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The problem of optical bistability (OB) and optical multistability (OM) is numerically investigated in a four-level inverted Y-type semiconductor quantum well (SQW) structure immersed in a unidirectional ring cavity. In the four-level SQW system under consideration, a closed loop configuration is coupled to the upper level through a tunable probe field. We show that the OB threshold intensity can be controlled via the intensity of coupling fields which gives rise to the absorption variation of the probe field. In addition, due to the existence of the closed-loop configuration, the OB and OM behaviors of the proposed SQW medium are dependent on the relative phase of the applied fields. It is found that the OB can be switched to OM or vice versa by properly adjusting the relative phase of the applied fields. The results may provide new possibilities in real experiments for realizing an all-optical switching or coding element in a solid-state platform.

We investigate the two-dimensional (2D) position-dependent probe absorption in a triple semiconductor quantum well (SQW) structure driven coherently by two orthogonal standingwave fields. Under weak probe field approximation and utilizing the density matrix approach, an analytical solution is presented for the probe linear susceptibility which clarifies directly the spatial-dependent nature of the probe absorption. Then, the distribution of probe absorption in 2D is numerically explored. It is found that 2D absorption spectra are very sensitive to the intensity and frequency detuning of standing-wave fields which can result in various 2D localization structures in one period of standing waves in the x-y plane.

A five-level X-type atomic scheme is proposed to elucidate the two-dimensional (2D) atom localization in sub-wavelength domain by using different coupling situations of the atom with standing wave fields. The scheme is a mixture of two upper V-and lower Λ-type usual three level systems which both are connected at a common intermediate level. The combination of the upper and lower systems can lead to different atom localization patterns as lattice-, chain-, crater-, wave-, and spike-like structures. We discuss how these structures may depend on different coupling conditions of the atom with the standing wave fields. Finally, an experimental implementation for such an atomic model is presented using the 87 Rb atoms.