Coupled nano-squares with optical response in nonlinear modes; Suitable substrate to control light by light for quantum applications

Nonlinear response of a nano-structure including two square quantum dots (QDs) of identical material but dissimilar sizes is discussed by considering possible quantum interferences. Density matrix approach is developed to extract physical characteristics of the system by considering Hamiltonians including couplings of the excitons to thermal bath and the possible intra-dot relaxations as well as the near field optical energy transfers (of Yukawa-type potentials) between the probable eight quantum states in subwavelength range. Realization of nonlinear behavior is studied systematically by putting the structure inside a unidirectional ring cavity and driving it by pair of dichromatic fields, that one provides a weak probe, while the other offers a strong driving component. It is shown that the absorption/dispersion properties of the probe field might be controlled by tuning the quantum interference via changing the structural features as well as the externally controlled parameters. Thus adjusting the optical bistability (OB) threshold, hysteresis cycle size or even transition from OB to multi-stability might be possible easily. Moreover, machine learning approach is proposed to evaluate how predictable are the responses of the suggested structure in various preliminary circumstances. Results clearly reflect high potential of the suggested structure for applications such as all-optical switches or memories.