Collective bunching of uncharged macroscopic particles via an FEL-like instability

A classical theory of the scattering of electromagnetic radiation by uncharged macroscopic particles is presented, in which the particles may interact cooperatively via common pump and probe radiation fields. The collective nature of the process is manifest both in the exponential growth of the bunching of the particles at the radiation wavelength scale to form a particle grating, and in the exponential growth of the counterpropagating probe radiation intensity. The equations describing this phenomenon are shown to be of the same form as those describing the Compton FEL.