Electron-beam-driven anomalous Doppler effects in Smith–Purcell radiation

The interaction between electrons and matter is an effective means of light emission,through mechanisms including Cherenkov radiation and Smith–Purcell radiation(SPR).In this study,we show that the superlight inverse Doppler effects can be realized in reverse Smith–Purcell radiation excited by a free electron beam with a homogeneous substrate.In particular,we find that two types of anomalous SPR exist in the homogenous substrate:special SPR and reverse SPR.Our results reveal that the electron velocity can be tuned to simultaneously excite different combinations of normal SPR,special SPR,and reverse SPR.The proposed manifold light radiation mechanism can offer greater versatility in controlling and shaping SPR.

Tunable optical topological transition of Cherenkov radiation

Approaches to generate and manipulate Cherenkov radiation(CR)are challenging yet meaningful.Optical topological transition(OTT)in novel materials and metamaterials is also promising for modern photonics.We study the OTT of CR in graphene-based hyperbolic metamaterials(GHMs)for the first time.In GHMs,conventional and hyperbolic CR can be switched when crossing the topological transition frequency.This frequency can be altered by metamaterial components and external optical elements.For instance,external ultrafast optical pumps cause an ultrafast OTT from the elliptical to the hyperbolic state.Then,hyperbolic CR can be excited by lowenergy electrons by leveraging the excellent photothermal properties of graphene.Hyperbolic CR vanishes when the GHM returns to its original state.Furthermore,graphene nonlocality occurs when the electron velocity is low enough,corresponding to a large wave vector.Concretely,when the electron velocity approaches the Fermi velocity of graphene,a nonlocality-induced OTT modifies the plasmonic properties of the GHM and brings a new lower velocity threshold of hyperbolic CR.Therefore,hyperbolic CR can only be induced in a limited velocity range.These findings pave the way for understanding CR properties in active plasmonic metamaterials and may be applied to complex photonic and polaritonic systems.