Bunch-type dipole spectrum of a weakly excited two-level system

Abstract

We derive a compact formula for the electric dipole induced in a weakly laser-driven two-level system. The dipole is shown to oscillate at frequencies being different combinations of even multiplicities of the transition frequency and odd multiplicities of the laser frequency. Applying this formula to laser frequencies much lower than the transition frequency in the system, we explain the numerical dipole spectra reported in literature, particularly the bunches of spectral lines grouped around even multiplicities of the transition frequency. These bunches form a part of spectrum additional to the ordinary odd laser-light harmonics.