Magnetic-field-induced two-colour frequency mixing in atoms: Polarization effects on a double-resonance cross section

Application of a steady magnetic field allows sum-frequency radiation generated via dipole-forbidden mixing of two-colour fields in an atomic medium to be emitted in the (otherwise forbidden) forward direction. As a result of resonance tuning at low-field strength (not exceeding 0.1 T) a fourth-order magnetic-dipole-induced amplitude for the sum-frequency process can be made comparable to the third-order field-free amplitude. We show that the interference between different terms of the field-dependent amplitude may cause the circular dichroism of the forward-scattering cross section for one of the two incident waves or the sum-frequency wave when the other two waves are linearly polarized. The dichroism effect is due to the dissipation processes determined by the resonant level width Gamma and depends strongly both on Gamma and the resonance detuning Delta. The effect may be used to control the frequency conversion process or to make possible a frequency mixing laser spectroscopy of excited atomic levels.