A diffraction grating mounted in the electron beam focal region of a scanning electron microscope has been used to produce superradiant emission over the 300–900 $\mu \mathrm{m}$ wavelength range. Feedback is provided by the grating itself and the electron beam is focused and positioned over the grating by the microscope's electron optical system. Extensions of this technique promise a new tunable, coherent, cw source for the difficult to access far infrared (30–1000 $\mu \mathrm{m}$) range of the spectrum.

• Received 2 July 1997

DOI:https://doi.org/10.1103/PhysRevLett.80.516