Smith-Purcell Research Articles

Free electron emission in vacuum assisted by photonic time crystals

The Cerenkov radiation and Smith–Purcell (SP) effect state that free electron emission occurs exclusively in dielectrics when the velocity of the particles exceeds the speed of light in the medium or in the vicinity of periodic gratings close to each other within a vacuum. We demonstrate that free electrons in a vacuum can also emit highly directional monochromatic waves when they are in close proximity to a medium that is periodically modulated temporally, suggesting the existence of the temporal SP effect. The momentum band gaps of time-varying media, such as photonic time crystals (PTCs), create new pathways for the injection of external energy, allowing the frequency, intensity, and spatial distribution of electromagnetic fields to be controlled. Moreover, the PTC substrate enables the conversion of localized evanescent fields into amplified, highly directional propagating plane waves that are only sensitive to the velocity of particles and the modulation frequency, which allows us to observe and utilize Cerenkov-like radiation in free space. Our work presents significant opportunities for the utilization of time-varying structures in various fields, including particle identification, ultraweak signal detection, and improved radiation source design.

Smith–Purcell Radiation from Highly Mobile Carriers in 2D Quantum Materials

AbstractTerahertz (THz) radiation has broad applications ranging from medical imaging to spectroscopy. One viable source of high‐intensity THz radiation is the Smith–Purcell (SP) effect, which involves charge carriers moving over a periodic surface. Conventional SP emitters use electron beams to generate charge carriers, necessitating bulky electron acceleration stages. Here, a compact design for generating THz SP radiation using mobile charge carriers within 2D materials is proposed. This circumvents the beam alignment and beam divergence challenge, allowing for a reduction in the electron‐grating separation from tens of nm to 5 nm or less, leading to more efficient near‐field excitation and a potentially chip‐level THz source. In such a configuration, it is shown that the optimal electron velocity and the corresponding maximum radiation intensity can be predicted from the electron‐grating separation. The numerical demonstration shows that hot electrons can excite SP radiation in graphene on a silicon grating, and the radiation intensity can be increased by graphene surface plasmons. This study can be extended to a broad variety of charge carriers in 2D materials, thus allowing for compact, tunable, and low‐cost THz sources.

Tunable Smith–Purcell radiation from MoS2-based grating

Different from the traditional tunable Smith–Purcell (SP) radiation in the graphene-based gratings in the terahertz band, we propose a tunable SP radiation generated from an electron beam passing through a single-layer molybdenum disulfide (MoS2) based grating in the visible band. The comparison between the simulation and the theoretical results shows good agreement. By varying the Fermi energy of MoS2 from 0.025 eV to 0.125 eV for the MoS2-based grating, we can not only control the radiation frequency but also can change the radiation magnitude. The radiation frequency, angle, and magnitude varying with the Fermi energy are also discussed, respectively. These properties would have potential applications in developing tunable visible SP radiation.

Polarization Shaping of Free‐Electron Radiation by Gradient Bianisotropic Metasurfaces

AbstractFree‐electron radiation phenomena facilitate enticing potential to create light emission with highly tunable spectra, covering hard‐to‐reach frequencies ranging from microwave to X‐ray. Consequently, they take part in many applications such as on‐chip light sources, particle accelerators, and medical imaging. While their spectral tunability is extremely high, their polarizability is usually much harder to control. Such limitations are especially apparent in all free electron based spontaneous radiation sources, such as the Smith−Purcell (SP) radiation. Here, anomalous free‐electron radiation phenomenon is demonstrated at the microwave regime from gradient bianisotropic metasurfaces, by using a phased dipole array to mimics moving charged particles. The phase gradient and the bianisotropy in metasurfaces provide new degrees of freedom for the polarization shaping of free‐electron radiation, going beyond the common spectral and angular shaping. Remarkably, the observed anomalous free‐electron radiation obeys a generalized SP formula derived from Fermat's principle.

Linearized theory of terahertz Smith–Purcell radiation from a cylindrical grating loaded with dielectric

An analysis of Smith–Purcell (SP) radiation is presented for a cylindrical grating filled with dielectrics and an annular electron beam propagating along the grating axis. The dispersion relations are obtained for the azimuthally symmetric modes when the transverse perturbation of the electron beam is included, and it is compared with the case when the beam motion is restricted in the z-direction. It is shown that the growth rate changes by the transverse perturbation. The effects of the grating parameter and dielectric constant on the growth rate up to second-order are found. The best parameters of the grating to maximize the growth rate of these modes are also obtained.

