I am learning quantum optics.
In the book written by Scully includes complex rabi frequency, when dealing with $\Delta$ level configuration, the Hamiltonian including laser-atom interacting concludes complex rabi frequency.
However, from the course I took as undergraduate, where rabi frequency are treated as real numbers (vectors), I cannot see why rabi frequency has imaginary part.
Could anyone help me with this? Why it has imaginary part? And how to deduce it?