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In this article, a method to enable efficient emission of coherent radiation by using an intense electron beam coupled with a quasi-bound state in the continuum (quasi-BIC) is investigated. We present an analytical solution providing an intuitive round-trip phase condition to explain the origin of quasi-BICs of dielectric gratings. Numerical study of the beam–wave interaction shows that the electrons can be bunched by the synchronous space harmonic enhanced by the quasi-BIC, resulting in self-excited coherent oscillation and consequently efficient Smith–Purcell radiation. This work presents an interesting solution for coherent radiation sources, and may find application in communications and physics. [ABSTRACT FROM AUTHOR]

In this article, a method to enable efficient emission of coherent radiation by using an intense electron beam coupled with a quasi-bound state in the continuum (quasi-BIC) is investigated. We present an analytical solution providing an intuitive round-trip phase condition to explain the origin of quasi-BICs of dielectric gratings. Numerical study of the beam–wave interaction shows that the electrons can be bunched by the synchronous space harmonic enhanced by the quasi-BIC, resulting in self-excited coherent oscillation and consequently efficient Smith–Purcell radiation. This work presents an interesting solution for coherent radiation sources, and may find application in communications and physics. [ABSTRACT FROM AUTHOR]

A dielectric loaded rectangular grating for Smith-Purcell devices is proposed in this paper. Regarding the electron beam as a moving plasma dielectric, a three dimensional (3D) linear theory of beam-wave interaction is developed. The first and second order growth rates are calculated, which are obtained by expanding hot dispersion equation at synchronous point. The results show that the cutoff frequency is affected by grating width. The dispersion curve becomes flatter and shifts towards lower frequency by loading dielectric in grooves. The simulation results, which are obtained by a 3D particle-in-cell code, are in good agreement with theoretical calculations. Compared the first and second order growth rate, it shows that the discrepancy is large when beam parameters are selected with high values. In this case, it is necessary to apply the second order growth rate, which can accurately describe the process of beam-wave interaction. [ABSTRACT FROM AUTHOR]

Abstract We investigate parametric X-ray radiation (PXR) under condition of the extremely asymmetric diffraction, when the ultra-relativistic electron bunch is moving in vacuum parallel to the crystal-vacuum interface, close to the crystal surface. This type of geometry coincides with the well known mechanism of generation of radiation, when the self-field of the particle beam interacts with the reflecting metal grating, namely the Smith-Purcell effect. We demonstrate that in this geometry the main contribution is given via a tail region of the beam distribution, which penetrates the crystal and X-rays are radiated along the normal to the crystal surface. We determine the electron beam characteristics, when this phenomenon can be observed. It is essential that in this geometry the majority of electrons does not undergo multiple scattering and consequently the characteristics of the particle beam are not changed, thus allowing the usage of the emitted X-rays for the purpose of non-destructive beam diagnostics, which can complement the traditional knife-edge method. [ABSTRACT FROM AUTHOR]

To meet the requirements of high power and broad-tunable terahertz sources, we proposed and investigated a modified Smith-Purcell free-electron laser (SP-FEL) driven by periodic electron-bunches (PEBs). We first designed an electron-gun with a photocathode, which generates a train of PEBs with tunable macro-bunching frequency. These PEBs then interact with surface waves on a grating and are further micro-bunched. The macro-bunching frequency together with its harmonics is superimposed on the micro-bunching one such that the multi-color superradiant Smith-Purcell radiation is achieved at the superimposed harmonics. By adjusting the macro-bunching frequency, the superradiant frequency can be tuned from 0.5 to 1 THz—a region hard to reach by conventional SP-FELs and vacuum electron devices. Compared to DC-beams, the PEBs have higher peak current density and generate higher peak power. Thus, the proposed scheme can be promisingly developed as high power and broad-tunable terahertz sources. [ABSTRACT FROM AUTHOR]

