In a typical three-level system, as depicted in Fig. 1, one can increase the nonlinearity by decreasing the de- tunings, especially the one from the intermediate state ji . that appears quadratically in x(3). However, this will in turn strongly increase the absorption of the inci-dent wave at va. Moreover, for very small detunings, the values for x(3) and Im[ x(1)] are determined by the decay rates, which imposes another limit on the size of the nonlinear effects. Finally, the system of Fig. 1also has a nonzero contribution to self-phase modula- tion that is not desirable if one is interested in the phase shifts of the wave at va that are induced by the wave at vb.
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