Study On Diffraction Radiation Of Nano-Scale Structure Excited By Free Electron And Its Application In THZ

Terahertz science and technology is developing rapidly, and the developments of the radiation sources are one of the key researches in this area. In this dissertation, the radiation of the micro-nano stucture excited by moving electrons is studied, and it shows that it is a good promising way to combine the electronics and photonics for the generation of terahertz radiation. Also, some new physical phenomena and mechanism are also studied in this work.The main achievements are as follows:1. The diffraction radiation of subwavelength structure is explored, and the two cases are discussed:subwavelength grating and subwavelength holes array. The mechanism and characteristics of diffraction radiation of subwavelength metallic grating structure are analyzed. It shows that the diffraction radiation in the upper half-space and lower half-space can also be explained by the formula of Simth-Purcell radiation. Then, the influences of the gap width and the thickness of the grating to the diffracton radiation are also analyzed. Based on the comparison for the asymmetric grating excited by electrons with different beam energies, the physical mechanism of the diffraction radiation in the lower half-space is discussed in further. Then the diffraction radiation of subwavelength holes array is studied, and the influences of the thickness and period of the array are discussed.2. Under the excitation of a line current of electron beam, a dielectric medium loading in the lower half space of the sub-wavelength holes array brings essential changes to the diffraction radiation. In the lower half space, the surface wave becomes radiation wave. The fundamental space harmonic is transformed into Cherenkov radiation with unique characteristics and the first negative space harmonic is converted into radiation wave with the angle of Smith-Purcell radiation. Consequently, in the lower half-space the interference happens due to these two kinds of diffraction radiation. The results of the numerical calculations based on the analytical theory agree well with the computer simulation. The subwavelength metal array structure could strongly support surface wave, and the frequency of the surface wave can be changed from microwave to THz by varying the period of the array. When the surface wave can be transformed into radiation wave, the THz radiation could be reasonably generated. A THz radiation source based on this mechanism is analyzed in detail. In the structure of a dielectric medium rod covered by subwavelength metal ring, the electron bunch would excite surface wave on the surface of subwavelength metallic array, and then the surface wave could be transformed into Cherenkov radiation in the dielectric medium.3. The surface polaritons excited by electrons moving along the metal surface are studied, and the surface polaritons of the bulk metal and metal film are analyzed by numerical calculations and computer simulations. Then, based on the mechanism of that the surface polaritons excited by a uniformly moving electron beam bunch can be transformed into Cherenkov radiation in a structure of nano-scale metal film with dielectric medium loading, the planar structure and cylindrical structure of surface polariton Cherenkov light radiation source (SPCLS) are studied. The results show that SPCLS can generate radiation from visible light to ultraviolet frequency regime and the radiation power density can reach or even excess108W/cm2depending on the beam energy and current density. It is a tunable and miniature light radiation source, and promising to be integrated on chip and able to build up a light radiation source array.4. With the aid of three-dimensional particle-in-cell code simulation, the enhancement of Smith-Purcell radiation with surface-plasmon excited by electrons is considered. In general, the surface wave excited by electrons lies in the non-radiation region, and the frequency of surface wave is always below the threshold for Smith-Purcell radiation. However, the frequency of the surface-plasmon mode excited by electrons is mainly determined by the beam energy and the metal. It can be larger than the threshold for Smith-Purcell radiation, and then it can be transformed into radiation. In the simulation, the model is a grating covered by an Ag film. The results demonstrate that, when the surface-plasmon mode is excited by electrons, the maximum radiation occurs at an observation angle depending on the surface-plasmon frequency, and the radiation power can be enhanced more than ten times. This work is of significance for developing radiation source based on Smith-Purcell radiation.