We report the observation and studies of selective excitation and control of terahertz (THz) coherent Smith-Purcell radiation (cSPr) from a train of sub-picosecond-period electron micro-bunches. The coherence of the radiation from such a train has been demonstrated. The spectrum of cSPr was measured and the selective excitation of the first and the second harmonics was observed, respectively. We also demonstrate experimentally that radiation pulses generated by each electron micro-bunch interfere coherently with the maximum intensity of cSPr observed at the frequency equal to the frequency of the micro-bunch spacing. The experimental results greatly contribute to the understanding of coherent Smith-Purcell radiation from the train of electron micro-bunches, as well as the development of THz diagnostics for accelerators. [ABSTRACT FROM AUTHOR]

Coherent Smith-Purcell radiation from multiple electron beam excitation in a three-dimensional open holes array structure has been proposed. The electron beam passes through the holes, which can be coupled with each other to enhance surface wave fields, so that the multiple electron beam injection can interact with the surface wave field efficiently. And the Smith-Purcell radiation will be enhanced due to an increasement in dipole oscillations occurring when the electrons pass through the holes. Therefore, a high beam-wave interaction efficiency is yielded and very strong coherent terahertz radiation can be generated. This multi-beam system is expected to provide a favorable way to excite the coherent Smith-Purcell radiation, which would be used to generate terahertz radiation. [ABSTRACT FROM AUTHOR]

We proposed and investigated, by theoretical analyses and simulations, a multi-color and multi-directional radiation by using sheet electron-beams to drive two-dimensional (2D) sub-wavelength hole arrays (SHAs). Each sub-wavelength hole is a resonant and radiative unit, which is successively excited by the electron-beam, such that the 2D SHA becomes a 2D array of radiating units and generates coherent Smith-Purcell radiation in specific directions where the radiation from all the units constructively interferes. The multi-color radiation is obtained when several resonant modes are excited. Each resonant mode could simultaneously radiate at several directions due to the multiple periodicity of the 2D SHA, and the radiation could be steered to any directions in the three dimensional space by adjusting the arrangement of 2D SHA and the electron energy. This radiation could promisingly be developed as radiation sources and devices with broad applications. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Extended interaction oscillators (EIOs) are high-frequency vacuum-electronic sources, capable to generate millimeter-wave to terahertz (THz) radiations. They are considered to be potential sources of high-power submillimeter wavelengths. Different slow-wave structures and beam geometries are used for EIOs. This paper presents a quantitative figure of merit, the critical unloaded oscillating frequency (fcr) for any specific geometry of EIO. This figure is calculated and tested for 2π standing-wave modes (a common mode for EIOs) of two different slow-wave structures (SWSs), one double-ridge SWS driven by a sheet electron beam and one ring-loaded waveguide driven by a cylindrical beam. The calculated fcrs are compared with particle-in-cell (PIC) results, showing an acceptable agreement. The derived fcr is calculated three to four orders of magnitude faster than the PIC solver. Generality of the method, its clear physical interpretation and computational rapidity, makes it a convenient approach to evaluate the high-frequency behavior of any specified EIO geometry This allows to investigate the changes in geometry to attain higher frequencies at THz spectrum. [ABSTRACT FROM AUTHOR]

We report on a novel type of spaser which is based on the nonlinear optical Raman gain rather than stimulated emission. It consists of a plasmonic nanoantenna as the cavity and a highly Raman-active diamond nanoparticle as the gain medium. Ultrasmall mode volume accompanied by strong field enhancement and high Raman gain enable Raman spaser-based nanolasing. By using the model describing the oscillation of Raman mode in micro/nanocavity and the calculations for plasmonic nanocavity characteristics, we theoretically study the dependency of spasing threshold on the structural parameters. We believe that our work will trigger the extensive studies of nonlinear optical spasers. [ABSTRACT FROM AUTHOR]

