中文摘要：

在Y型四能级原子系统中理论分析了外加相干控制场的强度变化对电磁感应透明和Kerr非线性效应的影响。结果发现：当外加的耦合场和抽运场的强度从相等变得不相等时，原子系统对探测场的吸收曲线从一个透明窗口逐渐变成了具有两个透明点的增益区间，透明点的位置、增益值的大小以及增益区间的宽度都与变化着的外场的拉比频率密切相关；当两个外场强度较弱并且相对强度减小时，Kerr非线性效应有所增强但同时伴随有吸收；而当两个外场的强度较强并且相对强度增大时，Kerr非线性的峰值也随之增大并且逐渐与透明点对应，即获得了无吸收巨Kerr非线性效应。研究还发现，抽运场的强度变化对Kerr非线性增强的影响作用较耦合场要大。

英文摘要：

A theoretical investigation is carried out into the effect of applied coherent fields on electromagnetically induced transparency （EIT） and Kerr nonlinearity in the four-level Y-type energy atom system. It is found that the absorption curve of the atom system changes from one EIT window to a gain interval which has two transparency points when the intensity of coupling field and pumping field changes from equality to inequality. The position of transparency points, the magnitude of gain value together with the width of gain interval are all closely connected with the changed Rabi frequency of applied fields. It is also found that Kerr nonlinearity is enhanced but accompanied by linear absorption when applied fields are all weak and their relative intensity reduces. When the intensity Of applied fields is enlarged, Kerr nonlinearity is enhanced significantly and its peak moves to the position of transparency point with the enlargement of their relative intensity. That is to say, giant Kerr nonlinearity accompanied by vanishing linear absorption is obtained. Finally, we find that the effect of the intensity variation of the pumping field on the enhancement of Kerr nonlinearity is more remarkable than that of the coupling field.