Effects of fourth-order coherence on ultrafast modulation spectroscopy

The effect of fourth-order coherence on ultrafast modulation spectroscopy with phase-conjugation geometry (PCUMS) in a cascade three-level system is investigated using chaotic and phase-diffusion models. It has been found that the temporal behavior of the beat signal depends on the stochastic properties of the lasers and the transverse relaxation rate of the transitions, and the modulation terms of the beat signal depend on the second-order coherence function, which is determined by the laser line shape. Since different stochastic models of the laser field affect only the fourth-order coherence function, they have little influence on the general temporal modulation behavior of the beat signal. The different roles of the phase fluctuation and amplitude fluctuation can be investigated in the time domain. The cases where the pump beams have either a narrow or broad band linewidth are considered and it has been found that for both cases a Doppler-free precision in the measurement of the energy-level difference between two states can be achieved. The difference between the PCUMS and the DeBeer, UMS [D. DeBeer et al., Phys. Rev. Lett. 56, 1128 (1986)] is discussed as well from a physical viewpoint.