TERAHERTZ GENERATION UTILIZING HOT ELECTRONS INJECTION ACROSS GRAPHENE PLANES

Abstract

Terahertz generation has been a subject of interest due to the expected impacts on biomedical and scientific applications initially. However, a compact tunable emission source in this frequency regime still remains as a challenging topic. In this report, it is explored the possibility of utilizing hot electrons injection across the graphene plane as a frequency tunable terahertz generation mechanism. Light generation from solid requires electron transition between conduction and valence band (inter-band transition). It has been known that, due to the unique electron energy dispersion of graphene, which is often described as massless Dirac fermion, the hot electrons injected along the graphene plane (parallel to graphene plane) can generate only intra-band transition, and inter-band transitions are strictly prohibited by selection rules. However, a recent theoretical development reveals that in the case of high energy electrons traveling across the graphene planes can generate inter-band transitions. Moreover, this transition energy can be precisely tuned by controlling the Fermi energy level of graphene. Our study shows that the scattering rate, which decides the efficiency of energy transfer from the hot electrons to the electrons in graphene, is a strong function of energy level and the incident angle of the injected hot electrons. We investigate the potential of this novel light generation mechanism and estimate their limits, theoretically. The efficiency among the different emitter materials will be compared to find the best emitter-graphene hybrid structures. Additionally, the tactics to improve emission efficiency will also be discussed.