Extended interaction oscillators (EIOs) are millimeter-wave to terahertz vacuum electronic sources, based on the Smith-Purcell (Cerenkov) interaction between an electron beam of low to moderate voltage and a resonant slow-wave structure. They have been analyzed using the power-balance and Particle in cell (PIC) methods. In this paper, based on power-balance method, a load-line concept is introduced to investigate the EIOs. By this approach, the analysis of EIOs is much faster than the PIC method, and a wider quantitative and qualitative insight into the interaction process is obtained compared with the common power-balance method. To achieve the optimum RF power from an EIO, a region on the defined “beam-line” curve is assigned. A “frequency-shift line” curve is also derived to obtain the electronic tuning of the oscillator. In addition, the correct steady state in the multivalued solution conditions can be selected, which is not evident in the power-balance approach. A 94-GHz sheet-beam EIO is analyzed and optimized in this way, and the results are compared with those of a PIC solver. [ABSTRACT FROM PUBLISHER]

The radiation resulting from the uniform motion of a charged particle by (or through) metal sphere is considered. The simple but rigorous description of the radiation process is developed for the case of non-relativistic particle and perfectly conducting sphere by the way of the method of images known from electrostatics. The spectral-angular and spectral densities of the diffraction and transition radiation on the single sphere are computed. The Smith–Purcell radiation caused by motion of the particle parallel to the periodic string of spheres is also considered. [ABSTRACT FROM AUTHOR]

We study the radiation for a charge rotating around a diffraction grating on a cylindrical surface with metallic strips parallel to the cylinder axis. The effect of the grating on the radiation intensity is approximated by the surface currents induced on the strips by the field of the rotating charge. The expressions are derived for the vector potential and for the angular density of the radiation intensity on a given harmonic. We show that the behavior of the radiation intensity on large harmonics can be essentially different from that for a charge rotating in the vacuum. For the geometry of diffraction grating the radiation intensity on higher harmonics does not vanish for small angles with respect to the cylinder axis. For given characteristics of the charge, by the choice of the parameters of the diffraction grating, one can have highly directional radiation near the normal to the plane of the charge rotation. With decreasing energy, the relative contribution of the synchrotron radiation decreases and the Smith-Purcell part is dominant. [ABSTRACT FROM AUTHOR]

In this work we construct the theory of THz polarization radiation from electrons passing corrugated dielectric tube under non-central propagation. Fully analytical description of spectral and angular properties of radiation is obtained. In calculations we used the method of polarization currents combined with theory of perturbations assuming small distances from the central axis of the tube. Smith-Purcell and Cherenkov mechanisms of radiation are considered and analysed numerically. [ABSTRACT FROM AUTHOR]

The theory of Smith-Purcell effect for a 2D photonic crystal is constructed from the first principles proceeding from Maxwell’s equations and microscopic characteristics of particles the crystal consists of. Two-dimensionality 2D is thought in two ways: i) the photonic crystal is an arranged system of particles disposed in a monolayer, ii) the periodicity is in two different directions. We derive the expression for the spectral and angular distribution of arising Smith-Purcell radiation. We analyse the features distinctive for the 2D photonic crystal and show that its spatial distribution differs drastically from the distribution of the radiation from conventional diffraction gratings. [ABSTRACT FROM AUTHOR]

In a nonstationary (self-modulation) regime, terahertz quasi-Cherenkov generators can operate at MHz–GHz repetition rates and megawatt power levels. Shot noise, intrinsic to electron beams, is the cause of statistical fluctuations of radiation in the generators. The shot-noise related spread of instability growth time imposes appreciable limitations on the possibility of coherent summation of electromagnetic oscillations from several quasi-Cherenkov generators operating in a nonstationary mode. [ABSTRACT FROM AUTHOR]

