Effects of finite laser bandwidths in two-photon spectroscopy

Discusses within a simple statistical model modifications of two-photon spectra caused by finite bandwidths of the light sources used for excitation. Field fluctuations are shown to affect the two-photon and step-wise contribution differently which provides a way of distinguishing them. The rate equation approximation, which ignores coherent two-photon processes, is shown to be accurate only when the frequency spread of the fields exceeds the detuning parameters and relaxation rates of the three-level system. In an inhomogeneous system satisfying the Doppler-limit criteria the step-wise contribution is efficiently cancelled away demonstrating the poor applicability of a rate description. Only when the field bandwidths are much larger than the Doppler widths are the coherence effects negligible.