Theoretical Study Of Surface Polariton Cherenkov Light Radiation Source

Terahertz science and technology is playing a more and more important role in modern science,technology,and industry,so it has a wide application prospect.In order to promote the further development of terahertz science and technology,the "terahertz gap" needs to be filled,and Surface Polariton Cherenkov Light Source(SPCLS)is one of the most powerful ways to solve this problem.Because of the combination of the principles in electronics and photonics,this new-concept radiation source can generate terahertz and even much higher frequency electromagnetic radiation.In order to push forward the development of SPCLS-related science and technology,it is urgent to carry out systematic theoretical study,explaining the physical mechanism thoroughly and quantitatively.Following the principle of “from shallow to deep”,“from classical to quantum” and “from phenomenological to essential”,in this dissertation,the theoretical study on SPCLS will be carried out from the following four levels: the field matching approach,the dynamic theory,the photon excitation theory of classical electron beam,and the relativistic theory of electron beam – photon interaction.The field matching approach: Based on the equivalent charge and current of the electron beam,as well as Maxwell's Equations,the field distribution in SPCLS will be solved.It will be obtained that mediated by the excitation of Surface Plasmon Polaritons,the electron beam can produce coherent,enhanced Cherenkov Radiation that can be tuned by the velocity of the electron beam.At the same time,based on the Dyadic Green's Function,a powerful mathematical tool,the dissertation could greatly simplify the results of the field matching approach,so that the approach can be more easily extended to different structures,and more suitable for dealing with the complex motion of electron beam.The dynamic theory: Based on electromagnetic theory,the dissertation will further introduce Boltzmann Equation to describe the electron motion.The obtained dynamic theory can study the effect of the electromagnetic field on the electron beam,which is not involved in the field matching approach.The dynamic theory predicts that when the electron number density of the electron beam is large enough,the evolution of SPCLS would deviate from the prediction of field matching approach and enter the "strong coupling" regime.At this point,according to the parameters of SPCLS,there are two possible interaction states of the electron beam and the electromagnetic field,referred as the o state and the m state,respectively.Based on the o state,it is possible to improve the SPCLS output significantly in the application scenario of high-current electron beam.The photon excitation theory of classical electron beam: The dissertation will further regard the electromagnetic field as photons,but still regard the electron beam as classical charge and current field,and calculate the evolution of the system through the “semi-quantized” coupling Hamiltonian.This theory deals with the interaction of electromagnetic field with low-density electron beam that is not covered by the dynamic theory.The study will find out that,regardless of the electron number density of the electron beam,when SPCLS is excited,the output signal will rise faster than that predicted by the classical theories(the field matching approach and the dynamic theory).This provides some advantages in the design of sensors.At the same time,because the quantum nature of electromagnetic field is considered,the theory can also be used to study the fluctuation of SPCLS output signal.It finds out that in the SPCLS output signal,there exists an excited state fluctuation due to the interaction between the electron beam and the Surface Plasmon Polaritons.Based on this fluctuation,it is possible to enhance the output of SPCLS utilizing stochastic resonance.The relativistic theory of electron beam – photon interaction: Based on the electromagnetic field quantization,the dissertation further will introduce the quantized Dirac spinor field to describe the electron beam,and then study the evolution of SPCLS through the fully quantized coupling Hamiltonian.This theory covers many physical effects that could not be studied in the photon excitation theory of classical electron beam,including the relativistic effect of electron beam,the position-momentum uncertainty,and the spin.The theory finds out that when the electron beam velocity approaches the Cherenkov threshold,the output will be affected by the uncertainty principle,and will be significantly reduced,while still maintaining coherence and tunability.These results not only solve the continuity paradox faced by the SPCLS signal when the electron beam velocity crosses the Cherenkov threshold,but also provide an experimental scheme to measure the quantum property of the Cherenkov Radation.When the theoretical study has reached the fourth level,the physical process in SPCLS can be described in a relatively complete way,revealing hidden information that cannot be explained by the classical theories.The above four levels of theory,as a whole,can not only deepen people's understanding of SPCLS from different aspects,but can also study the systems based on SPCLS in different applications scenarios.Viewing from a broader perspective,the theory of SPCLS is the theory that a parallel moving electron beam excites a system that converts electron energy into electromagnetic energy through local response.This type of physical phenomena is ubiquitous in many electronic and photonic devices,and the theory of SPCLS is a powerful tool for studying these physical phenomena.For example,the theory of SPCLS can be used to study Asymmetrical SPCLS,Reversed Cherenkov Source Based on Topological Insulator Hyperbolic Metamaterial,Electron Beam Excited Spintronic Terahertz Emitter,and X-ray Source Based on Graphene Quantum Cherenkov Effect,predicting the field distribution and output correctly,clarifying the physical process.In summary,the theoretical study of SPCLS actually covers a series of novel radiation sources that can be applied in different situations and work at different frequencies.