Cooperative effects in cone emission from laser-pumped two-level atoms

The cone emission obtained from pumped two-level atoms is related, by a perturbation theory, to time-dependent terms in the macroscopic Bloch equations. It is described as a cooperative superfluorescence radiation from collections of spatially distributed atoms, including in the theory the far-zone dipole-dipole interactions, in the presence of the strong monochromatic field. The distribution of the cone emission as a function of frequencies and cone angles is analyzed. A phase-matching condition is obtained for the wave vectors of the pump and conical beams that is similar to the Cherenkov condition. The frequency shift for the maximum of the cone light is found to be equal to the generalized Rabi frequency. The broadening of the cone emission is related to inhomogeneity in Rabi frequencies and decoherence effects. An approximate result is obtained for the ratio of intensities of the blue-shifted cone relative to the red-shifted cone so that the blue-shifted cone can appear only for extremely strong laser intensities.