We investigate the spectral-angular distribution for the radiation emitted by a point charge moving along a helical trajectory inside a cylindrical grating with conducting strips. Two types of the radiation processes are realized: undulator and Smith–Purcell radiations. Their relative contributions to the total radiation intensity are discussed in various asymptotic regions of the parameters describing the diffraction grating and for large harmonics. The region of the parameters is specified for which the interference effects between the undulator and Smith–Purcell radiations are essential. [ABSTRACT FROM AUTHOR]

We investigate the possibility to enhance the THz and X-ray sources based on interaction between an electron beam and a grating using a ribbon beam. Making the beam ribbon, we increase the area of effective interaction between the beam and the grating, which can lead to increasing the number of photons radiated through Smith-Purcell mechanism. The theory of this process is constructed; the fully analytical description is given; the numerical analysis shows that the radiation can be enhanced considerably with use of ribbon beams instead of cylindrical one in case when transversal part of coherent form-factor plays a role. [ABSTRACT FROM AUTHOR]

Grating transition radiation (GTR) emitted by electrons interacting with a diffraction grating was experimentally confirmed. This radiation appears when a charge intersects the grating at some inclination angle θ between the main grating plane and the charge velocity vector. If the condition θ = 0 is fulfilled the GTR tends to the conventional Smith-Purcell radiation. In this report we present the experimental investigation of spectral and orientation dependencies of a coherent GTR in sub-THz wavelength region emitted by 8 MeV electron at KEK LUCX facility. The coherent GTR spectral line shapes up to 5th diffraction order were measured for different inclination angles ( θ = 0–25 deg) with the grating period 4 mm. A pair of room-temperature Schottky barrier diode (SBD) detectors with sensitivity bands 60–90 GHz and 320–460 GHz were used. The estimation of radiation frequency dependence on the inclination angle using the dispersion relation justifies the usage of different narrow-band SBD detectors to investigate different diffraction orders of coherent GTR spectral lines. It was shown that the bandwidth of the GTR spectral lines decreases with the diffraction order. The comparison with coherent transition radiation in the same frequency range showed that the brightness of both radiation mechanisms are comparable. [ABSTRACT FROM AUTHOR]

In this report we present experimental investigations of the THz radiation generated from a corrugated and a non-corrugated capillary with reflectors, using a femtosecond electron beam of LUCX accelerator at KEK, Japan. We discuss measurements of the radiation angular distributions and their comparison with Particle In Cell simulations, and also investigate an off-central propagation of the beam in the capillaries based on experimental measurements and simulations. [ABSTRACT FROM AUTHOR]

The conical effect in Smith-Purcell radiation arising from electrons moving at non-zero angle to the direction of grating periodicity has been observed for the first time. It was found that the maximum of radiation intensity for ψ ≠ 0 shifts in both polar and azimuth angles. The experimental and theoretical results were compared, and the good agreement was shown. The experiment has been performed for 6 MeV electrons and at millimeter wavelengths. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

We proposed and investigated a novel terahertz free-electron laser, which is based on two gratings driven by two electron-beams. Two gratings are symmetrically arranged to form an open-cavity. Two electron-beams generate special Smith-Purcell radiations, respectively, from two gratings. When radiation interferes constructively, operation modes of the open-cavity are excited and then amplified by beam-wave interactions. By means of particle-in-cell simulations, we have shown that, with compact equipments and available electron-beams, this scheme can generate radiation with power and efficiency being higher than those of majority radiation sources in the vicinity region of 1 THz. It can promisingly be developed as a high-power, high-efficiency, and compact terahertz source for practice. [ABSTRACT FROM AUTHOR]

Frequency-locked coherent Smith-Purcell Radiation (SPR) is emitted when a train of pre-bunched electron beam passes close to the surface of a metallic grating, which develops an energy density spectrum restricted to harmonics of the bunching frequency. For the lamellar grating with narrow grooves, the radiation spectrum from a single electron can also have a feature of narrow band, which is related to the grating structure and the beam energy. The combination of them is proposed in this paper. By properly choosing the parameters, the peak frequency of single electron radiation can be overlapped with the harmonics of the bunching frequency of the electron bunch train, leading to the generation of extremely intense narrow-band THz radiation. [ABSTRACT FROM AUTHOR]