Proposal for elliptical and circular groove gratings for Smith–Purcell terahertz radiation

The goal of our paper is to show the design of a Smith–Purcell (SP) terahertz (THz) emitter with two circular and elliptical groove grating structures and its simulations. The SP source is a vacuum-state electronic device that is suitable for generating THz radiation. As a prerequisite for the main goal of the paper, a prebunched electron gun with Gaussian type has been used. Based on the design and simulations, the emitter can radiate THz waves with the frequency of <italic>f</italic> = 0.8 THz. Radiation frequency changes by filling grooves of gratings with different glasses. Also, the radiation frequency of emitted waves can be tuned by the types of groove structures and the energy of electrons emitted from electron gun. For simulating the emitter, 3-D particle-in-cell codes and eigenmode solver have been utilized.

Enhancement-limit of Smith–Purcell radiation with self-bunched keV-order electron beam

We studied enhanced Smith–Purcell (SP) radiation produced by a self-bunched keV-order electron beam based on a W-band surface-wave oscillator. The self-bunched beam is generated in the process of the beam interaction with the surface-wave mode. The bunched beam contains the second harmonic of the surface wave that enhances the SP radiation. The observed radiation shows that the enhancement is limited when the interaction transitions from the Compton regime to the Raman regime.

Novel radiation sources using relativistic electrons - Applications

Novel sources of radiation using relativistic electrons in periodic structures are being briefly discussed based on different mechanisms and producing radiation of very different characteristic.In this article we will focus among others on the Smith-Purcell effect and the many efforts made since their discovery 55 years ago. Especially Smith-Purcell (SP) free- electron lasers (FELs) using low energy electron beam are being seen as attractive option for a compact source of coherent terahertz radiation. The latter refers to the production of radiation in the THz gap region associated with many applications including very promi- sing and interesting results on imaging techniques in Medicine as well as applications in Biology, Molecular Physics etc. We will present calculations of Radiation Factors based on the Smith-Purcell radiation production with very interesting characteristics and discuss the possibility of calculating and designing a complete system of coherent THz radiation production based on a Smith-Purcell FEL.

Half-cylindrical grating structure for enhancing radiation power of Smith–Purcell emitter

We propose a new structure of Smith–Purcell (SP) emitter for increasing its power radiation. Injection of an electron beam passing over the metallic surface causes excitation of the surface mode for metal grating and subsequently electron acceleration and electromagnetic radiation. This radiation can be achieved in the terahertz spectrum applying suitable parameters in the structure. The grating is in half-cylindrical form, which helps to increase the interaction of a particle with the surface, and because the electron beam moves within the grating, intensity of radiation power, and electric field dramatically accumulate. The proposed geometry utilizes a single instead of the 10-point electron source with smaller dimensions and higher emitted power values compared to the traditional SP emitters. Furthermore, it comes with about 2- and 32-time increase, respectively, in radiation power and electric field. For our simulations, 3-D particle in cell and eigenmode solvers have been used.

Two-Color Terahertz Radiation Emission in Quasi-Periodic Smith–Purcell Structures

We study the Smith–Purcell (SP) radiation emission generated in terahertz (THz) frequency range by an electron beam passing close to the surface of a 3-D quasi-periodic array of metallic slots. When more than one electron beam is used in the SP-based periodic structures, the two or more colors can be generated. In this paper, two-color SP emission of radiation with variable frequency separations has been examined in a proposed quasi-periodic Fibonacci-class structure through 3-D-particle-in-cell (PIC) simulations. In spite of using an electron beam instead of implementing a double-electron beam streaming method, in this paper, a new method for successful two-color SP radiation generation in the THz frequency domain is reported in both quasi-spontaneous and quasi-super-radiant regimes. Finally, we show that quasi-superradiance simulations demonstrate the possibility of frequency separation controlling through repetition frequency of applied periodic bunches.

