Pump-induced stimulated superradiant Smith-Purcell radiation with ultra-narrow linewidth

Abstract Lasers with the gain medium of gas, liquid, semiconductor, and solid could generate coherent light with rather narrow spectral linewidth, which play an important role in the fields of communication, measurement, sensing, and so on. Although free electron lasers have been realized with their unique advantages, they face challenges in narrowing the spectral linewidth, owing to electron energy fluctuation, Coulomb effect, and other mechanism. Here we demonstrate the superradiant Smith-Purcell radiation (S-SPR) in terahertz frequency band with ultra-narrow and continuously tunable linewidth in a compact device. By proposing a new effect of pump-induced stimulated S-SPR (PIS-SPR), the spectral linewidth could be reduced to 0.3 kHz @ 291.7 GHz, which is about two–six orders of magnitude narrower compared with those obtained by accelerators and other electron devices. Meanwhile, the wide range of continuously tunable spectral linewidth spanning 0.3–900 kHz is observed for the first time. This work provides a way to greatly narrow the spectral linewidth of free electron radiation and to achieve high-order harmonic of S-SPR in a compact device, and offers a platform to study the interaction between free electron bunches and different micro-&nano-structures.