We propose a novel scheme of terahertz (THz) radiation generation from the beat frequency modulation of an electron beam by two co-propagating lasers and the generation of terahertz radiation by the modulated beam passing over a periodic metallic grating. The lasers cause velocity modulation of the beam by exerting a longitudinal ponderomotive force on it. In the drift space between the modulator and metallic grating, the velocity modulation translates into density and current modulation. The modulated beam, propagating over the grating of specific wave number, induces space periodic image current in the conductor that emits beat frequency Smith-Purcell radiation. With 1 μm, 4 x 1016 W/cm² lasers, beam current modulation of the order of 50% can be achieved at optimum lengths of the modulator and drift space. Employing 10 mA, 0.5MeV shortperiod electron beam, propagating at a height of 50μm above the grating of period 150 lm, one may obtain THz radiated power of the order of 6 mW at 10 THz. [ABSTRACT FROM AUTHOR]

The special Smith-Purcell radiation (S-SPR), which is from the radiating eigen modes of a grating, has remarkable higher intensity than the ordinary Smith-Purcell radiation. Yet in previous studies, the gratings were treated as perfect conductor without considering the surface plasmon polaritons (SPPs) which are of significance for the nano-scale gratings especially in the optical region. In present paper, the rigorous theoretical investigations on the S-SPR from a nano-grating with SPPs taken into consideration are carried out. The dispersion relations and radiation characteristics are obtained, and the results are verified by simulations. According to the analyses, the tunable light radiation can be achieved by the SSPR from a nano-grating, which offers a new prospect for developing the nanoscale light sources. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The recently uncovered special Smith-Purcell radiation (S-SPR) from the rectangular grating has significantly higher intensity than the ordinary Smith-Purcell radiation (SPR). Its monochromaticity and directivity are also much better. Here we explored the mechanism of the S-SPR by applying the fundamental electromagnetic theory and simulations. We have confirmed that the S-SPR is exactly from the radiating eigen modes of the grating. Its frequency and direction are well correlated with the beam velocity and structure parameters, which indicates its promising applications in tunable wave generation and beam diagnostic. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM PUBLISHER]

Over the past year, a collaborative effort between Advanced Energy Systems and Commissariat à l’Energie Atomique/Centre d’Etudes Scientifiques et Techniques d’Aquitaine has achieved the first lasing of a cylindrical Smith–Purcell free-electron laser (CSPFEL). These proof-of-principle experiments employed an 80-keV 45-A 200-ns full-width at half-maximum thin annular electron beam propagating just above a cylindrical grating. The interaction of the beam with the evanescent grating mode resulted in strong beam bunching and generation of high-power millimeter-wave radiation. First lasing of the CSPFEL was achieved with a grating period of 2 cm (5-GHz fundamental). Further experiments were performed with a 6-mm period grating (15-GHz fundamental) yielding 10 kW at the second harmonic, 30 GHz. This paper discusses only the second experiment. Comparison of the experimental results with particle-in-cell (PIC) simulations shows good agreement. An experiment at 100 GHz is in progress. The basic properties of Smith–Purcell radiation and the evanescent grating mode will be presented first. Then the experimental configuration and results will be discussed. The next section will cover the PIC simulations and comparison with the experiment. The final section will summarize the results and discuss the future directions for this research. The long-term goal is the development of compact terahertz devices with sufficient power to address applications that are impractical or impossible with the existing sources. [ABSTRACT FROM PUBLISHER]

