Direction controllable inverse transition radiation from the spatial dispersion in a graphene-dielectric stack

Transition radiation(TR) induced by electron–matter interaction usually demands vast accelerating voltages, and the radiation angle cannot be controlled. Here we present a mechanism of direction controllable inverse transition radiation(DCITR) in a graphene-dielectric stack excited by low-velocity electrons. The revealed mechanism shows that the induced hyperbolic-like spatial dispersion and the superposition of the individual bulk graphene plasmons(GPs) modes make the fields, which are supposed to be confined on the surface, radiate in the stack along a special radiation angle normal to the Poynting vector. By adjusting the chemical potential of the graphene sheets, the radiation angle can be controlled. And owing to the excitation of bulk GPs, only hundreds of volts for the accelerating voltage are required and the field intensity is dramatically enhanced compared with that of the normal TR. Furthermore, the presented mechanism can also be applied to the hyperbolic stack based on semiconductors in the infrared region as well as noble metals in the visible and ultraviolet region.Accordingly, the presented mechanism of DCITR is of great significance in particle detection, radiation emission,and so on.

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.

A new type of wave in waveguide filled with magnetized plasma

A new type of wave containing quasi-static part is found and studied. Characteristics of this wave are analyzed, and the theoretical analysis is consistent with the numerical calculation.

PIC simulation study of electron gun with rotational surface cathode

The electron gun with rotational surface cathode is presented to improve the laminarity of the converging electron beams in this paper,and the function expression of the rotational surfaces is given.The results of particle in cell(PIC)simulation indicate that anode-hole spherical aberration is the major cause for the nonlaminarity of the electron beams.By properly choosing the size of the shape,rotational surface cathode can effectively counteract the effect of the anode-hole spherical aberration and enhance the laminarity of the electron beams.The theoretical analysis was carried out for the explanation of the phenomenon that appeared in the PIC simulation.

Study on a 60 kV/5 A magnetron injection gun for 200 GHz electron cyclotron master

To develop 200 GHz electron cyclotron maser(ECM)in University of Electronic Science and Technology of China,a magnetron injection gun(MIG)with 60 kV/5 A has been designed.The behavior of the designed magnetron injection gun with variations of relevant parameters is given in detail by means of computer simulations.The preliminary and operation parameters of the gun are reported.The optimization by using the EGUN code shows that the ratio of the perpendicular velocity to the parallel velocity of the electron beam is 1.63,and the perpendicular and parallel velocities spread are 2.62%and 6.63%,respectively.