Smith–Purcell radiation from highly mobile carriers in 2D quantum materials

Abstract

Terahertz (THz) radiation has broad applications ranging from medical imaging to spectroscopy due to its useful properties, such as strong absorption by organic materials. One viable source of high-intensity THz radiation is the Smith-Purcell (SP) effect, which 1 involves charge carriers moving over a periodic surface. Conventional SP emitters use electron beams to generate charge carriers, necessitating bulky electron acceleration stages. Here, we propose a compact design for generating THz SP radiation using mobile charge carriers within 2D materials. This circumvents the beam alignment and beam divergence challenge, allowing for a reduction in the electron-grating separation from tens of nm to 5 nm or less, leading to more efficient near-field excitation and a potentially chip-level THz source. In such a configuration, we show that the optimal electron velocity and the corresponding maximum radiation intensity can be predicted from the electron-grating separation. For graphene on a silicon grating, we numerically demonstrate how SP radiation is excited by hot electrons, including how the radiation intensity can be enhanced by graphene surface plasmons and modified by tuning the Fermi level in the graphene sheet. Due to the high carrier concentration in graphene, the radiation intensity may be further enhanced through coherent interference. This study can be extended to a broad variety of charge carriers in 2D materials, including electrons, holes, and trions, mobilized through various means such as photoexcitation and external electric fields. Utilizing intrinsic mobile carriers in 2D materials may thus allow for compact, tunable, and low-cost THz sources.