid development in recent years.Nowadays the development of the micro-vacuum electronic devices shows a tendency of high-power, ultra-high frequency and miniaturization.

The working frequencies of the devices have been now extending to millimeter wave, sub-millimeter wave and even to terahertz (THz) band. THz technologies have acquired various significant applications, such as biological imaging, medical diagnosing and wide band communication. Micro-vacuum electronic devices will find more wide and more predominant applications in the THz field. It has been approved that devices such as backward wave oscillators, nano klystrons and nano travelling wave tubes can produce higher radiation power in THz band.

Along with the reduction of the size and the enhancement of the working frequency of the devices, series problems will come into being. For example, the power capability will fall down, the efficiency will decline, the working modes will become more complicated, and furthermore the oscillation among various modes will be more significant. These troubles will greatly restrict the development of micro-vacuum electronic devices. The photonic ban-gap (PBG) structure is a new pattern artificial materials. By introducing the PBG structure to the micro-vacuum electronic devices, people may find new route to resolve the above problems.In this thesis, one-dimensional (1D) and two-dimensional (2D) PBG structures are introduced to the design of high frequency resonator and slow-wave system.

By using theoretic analysis and numerical calculation, resonant characteristics in PBG resonator, dispersion relation and coupled impedance in slow wave system with PBG structure have been discussed. The main contents of this thesis are as follows:Various micro-vacuum electronic devices have been briefly summarized and their promising future and present difficulties have been predicted. Then the background of this thesis has been described.After a short review of the theory of photonic crystal, two kinds of numerical calculating software have been introduced. Based on the method of plane wave expansion, energy band of 1D coaxial photonic crystal and 2D photonic crystals with square lattice and hexagonal lattice have been calculated by using the software developed by the R-soft Corporation in German.

The calculating results indicate that reducing the thickness of medium ring in 1D coaxial photonic crystal will be favorable to widen the band gap. For the same parameters in 2D photonic crystals, the band gap of hexagonal lattice is significantly wider than that of square lattice.The electromagnetism analysis software of MAFIA and MWS has been introduced and the procedure of simulation has been illustrated. The resonant properties of 1D coaxial photonic crystal micro-cavity and cold characteristics of slow wave system with 1D PBG structure have been simulated by MWS. The effect of geometrical parameters on the characters of micro-cavity and slow wave system has been analyzed. The results indicate that a single defect mode TM_(010) can be gained in the micro-cavity, and that the resonance character can be sensitively affected by the longitudinal length and the dielectric constant. This slow-wave system presents a good dispersion property.

Increasing the electron beam radius and decreasing the periodic length will result in higher operating frequency and wider bandwidth.THz band resonators and slow wave system with two-dimensional PBG structure have been constructed. By using MWS and MAFIA, the field-pattern of various resonant modes has been simulated, the quality factor (Q-factor) and the power loss of the resonators have been calculated. The dispersion relation and coupled impedance of the slow wave system have also been calculated by the same methods. The affection of geometrical parameters on the cold characters of resonator and slow wave system been analyzed in detail. The numerical results indicate that a single and stable mode can be gained in the cavity and the larger the transverse size of cavity, the higher the order of the defect mode.

The Q-factor is sensitively affected by the longitudinal length of the cavity. For instance, the Q-factor of TM330 mode has been above 12000 when the longitudinal length was 1mm. The concerned slow wave system can be used for backward wave operation. The working mode keeps stable and the bandwidth has been about 10%. The slow-wave ratio curve is flat and meanwhile the dispersion relation is acceptable. The highest coupled impedance can reach tens or even hundred ohm. The coupled impedance increases and the bandwidth decreases while increasing the length of the period unit or decreasing the radius of electron beam.