Some Advances in Theory and Experiment of High-Frequency Vacuum Electron Devices in China

The advances of high-frequency vacuum electron devices (VEDs) in China are reviewed in this paper, including traveling-wave tubes (TWTs) based on novel slow-wave structures (SWSs), high harmonic TWT amplifier, gyrotrons, Smith-Purcell (SP) radiation sources, surface plasmon polaritons (SPPs) sources, and terahertz (THz) radiation sources in ion-focused beam-plasma system. As one of the most widely used VEDs, TWTs based on folded type SWSs, planar SWSs, and grating type SWSs are studied theoretically and experimentally from Ka-band to R-band. Using folded type SWSs, two TWT amplifiers are developed. As high-power millimeter wave and THz wave sources, gyrotron TWTs, gyrotron oscillators, and gyroklystrons are presented. Moreover, this paper shows SP radiation sources, SPPs sources, and THz radiation sources in ion-focused beam-plasma system open good prospects for developing high-frequency VEDs with new mechanisms, which undoubtedly brings about innovative changes for the development of VEDs.

Smith–Purcell Radiation from Low-Energy Electrons

Recent advances in the fabrication of nanostructures and nanoscale features in metasurfaces offer new prospects for generating visible light emission from low-energy electrons. Here we present the experimental observation of visible light emission from low-energy free electrons interacting with nanoscale periodic surfaces through the Smith–Purcell (SP) effect. We demonstrate SP light emission from nanoscale gratings with periodicity as small as 50 nm, enabling the observation of tunable visible radiation from low-energy electrons (1.5 to 6 keV), an order of magnitude lower in energy than previously reported. We study the emission wavelength and intensity dependence on the grating pitch and electron energy, showing agreement between experiment and theory. Our results open the way to the production of SP-based nanophotonics integrated devices. Built inside electron microscopes, SP sources could enable the development of novel electron–optical correlated spectroscopic techniques and facilitate the observatio...

Enhancement of Smith-Purcell radiation with temporal harmonics of surface wave on cylindrical corrugated waveguide

We experimentally studied an intense Smith-Purcell (SP) radiation based on a cylindrical corrugated waveguide driven by a weakly relativistic electron beam. The corrugated waveguide has a rectangular periodic structure, and an annular electron beam with energy less than 100 keV and current of the order of 100 A is injected. We observe both intense surface-wave radiation at 25 GHz and intense SP radiations up to 150 GHz. The observed SP radiations are enhanced by the sixth and fourth temporal harmonics of the surface wave. The characteristics of the enhanced SP radiation depend on the periodic number of the structure. In the experiment with the use of the 60-period structure, the observed power of the SP radiation at the fourth harmonic is higher than that of the radiation at the sixth harmonic. In the case of the 30 period-structure, the power level of the radiation at the sixth harmonic is comparable to that at the fourth harmonic.

Coherent radiation at the fundamental frequency by a Smith-Purcell free-electron laser with dielectric substrate

This letter reports an approach to realize coherent radiation at the fundamental frequency of the wave that is resonant with the electron beam in a Smith-Purcell (SP) free-electron laser (FEL). We found that a two-dimensional grating structure equipped with a dielectric substrate could provide an electromagnetic mode which shows the property of a surface wave in the vacuum region where the beam-wave interaction occurs, and operates as a radiative wave in the dielectric region. Thus, an electron beam holding medium energy resonant with the mode could radiate at the fundamental frequency, which is different from the typical Smith-Purcell free-electron laser that radiates only at harmonics of the resonant frequency.

Start current of dielectric-loaded grating in Smith-Purcell radiation

In this paper, a three-dimensional dielectric loaded grating (DLG) is proposed for the Smith-Purcell (SP) device. Taking into the considerations of thickness and width of electron beam, the dispersion equation is derived by using field matches method. The complex frequency is obtained by the numerical solution of dispersion equation, in which the imaginary part represents linear growth rate. The impacts of the electron beam filling factor (EBFF) on growth rate are discussed under the condition that the beam current and beam current density are kept as constants, respectively. In addition, the start current for SP oscillator is obtained by using the dispersion relation combined with boundary conditions. The relationship between the start current and other parameters is discussed and compared with the conventional metal grating. The results show that with the increasing of EBFF, the peak growth rate increases rapidly firstly and then decreases slowly, in which the current and current density are kept as constants, respectively. For the SP oscillator, the start current is increased with the shifting up beam voltage, but it is decreased with the improved EBFF, and only it has a slightly increasing trend when EBFF is close to 1. In addition, the start current is decreased with the increasing of relative dielectric constant, which indicates that by introducing DLG, the start current can be effectively reduced. Theoretical results are in good agreement with that of the simulations.

