First observation of Smith-Purcell radiation from relativistic electrons

Abstract

A beam of 3.6-MeV electrons has been used to study the generation of radiation in the far infrared (FIR) by the Smith-Purcell mechanism. The dependence of wavelength on angle of emission, over angles from 56° to 150° and wavelengths from 350 to 1860 /xm, is in excellent agreement with the SmithPurcell dispersion relation. Comparison of the yield with that from a 5000-K source suggests that the spontaneous Smith-Purcell effect offers an easily tunable alternative to the synchrotron as a coherent FIR source, and that it could also form the basis of an inexpensive, compact free-electron laser.PACS numbers: 41.75.Ht, 41.60.Cr We report the first observations of Smith-Purcell (SP) radiation in the submillimeter-far-infrared region of the spectrum. Unlike both early [1] and more recent work [2], which relied on low-energy electron beams and concentrated on emission in the visible, here a relativistic electron beam was employed and the emphasis is on a spectral region where the population of sources is sparse.Comparison of the measured value of the emitted wavelength (Xsp) and the emission angle iO) for a series of gratings confirms in detail that the process obeys the celebrated SP formula:(1)The parameter XQ is the grating period and p is the relative velocity of a beam electron. In the spectral region where the radiation was observed, the level is comparable to that produced by an infrared beam line on a synchrotron. The emission from compact, linear-acceleratordriven gratings would be much greater. Thus, the process provides a means whereby Fourier-transform spectroscopic techniques can be extended into the technologically important long-wavelength spectral region. The grating is also a much simpler structure than a magnetic undulator and with adequate feedback it offers another basis for the free-electron laser (FEL) [3].The inverse SP process has also been considered as a possible means of accelerating electrons to high energy I4]. Although in the present work acceleration was not the object, coupling of near speed of light electrons and gratings has been demonstrated.The electrons were accelerated in a modified 10-MV Van de Graaff accelerator formerly used for a program of nuclear structure research at Oxford. Conversion to accelerate electrons had been started in preparation for a FEL project which was then not funded. Work necessary to enable efficient electron transmission [5l was completed and an electron gun was kindly lent by the Department of Physics, University of Glasgow.First, acceleration of the electrons was obtained at an energy of 3.6 MeV. The grid of the gun was pulsed, giving 6-//S bursts of electrons at 1 Hz. Considerable difficulties were experienced with both the yield and short lifetime of the cathodes; these were not fully overcome. Another pair of unsolved problems were a slight positional jitter in the beam and a beam size slightly larger than expected. The beam size, shape, and transverse position could be controlled using the up-stream deflecting magnets and quadrupole (Fig. l).The electr...

Light generation via quantum interaction of electrons with periodic nanostructures

“…We shall call this result the conventional Smith-Purcell effect (CSP), and note that it can be explained in many other ways: the diffraction of a charged particle self-field from a * Corresponding author: kaminer@tx.technion.ac.il grating or other periodicity [20,21]; as a product of induced currents [22], or an oscillating dipole moment, created by a mirror charge [17]; as resonant diffraction radiation [23]; in the Green's function formalism [24]; and in a quantum mechanical formalism [25]. All of these approaches match the findings of the Smith-Purcell experiment, as well as many experiments that followed (e.g., [26][27][28][29][30]), ever confirming the conventional theory represented by Eq. [1].…”

Maximal spontaneous photon emission and energy loss from free electrons

“…Consider an electron at velocity β = v/c traversing a structure with periodicity a; it generates far-field radiation at wavelength λ and polar angle θ, dictated by where m is the integer diffraction order. The absence of a minimum velocity in equation (1) offers prospects for threshold-free and spectrally tunable light sources, spanning from microwave and terahertz [14][15][16] , across visible [17][18][19] , and towards X-ray 20 frequencies. In stark contrast to the simple momentum-conservation determination of wavelength and angle, there is no unified yet simple analytical equation for the radiation intensity.…”

Smith-Purcell radiation emission in aperiodic arrays

“…This condition is analogous to the Huygens construction, but now also involving the time delay used by the electron to interact with consecutive grating periods. Following the initial demonstration and explanation of the SP effect, 1 subsequent studies have further corroborated the dependence of the emission on electron energy and grating period, 2,3 confirming that it occurs over a wide spectral range, including X-rays, 4 UV 2 and visible 1,5 light, NIR, 6 FIR, 7 and THz. 8,9 The SP effect is the basis of free-electron lasers, 10,11 whereby the emission intensity produced a large number of electrons bunched within a small spatial region compared with the emitted light wavelength is proportional to the square of the number of electrons 8,[12][13][14] .…”