Smith–Purcell radiation (SPR) from a lamellar grating with vacuum gaps was calculated using Computer Simulation Technology (CST) Particle In Cell (PIC) solver. The shapes of the radiation distributions were compared with those of Resonant Diffraction Radiation theory. Study of calculation domain meshing was performed. Influence of a transverse bunch size on the calculation accuracy and an SPR intensity distribution was investigated. Dependencies of the SPR yield on Lorentz factor and grating strip depth were calculated and compared with previously reported theoretical and experimental studies. [ABSTRACT FROM AUTHOR]

Simulation of radiation from a channel with variable radius in the THz region is performed using Particle In Cell (PIC) solver of the Computer Simulation Technology (CST) software package. Simulation results are compared with theoretical expressions, similarities and differences between theoretical model and computer simulation are discussed. [ABSTRACT FROM AUTHOR]

In this paper, a novel method of getting the dispersion relations in planar grating with arbitrary grooves for terahertz Smith-Purcell radiation is investigated analytically. The continuous profile of the groove is approximately replaced by a series of rectangular steps. By making use of field matches method and the continuity of transverse admittance, the universal dispersion equation for grating with arbitrarily shaped grooves is derived. By solving the dispersion equation in presence of electron beam, the growth rate is obtained directly and the dependence on beam parameters is analyzed. Comparisons of the dispersion characteristics among some special groove shapes have been made by numerical calculation. The results show that the rectangular-step approximation method provides a novel approach to obtain the universal dispersion relation for grating with arbitrary grooves for Smith-Purcell radiation. [ABSTRACT FROM AUTHOR]

Generation of THz radiation based on the Smith-Purcell effect in graphene is investigated numerically. The specific device geometry considered involves an electrically biased single-layer sheet of graphene deposited on a periodic array of holes in a solid substrate. Rigorous electrodynamic simulations combined with a basic model of charge transport are presented, showing that technologically signifcant output power levels can be obtained at geometrically tunable THz frequencies. These results suggest that graphene is a uniquely suited materials platform for the demonstration of THz electron-beam radiation mechanisms in compact solid-state systems. [ABSTRACT FROM AUTHOR]

It is shown that lasing action at subwavelength scales can be achieved in realistic plasmonic systems supporting long-range surface plasmons (LRSPPs). To this end, a general numerical framework has been developed that is able to accurately account for the full spatio-temporal lasing dynamics and the vastly different length- and time-scales featured by this class of systems. Starting from a loss compensation regime for propagating LRSPPs, it is shown how the introduction of an optical feedback mechanism induces the formation of a self-sustained laser oscillation at moderate pump intensities. The simplicity of the proposed subwavelength scale laser offers significant potential as a novel class of planar light sources in complex plasmonic circuits. [ABSTRACT FROM AUTHOR]

We introduce an optical fiber platform which can be used to interrogate proximity interactions between free-electron evanescent fields and photonic nanostructures at optical frequencies in a manner similar to that in which optical evanescent fields are sampled using nanoscale aperture probes in scanning near-field microscopy. Conically profiled optical fiber tips functionalized with nano-gratings are employed to couple electron evanescent fields to light via the Smith-Purcell effect. We demonstrate the interrogation of medium energy (30-50 keV) electron fields with a lateral resolution of a few micrometers via the generation and detection of visible/UV radiation in the 700-300 nm (free-space) wavelength range. [ABSTRACT FROM AUTHOR]

Abstract: Coherent Smith–Purcell radiation has the potential of providing information on the longitudinal profile of an electron bunch. The E-203 experiment at the FACET User Facility measures bunch profiles from the SLAC linac in the hundreds of femtoseconds range and the SPESO collaboration at Synchrotron SOLEIL is planning to make an accurate 2D map of the Coherent Smith–Purcell Radiation emission. [Copyright &y& Elsevier]

