Effects of fourth-order coherence on ultrafast modulation spectroscopy - art. no. 023809

The effect of fourth-order coherence on ultrafast modulation spectroscopy w ith phase-conjugation geometry (PCUMS) in a cascade three-level system is i nvestigated using chaotic and phase-diffusion models. It has been found tha t the temporal behavior of the beat signal depends on the stochastic proper ties of the lasers and the transverse relaxation rate of the transitions, a nd the modulation terms of the beat signal depend on the second-order coher ence function, which is determined by the laser line shape. Since different stochastic models of the laser field affect only the fourth-order coherenc e function, they have little influence on the general temporal modulation b ehavior of the beat signal. The different roles of the phase fluctuation an d amplitude fluctuation can be investigated in the time domain. The cases w here the pump beams have either a narrow or broad band linewidth are consid ered 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 ac hieved. 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.