We experimentally study the nonlinear optical behaviors of N-type four-level atomic systems inside an optical cavity. These nonlinear effects include two-photon absorption, splitting of dark polariton, modified self-Kerr nonlinearity, Raman amplification, and white-light cavity.
The self-Kerr-nonlinearity of a four-level N-type atomic system is studied both experimentally and theoretically. The self-Kerr-nonlinear coefficient n2 of the probe beam is greatly enhanced near atomic resonance, and can be dramatically modified by changing the switching laser power. With an increasing switching laser power, the slope of the Kerr nonlinear coefficient n2 around the atomic resonance changes from negative to positive, and the value of n2 varies from \ensuremath{\sim}--4 \ifmmode\times\else\texttimes\fi{} 10${}^{\ensuremath{-}6}$ to 4 \ifmmode\times\else\texttimes\fi{} 10${}^{\ensuremath{-}6}$ cm2/W at certain near-resonant probe frequency detunings. Such controllable Kerr nonlinearity can find applications in optoelectronic devices, such as all-optical switching and logic gates.
I analyze a scheme that is capable of producing large Kerr nonlinearities on cavity-atom polaritons in a cavity quantum electrodynamics system consisting of multiple three-level atoms confined in a cavity mode. A weak control laser coupled to the atoms from free space induces destructive quantum interference in the polariton excitation of the coupled cavity-atom system and creates large Kerr nonlinearities on the intra-cavity light field. The scheme can be used for optical switching or cross-phase modulation of the cavity-atom polariton at ultralow light levels.
We analyze the absorptive and hybrid absorptive–dispersive double-cavity optical bistability behavior in a four-level N-type atomic system driven by two optical fields circulating inside two independently unidirectional ring cavities and a trigger field outside the cavities. In this system, the two-photon absorption and cross Kerr nonlinearity play significant roles in the formation of bistability and can be controlled efficiently due to the quantum interference effect, and then the resulting nonlinear behavior exhibits different features from the two/three-level schemes. It is shown that the threshold values and hysteresis cycle width of the bistable curve can be manipulated via the adjustment of the system parameters. Meanwhile, we also obtain double-beam optical multistability. When adding positive and negative pulses to the cavity input, all-optical steady-state switching for the two cavity outputs can be achieved simultaneously. Therefore, the proposed scheme may have potential applications in all-optical communication and computing.