ENEA has a long term expertise in the development of powerful short-pulse mm-wave and THz radiation sources driven by free electrons. Various electron-wave interaction schemes were successfully tested in the past, including Cerenkov and Smith-Purcell radiators as well as undulator devices. Two THz-FEL sources are currently available, covering altogether the spectral range from 90 GHz to 0.7 THz. Recently a novel Electro-Magnetic pulser, capable of providing both nanosecond THz electromagnetic (EM) radiation pulses and electrostatic (ES) pulses with identical time duration in one device has been designed and is currently under development. The pulser will allow, for the first time, direct comparison of EM and ES pulses on biological systems with peak electric fields that are significantly higher than any reported to date. [ABSTRACT FROM AUTHOR]

Absorption of a photon by an electron moving parallel to a rough surface is studied. In the weak scattering regime we have evaluated the absorption probability of absorption of a single photon of energy ?. It is shown that the absorption probability with diffusional contribution becomes large by a lin/l » 1 factor compared to the analogous result with the single scattering contribution. The maximum of probability takes place for the infrared wavelengths and strongly depends on the particle energy. We also discuss the case of two-dimensional periodical surface profile and indicate optimal conditions for maximal absorption probability. The results can be used in electron energy gain spectroscopy and in laser-driven acceleration. [ABSTRACT FROM AUTHOR]

A proof-of-principle experiment demonstrating dielectric laser acceleration of nonrelativistic electrons in the vicinity of a fused-silica grating is reported. The grating structure is utilized to generate an electromagnetic surface wave that travels synchronously with and efficiently imparts momentum on 28 keV electrons. We observe a maximum acceleration gradient of 25 MeV/m. We investigate in detail the parameter dependencies and find excellent agreement with numerical simulations. With the availability of compact and efficient fiber laser technology, these findings may pave the way towards an all-optical compact particle accelerator. This work also represents the demonstration of the inverse Smith-Purcell effect in the optical regime. [ABSTRACT FROM AUTHOR]

With the aid of a three-dimensional particle-in-cell code simulation, the enhancement of Smith-Purcell radiation with a surface-plasmon mode excited by a single electron bunch and by a premodulated electron beam is considered in the paper. In the simulation, the model is a grating covered by Ag film. The results demonstrate that when the surface-plasmon mode is excited by a single electron bunch, the maximum radiation occurs at an observation angle depending on the surface-plasmon frequency, and the radiation power can be enhanced more than ten times. And for pre-bunched electron beam excitation, when one of the harmonics of the bunching frequency is resonant with that of the surface-plasmon mode, the radiation power is twenty times more than that from a perfectly conducting grating excited by the same premodulated electron beam. [ABSTRACT FROM AUTHOR]

By using ultrathin nonlinear media, the phase velocity of nonlinear polarization waves can be continuously adjusted. We realized nonlinear Cherenkov radiation (NCR) in an anomalous dispersion medium experimentally, which breaks the minimum speed limit of NCR. This modified nonlinear Cherenkov radiation has two new special states with respect to its prior, i.e., the degenerate state and extreme state. Also, periodically arranged ultrathin nonlinear media can enhance NCR and realize the nonlinear Smith-Purcell effect. [ABSTRACT FROM AUTHOR]

Abstract: Smith–Purcell radiation occurs when an electron passes close to the surface of a metallic grating. The radiation becomes coherent when the length of the electron bunch is smaller than the wavelength of the radiation. A train of periodic bunches can enhance the spectral intensity by changing the angular and spectral distribution of the radiation. This is called super-radiant Smith–Purcell radiation, and has been observed in experiments and particle-in-cell simulations. In this paper, we introduce a new method to study this effect by calculating the reflected waves of an incident evanescent wave from periodic line charges. The reflection coefficients are numerically computed, and the spectral distributions of the super-radiant radiation are demonstrated. These analytical results are in agreement with those obtained through part-in-cell simulations. [Copyright &y& Elsevier]

This letter reports a theory to calculate the growth rate and start current of a Smith-Purcell free-electron laser, which is a promising radiation source in the terahertz domain. A two-dimensional model was used to investigate the interaction between a sheet electron beam and the surface wave above a lamellar grating. After deriving the growth rate from the dispersion equation, the start current was carefully estimated by considering the power flow above the grating. The agreement between the predictions of our theory and the results from the particle-in-cell simulations is acceptable. [ABSTRACT FROM AUTHOR]

