Research On Terahertz Smith-Purcell Radiation Based On Silicon Gratings Structure

Terahertz waves have broad application prospects in communication,radar,nondestructive testing,security check,material research,biomedicine and other fields because of its unique advantages in electromagnetic spectrum.The lack of compact,high-efficiency and highpower terahertz radiation sources is one of the important reasons restricting the development of terahertz technology.In this thesis,aiming to provide a series of high-efficiency and highpower,compact terahertz radiation sources,a series of terahertz studies of Smith-Purcell radiation based on silicon gratings are carried out,and the radiation mechanism is discussed in detail through theoretical analysis and PIC simulation.Specific studies include:Firstly,two numerical analysis methods are introduced and the smith-Purcell radiation mechanisms of silicon gratings are analyzed in detail by using Waveguide array(WGA)method and Rigorous Coupled Wave Analysis(RCWA).WGA is used to analyze the influence of electron beam and grating parameters on the efficiency of Smith-Purcell radiation,and to analyze the coupling process between the evanescent field generated by the electron beam,the WGA mode inside the grating,and the Smith-Purcell radiation outside the grating,and to explain the generation mechanisms of high-efficiency Smith-Purcell radiation;RCWA is used to analyze the effect of electron beam and grating parameters on the resonance modes and quality factors of silicon-based double gratings.A series of high-efficiency Smith-Purcell radiation sources based on silicon gratings are designed by using WGA and RCWA.Secondly,a Smith-Purcell superradiat emission model excited by a train of electron bunches based on silicon grating’s bound states in the continuum(BICs)is designed,and the Smith-Purcell superradiat emission is significantly enhanced by the BIC of silicon grating.The interference effect of WGA modes and diffraction fields at grating boundary is analyzed,and the formation condition of BICs is revealed.The effect of BICs on the superradiat emission of silicon gratings excited by a train of electron bunches is studied.The results show that BICs of silicon gratings can effectively excited by appropriate grating parameters and improve the efficiency of Smith-Purcell superradiat emission.Then,a Smith-Purcell radiation model based on two-section silicon gratings excited by continuous electron beam is designed.The first section is used to prebunch electron beam,and the second section is used to generate coherent Smith-Purcell radiation.the WGA method is used to analyze the physical mechanism of the interaction between the continuous electron beam and the first section grating to prebunch electron beam.Then,the Smith-Purcell radiation based on two-section silicon gratings is simulated by using particle simulation software.The model has the advantages of compact structure and low starting current density,and is easy to realize miniaturized and high-efficiency terahertz radiation sources.Finally,a Smith-Purcell radiation model with silicon-based double gratings excited by continuous electron beam is designed.Two high reflectivity gratings are used to form a high quality(Q)factor FP resonator to generate self-excited oscillation,thus generating coherent Smith-Purcell radiation.The effect of grating parameters on reflectivity and reflection phase of single grating is analyzed,and the FP resonant mode of silicon-based double grating is excited when the phase matching condition is satisfied.Then,silicon single grating and double grating were fabricated by micro-machining and bonding technology,and terahertz time-domain spectroscopy(THz-TDS)platform was built to conduct cold measurement of grating structure,and the test results verified the feasibility of grating design.the reflection spectrum and Q value of silicon gratings are measured by using terahertz time-domain spectroscopy(THz-TDS)system platform to verify the feasibility of the grating design,which provides theoretical support and experimental reference for the development of practical devices.It is helpful to promote the practical development of compact,high frequency and high-efficiency radiation sources.