1. Field of the Invention
This invention relates to electromagnetic radiation sources, and more specifically to radiation sources based on the Smith-Purcell effect with particular application in the THz regions of the spectrum.
2. Description of the Related Art
THz-frequency radiation, in the frequency region from 0.3 THz to 10 THz, has been relatively unexploited compared to the adjacent radio frequency (RF) and IR spectral bands that lie on either side of it. This is largely because of transmission difficulties due to absorption by atmospheric water vapor but also due to a lack of practical radiation sources and detectors. In recent years there has been a significant growth of interest in applications of this previously underutilized portion of the electromagnetic spectrum in the areas of active short range imaging systems, space-based communications, imaging, and spectroscopy.
Prior work by Urata et al. of Dartmouth College (Urata, J., et al., “Superradiant Smith Purcell Emission”, PRL 80#3, January 1998), which is hereby incorporated by reference, has established that the classic Smith Purcell effect can be used as a source of radiation in the far infrared (FIR) to terahertz (THz) spectral regions. As illustrated in FIG. 1, the beam 10 from an electron microscope is focused over a properly sized metal diffraction grating 12 thereby producing electromagnetic radiation 14. The wavelength of the radiation is determined from the expression
  λ  =            l                      n                      ⁢          (                        1          β                -                  sin          ⁢                                          ⁢          θ                    )      where l is the grating period, β≡v/c is the electron velocity v relative to the speed of light c, θ is the angle of emission measured from a direction normal to the surface of the grating, and n is the spectral order. Urata et al. conclude “extensions of this technique promise a new tunable coherent CW source for the difficult to access . . . 30-1000 micron range of the spectrum”. As shown in FIG. 2, limited tunability of the emission spectrum 16 can be achieved by varying the voltage, hence velocity of the electron beam. Vermont Photonics (in collaboration with a Dartmouth group) has recently developed a source of THz radiation using this technique.
Subsequent work by Yamaguti et al. (Yamaguti et al., “Photonic crystals versus diffraction gratings in Smith-Purcell radiation”, Physical Review B 66, 195202 (2002)) demonstrated that the use of a two-dimensional photonic crystal in place of the one-dimensional diffraction grating created resonant structures of enhanced intensity whose peak heights were an order of magnitude larger than the emission intensity of the one dimensional Bragg diffraction grating. Yamaguti et al. conclude “the results of calculations thus show a possible advantage of photonic crystals over diffraction gratings as converter of the evanescent field from a running charge into propagating waves.”
Although Smith-Purcell sources hold great promise for THz sources, further advancements are needed to increase their intensity and efficiency to a point where commercialization is practicable.