The present invention relates to radiation sources employing an electron beam reflecting from a conductive grating.
Radiation from electrons interacting with a diffraction grating has been studied both theoretically and experimentally since the early work of W. W. Salisbury described in U.S. Pat. No. 2,634,372 and J. Opt. Soc. Am., Vol. 60, page 1279 et seq. (1970), and the work of S. J. Smith and E. M. Purcell as described in Phys. Rev., Vol. 92 page 1069 et seq. (1953). As described in these references, an electron beam interacting with the surface of a metallic grating has been shown to generate electromagnetic radiation. This radiation source is easily tunable, with the wavelength .lambda. depending on the grating period a, electron speed v.sub.o and the angle .theta. between the electron velocity and observation direction as shown in Equation 1, where c is the velocity of light. ##EQU1##
Past theoretical treatments of the radiation from an electron beam interacting with the surface of a metallic grating are understood to have assumed that the electrons do not collide with the grating. With this assumption, both incoherent and coherent radiation have been discussed.
The incoherent radiation has been discussed in terms of dipole radiation from oscillating image changes (S. J. Smith and E. M. Purcell, Physics Review, Vol. 92, page 1069 (1953)), grating scattering of the evanescent waves tied to the electrons (E. Labor, Physics Review, Vol. A7, page 435 (1973); G. Toraldo di Francia, Nuovo Cimento, Vol. 16, page 61 (1960)), and rigorous Green's function formulations of the electromagnetic fields generated in the half-space grounded by the grating (C. W. Barnes and K. G. Dedrick, J. Appl. Phys., 37, 411 (1966); P. M. Van den Berg and T. H. Tan, J. Opt. Soc. Am., 64, 325 (1974)).
The generation of coherent radiation has been discussed in connection with the situation when the electron beam and grating are placed within a resonant cavity. The resulting device has been called the oratron or ledatron. (F. S. Rusin and G. D. Bogomolov, JETP Lett., 4, 160 (1966); K. Mizuno, S. Ono. and Y. Shibata, IEEE Trans. Electron Devices, ED-20,749 (1973)). In that case, the coherent radiation produced has been treated by calculating the power transferred from an electron beam to a cavity mode which is perturbed by the periodic grating. R. P. Leavitt, D. E. Wortman, and C. A. Morrison, Appl. Phys. Lett., 35, 363 (1979); R. P. Leavitt and D. E. Wortman, J. Appl. Phys., 54, 2219 (1983).
In none of the foregoing analyses are the electrons assumed to collide with the grating. There is, however, some experimental work indicating that electron collisions with the grating should make an appreciable difference. The early experiments of W. Salisbury with low divergence beams scattering off the grating, discussed in the paper "Generation of Light from Free Electrons", Winfield W. Salisbury, Journal of the Optical Society of America, Winfield W. Salisbury, Vol. 60, No. 10, October 1970, pp. 1279-1284, disclosed the following significant differences from the Smith-Purcell-type experiments in which no electron collisions with the grating occurred.
1. The radiation intensity was much larger with very bright colors appearing even when overhead illumination was present;
2. electrons which were 1 mm from the grating contributed as much as electrons within a grating spacing of the grating; and
3. the radiation intensity was largest when the numbers of scattered and unscattered electrons were comparable.
The second finding above is in direct contradiction to theoretical calculations which assume that no collisions with the grating occur. These calculations indicate that electrons which are farther away from the grating than one grating spacing should produce negligible radiation.
Insofar as is known to applications, the above-referenced research efforts have not resulted in explanations of the underlying radiation phenomena, which has, in turn, limited the usefulness of the phenomena.
It is, therefore, an object of the present invention to provide radiation source resulting from an understanding of the emission of radiation from the reflection of electrons from a grating.
It is another object of the present invention to provide a radiation source which is easily tunable over a broad band of wavelengths and which can provide either coherent or noncoherent radiation.
It is another object to provide a radiation source which may be easily modulated both in frequency and in amplitude.