Study of tunable and enhanced transition radiation from graphene in terahertz band

Abstract

Terahertz (THz) radiation has many special properties, such as low quantum energy, high signal-to-noise ratio and wide frequency, so that it possesses important application prospects in various fields. With the development of THz technology and its application fields, it is an urgent need to develop all types of terahertz source. It is as well-known that the radiation generated by traditional electronics methods has high power with low frequency, while by traditional photonics method has relatively high frequency with low power. Fortunately, the method of generating radiation based on the combination of photonics and electronics can overcome the above disadvantages, so it can solve the problem encountered by the terahertz source. In this paper, the integrated method by using electron beam to excite surface plasmon waves and generate transition radiation is adopted to obtain terahertz waves. Although the output power by this method is not remarkably high, the method is flexible and easy to implement. Due to the merit of graphene plasmon in THz band, a physical model of a moving electron incidence on a monolayer graphene at arbitrary angle is established in this paper to generate transition radiation in THz band. The field of the transition radiation is fully deduced by Maxwell’s equations combined with the special boundary conditions due to graphene’s conductivity. Then the expression of spectral-angular distribution is also derived in order to show the radiation energy characteristic more intensively in three-dimensional space. The characteristic of the radiation is numerically calculated by MATLAB. We focus on the transition radiation generated by a charged particle with a moderate energy and explain some contributions to the radiation, such as the incident angle, the conductivity or the Fermi level in graphene and the substrate dielectric characteristic. The pattern and intensity of the radiation field are numerically analyzed in the upper and lower half-spaces. We plot the figures of field distribution and the spectral angular distribution related to these variables mentioned above to analyze their influences on the radiation. In the case of the normal incidence, the transition radiation exhibits strong symmetry and considerably more intensive than that at grazing and oblique incidence. The results of the spectral angular distribution display that the radiation intensity increases strongly with the incident angle, and that the radiation energy focuses in backward and forward directions in the case of normal incidence. This study summarizes that the parameter adjustment mechanism is favorable for the performance of terahertz source. Since the conductivity of graphene can be adjusted by applying a bias voltage and doping, the radiation frequency and intensity can be tuned by the conductivity of graphene. Meanwhile, the substrate, electron energy and incident angle are conveniently controlled, therefore a more flexible way is obtained to develop tunable terahertz radiation sources.