Investigations On The Coherent Terahertz Diffraction Radiation Using Electron Beams To Excite Resonator Arrays

Recently,THz radiation has been increasingly developed and widely used in medical and biological detection,material analysis,wireless communication,atmospheric monitoring and other aspects.THz source is the key device to support the application and development.However,THz source based on traditional electronics and photonics is difficult to meet the practical requirements in terms of output power,frequency and portability,due to the middle region of THz wave between electronics and photonics.The traditional THz source based on free electron beams can achieve high power output,but rely on high energy electrons,which are bulky,expensive and on a large scale.Therefore,the compact THz source with high power has become a research hotspot.Diffraction radiation,which is excited by the induced field of charged particles moving near the mutated metal or dielectric structure,is an effective way to generate THz radiation.Moreover,the electron energy required is low,which provides favorable conditions for the development of portable high power THz sources.In this thesis,we illustrate several coherent THz diffraction radiation schemes using electron beams to excite resonator arrays based on the diffraction radiation theory,including coherent Smith-Purcell radiation and coherent Cherenkov diffraction radiation,which provides references for the development of free-electron THz sources for practices.The main work is summarized as follows:1)Firstly,we use an electron beam to parallelly excite a Fabry-Perot-liked resonant mode formed by a cluster of coupled grooves with low-aspect-ratio.This scheme not only reduces the machining difficulty but also has higher storage energy,higher quality factor and higher radiation power compared with the one formed by a single uncoupled groove in the existing paper.Coherent Smith-Purcell radiation has been realized by using electron beams to excite the resonant mode of periodically arranged coupled grooves,where the radiation intensity is more than one order of magnitude higher than the Smith-Purcell radiation generated by ordinary diffraction gratings.Besides,based on the surface plasmon properties of metamaterials in THz band,the resonant mode properties of metamaterials with subwavelength holes in THz band are studied in this thesis.The periodically arranged hole modes and SPs modes excited by electron beams further generate the coherent Smith-Purcell radiation.Compared with the PEC subwavelength hole array,the intensity of coherent THz radiation generated by metamaterial subwavelength hole array with surface plasmon is increased by one order of magnitude.2)In order to further improve the radiation intensity,this thesis proposes a scheme of enhanced coherent Smith-Purcell radiation by using a flat electron beam to obliquely excite coupled groove resonant mode array,which overcomes the limitation of decay length of electron beam and improves the utilization rate of low energy electron beam.Compared with the case of parallel excitation,the radiation intensity can be increased by nearly one order of magnitude under the inclined excitation.In addition,we prove through theory and simulation that the radiation excited by oblique electron beams is dominated by coherent Smith-Purcell radiation,namely diffraction radiation,rather than the coherent transition radiation generated by electron beam impacting periodic grating structure.3)We propose a special diffraction radiation generated by using a sheet electron beam to excite an inclined groove arrays,which is produced by the Cherenkov effect rather than Smith-Purcell effect.Meanwhile,different from the traditional Cherenkov produced in the medium,this radiation is a diffraction radiation,namely Cherenkov diffraction radiation,where the array structure does not have to be periodic and the radiation elements in the array does not have to be uniform,differing from Smith-Purcell radiation.In this thesis,we use an inclined array of tapered grooves(non-uniform)to generate a single pulse THz radiation with a wide spectrum.Furthermore,by using several arrays of tapered grooves,a train of ultrashort THz pulses can be further obtained,and the radiation intensity is increased.4)In order to obtain high power THz radiation,we propose a coherent THz radiation,which uses pre-bunched electron beams to excite resonator arrays.Based on the optical platform of Taiwan"Tsinghua University",a train of laser pulses is designed and implemented to drive a photocathode to generate a train of periodic electron bunches.In this thesis,a photocathode DC electron gun structure has been designed;a Michelson interference optical path for detecting THz radiation has been built;a resonant array with coupled grooves which work at the frequency of 0.42THz has been designed and fabricated;a flat electron beam based on a DC electron gun at Shanghai Jiao Tong University has been implemented.We use it to excite coupled resonator arrays and generate enhanced coherent THz radiation in experiments.The study in this thesis provides theoretical and experimental references for the development of high power and miniaturized THz sources.