A non-stationary two-dimensional theory of interaction of a surface wave and a wide sheet electron beam is developed. This theory is used for studying the problem of mode competition and non-coherent transverse distribution of the microwave field in the surface-wave generation process. According to numerical simulations based on a quasi-optical equation, transverse diffraction of surface wave provides stable stationary generation in a wide range of beam widths and currents. The developed theory can be applied both to surface-wave oscillator and to a Smith-Purcell free-electron laser, where surface pi-type eigenmode excitation provides the beam bunching and enhances the power of the Smith-Purcell radiation at the frequency harmonics of the surface wave. [ABSTRACT FROM AUTHOR]

Polarization radiation generated when a point charge moves uniformly along a straight line in vacuum in the vicinity of media with a finite permittivity ɛ(ω) = ɛ′ + iɛ″ and sharp boundaries is considered. A method is developed in which polarization radiation is represented as the field of the current induced in the substance by the field of the moving charge. The solution to the problem of radiation induced when a charge moves along the axis of a cylindrical vacuum channel in a thin screen with a finite radius and a finite permittivity is obtained. Depending on the parameters of the problem, this solution describes various types of radiation (Cherenkov, transition, and diffraction radiation). In particular, when the channel radius tends to zero and the outer radius of the screen tends to infinity, the expression derived for the emitted energy coincides with the known solution for transition radiation in a plate. In another particular case of ideal conductivity (ɛ″ → ∞), the relevant formula coincides with the known results for diffraction radiation from a circular aperture in an infinitely thin screen. The solution is obtained to the problem of radiation generated when the charge flies near a thin rectangular screen with a finite permittivity. This solution describes the diffraction and Cherenkov mechanisms of radiation and takes into account possible multiple re-reflections of radiation in the screen. The solution to the problem of radiation generated when a particles flies near a thin grating consisting of a finite number of strips having a rectangular cross section and a finite permittivity and separated by vacuum gaps (Smith-Purcell radiation) is also obtained. In the special case of ideal conductivity, the expression derived for the emitted energy coincides with the known result in the model of surface currents. [ABSTRACT FROM AUTHOR]

I discuss the life, education, personality, and contributions of Edward Mills Purcell (1912-1997) to physics, radio astronomy, astrophysics, biological physics, physics teaching and education, and to the nation. [ABSTRACT FROM AUTHOR]

Abstract: A terahertz (THz) Smith–Purcell (SP) backward-wave oscillator with an inverse wet-etched grating based on silicon has been proposed to enhance radiation intensity. This grating strengthens the interactions between an electron beam and the evanescent wave due to the adjacent surface structure between gratings that improves the magnitude of the electric field up to 1.7 times compared to the conventional rectangular gratings. A two-dimensional particle-in-cell (PIC) simulation shows that the radiated power is increased up to 2.3 times higher at the radiated frequency of 0.66 THz for an electron-beam energy of 30 keV. [ABSTRACT FROM AUTHOR]

Excitation of induced coherent Smith-Purcell (SP) radiation by relativistic magnetized electron beam in the absence of the resonator is considered within the framework of the dispersion equation. We have found that the dispersion equation for the SP instability is a quadratic equation for frequency. The first-step approximation for solution of the dispersion equation, giving the SP-spectrum of frequency, corresponds to the mirror boundary case, when the electron beam propagates above a plane metal surface (mirror). It was found that the conditions for both the Thompson and the Raman regimes of excitation do not depend on beam current and depend on the height of the beam above the grating surface. The growth rate of the instability in both cases is proportional to the square root of the electron beam current. No feedback is needed to provide the coherent emission. [ABSTRACT FROM AUTHOR]