Spectrum discretization of Smith-Purcell radiation for efficient multicolor terahertz emission

The Smith-Purcell radiation (SPR) is a promising way for developing compact and high power terahertz sources with broad application prospects. Yet, the conventional SPR is largely restricted by its low efficiency and incoherency. Here we illustrate an enhanced coherent SPR from a compound array of deep-narrow-rectangular-grooves (DNRGs). We show, by detailed theoretical analysis and simulations, that each DNRG is an effective resonant cavity with low radiating Q-factor and a cluster of closely adjacent DNRGs can get enhanced coherent emission. More notably, by setting clusters of DNRGs with gradient shapes to be periodically arranged in an array, we can change the conventional incoherent SPR to be a coherent emission with a series of specified frequencies at specified directions, namely, the radiation is discretized. The total radiation efficiency can be increased by more than an order of magnitude. Thus, it affords an efficient way for high power multicolor terahertz generations.