【作者】 李星；

【导师】 胡响明；

【作者基本信息】 华中师范大学 ， 理论物理， 2005， 硕士

【摘要】 光学介质的吸收和色散性质在量子光学与非线性光学中有着广泛的应用。近年来,人们非常关注利用原子相干抑制介质吸收、改变色散性质。主要原因在于,吸收性质往往会限制色散性质的利用,只有当介质的吸收或者增益相对较小时,色散性质才能获得有效利用。当用强相干场驱动原子介质时,可令介质的吸收-色散关系发生极大改变:由于量子相干与干涉效应,使原子处于相干叠加态,在一定条件下,能使不透明介质变成透明,此时介质对探测光(尤其重要的是对弱探测光)的吸收为零,吸收受到抑制的同时介质的色散性质则得到增强。通过改变介质的色散性质人们能控制光学介质中光的传播速度。由此,出现许多新的光学现象,如,电磁诱导透明、无反转激光、光的超慢传播或者超光速传播,无吸收折射率增强等。 本文主要研究了为强相干场驱动三能级级联型系统的吸收-色散关系,建议利用此系统介质实现从正常色散到反常色散的变换。在三能级级联型介质中,两个偶极跃迁分别与两个不同的相干场耦合。探测场或者与上跃迁耦合,或者与下跃迁耦合。利用谐振展开方法把介质密度矩阵元方程组转换成在定态条件下系数不依赖时间的方程组,然后用逆矩阵求解,并用此解求得介质的吸收与色散谱。结果表明,对于每一种情形,在同一频率区域系统既能呈现正常色散,也能呈现反常色散。通过改变耦合到探测跃迁的驱动场强度,可以实现从正常色散到反常色散的转换。级联系统与∧和∨系统之间呈现两个差别:第一,如果利用∧和∨系统的两个跃迁同时与探测场耦合获得色散转换,则需要利用近简并能级,而本方案利用缀饰态获得等价的近简并能级,其间距由相应驱动场强度进行控制。第二,如果利用∧和∨系统的单个跃迁耦合探测场建立色散转换,当退耦合态起主要作用时,系统不能产生色散更多还原

【Abstract】 The dispersion and absorption properties of optical medium have been extensively applied in quantum optics and nonlinear optics. Recently great interest has been directed to the modification of dispersion and absorption properties via atomic coherence induced by coherent fields. In fact, the use of the dispersive properties is often limited by the absorption. The dispersive properties can be effectively used when the absorption or gain of the medium is small. Based on the effect of quantum coherence and interference, the dispersion-absorption relations can be rapidly modified by the application of a strong coherent field. The resonant absorption was eliminated and the dispersive properties of the medium can be enhanced. Manipulation of light speed in an optical medium can be realized by changing the dispersive properties of the medium. These have led to the observation of many new effects and new techniques in quantum optics and atomic physics. Examples include Electromagnetically Induced Transparency, Lasing Without Inversion, the subluminal and superluminal light propagation, and Enhancement of the Index of Refraction Without Absorption.We suggest that a three-level cascade medium is used to achieve the switching from normal to anomalous dispersion. In the cascade medium, two dipole-allowed transitions are coupled by two different coherent fields, respectively. A probe field is applied either to the upper transition or to the lower transition. The equations for density matrix elements are transformed into the set of equations with time-independent coefficients at the steady state via harmonic expansion, which are solved by matrix inversion. By the solutions we obtain the absorption and dispersion spectra. It is shown that for each case, the mediumexhibits either normal or anomalous dispersion at the same frequency regime. The switching from normal to anomalous dispersion is obtained by varying the intensity of one of two driving fields. The two differences lie between the proposed cascade system and the A and V systems. Firstly, near degenerate levels are required for the A and V systems when the probe field is simultaneously coupled to the two transitions to achieve dispersion switching. The present scheme is based on an equivalent realization of near degeneracy in the dressed picture, which is controlled by varying the intensity of the corresponding driving field. Secondly, dispersion switching does not occur in the A and V systems when the probe field is only coupled to either of two transitions and the decoupled state plays a dominant role. In contrast, the present scheme does not involve the decoupled state.On the other hand, The dispersion-absorption relations of the two-level atom system tell us that at or near the resonance frequency, the index of refraction is large but absorption is equally large. Far from resonance, the two-level medium gets transparent while the index is not desirable. In this paper, we investigate the dispersive properties in the same three-level cascade medium. We found that in the three-level atomic mediums, the refractive-index enhancement of two probe fields without absorption can be obtained by controlling the intensity of driving fields.更多还原