Cherenkov terahertz radiation from Dirac semimetals surface plasmon polaritons excited by an electron beam**Project supported by the National Key Research and Development Program of China (Grant No. 2017YFA0701000), the National Key Scientific Instrument and Equipment Development of China (Grant No. 2018YFF01013001), and the National Natural Science Foundation of China (Grant Nos. 61701084 and 61505022).

Abstract

We demonstrate a physical mechanism for terahertz (THz) generation from surface plasmon polaritons (SPPs). In a structure with a bulk Dirac semimetals (BDSs) film deposited on a dielectric substrate, the energy of the asymmetric SPP mode can be significantly enhanced to cross the light line of the substrate due to the SPP-coupling between the interfaces of the film. Therefore, the SPPs can be immediately transformed into Cherenkov radiation without removing the wavevector mismatch. Additionally, the symmetric SPP mode can also be dramatically lifted to cross the substrate light line when a buffer layer with low permittivity relative to the substrate is introduced. In this case, dual-frequency THz radiation from the two SPP modes can be generated simultaneously. The radiation intensity is significantly enhanced by over two orders due to the field enhancement of the SPPs. The radiation frequency can be tuned in the THz frequency regime by adjusting the beam energy and the chemical potential of the BDSs. Our results could find potential applications in developing room temperature, tunable, coherent, and intense THz radiation sources to cover the entire THz band.