Free-electron lasing without inversion by interference of momentum states

Abstract

It is shown that lasing in free-electron devices can be attained without the standard population inversion between the two portions of the electron momentum distribution that contribute to stimulated emission and absorption, respectively. Coherent superpositions of two electronic states in appropriately designed wigglers can strongly suppress stimulated absorption without hampering stimulated emission. The resulting gain curve is symmetric about the emission resonance, and yields a much larger gain than the antisymmetric gain curve of a standard free-electron laser with the same parameters.