Smith–Purcell radiation in the terahertz regime using charged particle beams from laser–matter interactions

AbstractWe report on the experimental observation of Smith–Purcell (SP) radiation generation by charged particle beam from laser–matter interactions. High-power laser pulses were focused onto a thin metal foil target to generate proton beams with energies up to 1.7 MeV via the target normal sheath acceleration (TNSA) process. The particle beam from the TNSA process was sent close to a periodic structure to generate SP radiation. Sub-μJ terahertz pulses were recorded using a pyroelectric detector. Simultaneous measurement of the ion spectra allowed us to estimate the power of the emitted radiation and compare it with the experimental results. The distance between the grating and the particle beam was varied and its effect on the emitted radiation was studied.

3-D Numerical Analysis of Smith–Purcell-Based Terahertz Wave Radiation Excited by Effective Surface Plasmon

At lower frequency regime, altering the surface structure modifies the characteristics of propagating effective surface plasmons (ESPs), particularly their interaction with light. Movement of an electron beam in the proximity of the surface of a structured metallic grating excite these ESPs, which due to existence of a velocity phase match, they are able to emit terahertz (THz) waves known as Smith–Purcell (SP) radiation. By introducing a femtosecond perturbation (a single electron bunch), the important characteristics of a desired grating are revealed. Through our 3-D investigations, we find the incoherent THz radiation frequency span, and moreover we confirm that the frequency of the ESP is always less than the minimum SP frequency. Additionally, we find that the maximum of signal amplitude is distributed around 90°, only if the grating width is comparable to the longest SP wavelength. Also we learned that by increasing the grating length, the magnitude and spectral resolution of the radiation increases too. In order to study the coherent (superradiant) radiation, we use a train of electron bunches with variable bunch-to-bunch distances, and we calculate the radiation angle of the coherent SP signal. Simulations on the generation of SP radiation at THz frequencies are performed with the help of the 3-D particle-in-cell (PIC) finite integral (FI) method, in which the results agree very well with previously reported 2-D simulations.

Smith–Purcell radiation amplifier

In the framework of the earlier derived dispersion equation, we study induced Smith–Purcell (SP) radiation of relativistic electron beam in the absence of a resonator. We offer a new method for calculation of coefficients for partial amplitudes in the case of rectangular grating. Using this method, as an important example of applications of our earlier derived results (Klochkov D N, Artemyev A I, Oganesyan K B, Rostovtsev Y V and Hu C K 2010 J. Mod. Opt. 57 2060–8), we calculate the growth rate of the SPFEL with a rectangular grating. The growth rate dependences on harmonic number, observation angle and relativistic factor are presented.

Generation of induced Smith-Purcell radiation: free-electron laser in open system

We have used the framework of the dispersion equation to study coherent Smith–Purcell (SP) radiation induced by a relativistic magnetized electron beam in the absence of a resonator. As an important example of the application of the obtained results of our previous paper JMO v.57, 2060, (2010) the growth rate of SP FEL in the case with a rectangular grating was calculated. The growth rate of the instability is proportional to the square root of the electron beam current. The calculated results are consistent with the experimental data obtained by Urata et al. [Phys. Rev. Lett. 80, 516 (1998)].

Enhance the terahertz Smith-Purcell superradiant radiation by using dielectric loaded grating

A dielectric loaded grating (DLG) for terahertz Smith-Purcell (SP) device is proposed to enhance the radiation intensity. By using the theoretical analysis and particle-in-cell simulations, the dispersion characteristics and SP superradiant radiation are investigated. Compared with the general metal grating, the usage of DLG can improve the magnitude of electric field and, consequently, strengthen the interaction of the evanescent wave with electron beam, which can improve the growth rate, enhance the SP superradiant radiation, and lower the start current for the operation of SP free-electron laser.