Enhancement of coherent Smith-Purcell radiation at terahertz frequency by optimized grating, prebunched beams, and open cavity

Abstract

This paper studies various mechanisms to enhance the coherent Smith-Purcell radiation (SPR) at terahertz frequencies using particle-in-cell (PIC) simulations. A simple analytical dispersion relation is derived to predict the frequency of the evanescent surface wave of the open grating, which is excited by a backward-wave oscillation mechanism and emits radiation at harmonics of this frequency, and the cavity mode frequencies of the Orotron-like device, all in excellent agreement with the simulation data. The grating parameters are optimized to minimize the starting current of a continuous electron beam for exciting coherent SPR. It is demonstrated that a prebunched beam can be used to selectively excite coherent SPR at desired SPR angle and frequencies that are higher harmonics of the beam bunching frequency, where the starting current requirement is eliminated. Our PIC simulations show the dynamics of modes development and competition for the Orotron-like open cavity structure, which owns a lower starting current than the surface mode process. This oscillation mode dominates at steady state and enhances the radiated power by more than 2 orders of magnitude.

Spectrally and Spatially Resolved Smith-Purcell Radiation in Plasmonic Crystals with Short-Range Disorder

“…The Smith-Purcell effect critically depends on precise fabrication of such a periodic structure, which is why the substantial progress in nanofabrication techniques in recent years has revealed a large variety of new applications (e.g., Refs. [14][15][16][17][18][19][20]) based on the SmithPurcell effect. Interestingly, the inverse Smith-Purcell effect has also shown exciting possibilities [21,22], which consequently led to more advanced photonic crystal structures being used as promising platforms for particle acceleration by light excitation of a periodic structure [23][24][25].…”

Absolute Instability near the Band Edge of Traveling-Wave Amplifiers

“…An absolute instability will occur if the dispersion diagram is convex at that point if the beam current exceeds a certain threshold value, (or if the beam line intersects the backward wave side of the circuit dispersion curve). These general conclusions are applicable to novel TWT's of current interest, including meta-materials TWT [34,35,36], and the THz Smith-Purcell radiation sources [37,38,39]. Recent simulations indeed reveal an absolute instability at the upper band edge of a TWT that uses a disk-on-rod slow wave structure [40].…”

Spectral Interferometry with Electron Microscopes

“…Cherenkov radiation 24 , Smith−Purcell radiation 25 26 , and transition radiation 27 , to only name a few. The Smith−Purcell effect is commonly used to design free electron lasers in the THz frequency range 28 29 . Because of the interaction of the electrons with gratings rather than with a single scatterer, coherent electromagnetic radiation is formed in the far-field by the interference of the transition radiation from each individual element in the grating in a phase-matched way.…”