Abstract
By investigating the third-order nonlinearity of a four-level ladder-type atomic system, it is found that, with spontaneously generated coherence (SGC) present, the nonlinear absorption or refraction can be significantly enhanced with vanishing linear absorption. We attribute the enhancement of nonlinearity mainly to quantum interference in the two decay pathways from the two upper closely lying levels. With a standing-wave trigger field, absorption or phase grating, which effectively diffracts a weak probe into high-order direction, can be induced by the SGC-enhanced absorptive or refractive nonlinear modulation. In contrast to the schemes for enhancing nonlinearity, no additional coupling field is required. Moreover, the present gratings result from the nonlinear modulation, which differs from the recent investigations based on linear modulation.
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