A theoretical investigation is carried out into the effect of spontaneously generated coherence on the Kerr nonlinearity of general three-level systems of ⌳, ladder, and V-shape types. It is found, with spontaneously generated coherence present, that the Kerr nonlinearity can be clearly enhanced. In the ⌳and ladder-type systems, the maximal Kerr nonlinearity increases and at the same time enters the electromagnetically induced transparency window as the spontaneously generated coherence intensifies. As for the V-type system, the absorption property is significantly modified and therefore enhanced Kerr nonlinearity without absorption occurs for certain probe detunings. We attribute the enhancement of Kerr nonlinearity mainly to the presence of an extra atomic coherence induced by the spontaneously generated coherence.
A scheme for giant enhancement of the Kerr nonlinearity in a four-level system with double dark resonances is proposed. Compared with that generated in a single-dark-resonance system, the Kerr nonlinearity can be enhanced by several orders of magnitude with vanishing linear absorption. We attribute this dramatic enhancement to the interaction of dark resonances.
The nonreciprocal propagation of light at the single-photon level is essential for building a quantum network. Bulk optical schemes are lossy and difficulty to integrate onto a chip. We propose a single-photon optical diode and a three-port circulator without a magnetic field by coupling an unbalanced quantum impurity to a passive, linear optical waveguide or a whispering-gallery-mode microresonator which supports a locally or globally circularly polarized photon. Thanks to the unbalanced quantum Jaynes-Cummings coupling, the optical nonreciprocal propagation of single photons can be achieved without an external magnetic field. In particular, the three-port single-photon circulator can be accomplished using the existing experimental technology. The optical isolation can be reversed by selectively populating the initial state of the quantum impurity. Moreover, by using an ensemble of identical atoms filled in a hollow-core microbottle resonator, nonreciprocal propagation of weak light pulses can be achieved.
We propose a scheme for obtaining an electromagnetically induced grating in an asymmetric semiconductor quantum well (QW) structure via Fano interference. In our structure, owing to Fano interference, the diffraction intensity of the grating, especially the first-order diffraction, can be significantly enhanced. The diffraction efficiency of the grating can be controlled efficiently by tuning the control field intensity, the interaction length, the coupling strength of tunneling, etc. This investigation may be used to develop novel photonic devices in semiconductor QW systems.
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