nonlinear [9], and a planar metamaterial with broken structural symmetry on a nonlinear substrate [10]. The bistable optical response in these nanostructures commonly occurs under a relatively strong light illumination or by assisting with some large nonlinearity. Very luckily, metal-semiconductor hybrid nanostructures, which are expected to overcome those weaknesses in metal nanostructures, have been synthesized and provided a new way to explore the physical mechanism of optical bistability via the interaction between excitons in semiconductors and plasmons in metal nanostructures.The exciton-plasmon interaction can significantly modify the optical properties of metal-semiconductor hybrid nanosystems, such as nonlinear Fano effect [11], coherent molecular resonances [12], electromagnetically induced transparency [13], energy transfer [14, 15], slow light effect [16, 17], superradiance [18], enhanced thirdorder nonlinear optical response [19], intensity-dependent enhancement of saturable absorption [20], enhancement of the second-harmonic generation [21], strongly modified four-wave mixing, and four-wave parametric amplification [22, 23]. To date, some inspiring results around optical bistability have been achieved, showing that an optical bistability regime occurs as the coupling between semiconductor quantum dot (SQD) and metal nanoparticle (MNP) becomes stronger by increasing the sizes of both SQD and MNP [24]. Then, Malyshev et al. [25] reported that a coupled SQD-MNP nanosystem could manifest bistability by measuring optical hysteresis of the Rayleigh scattering. Malyshev also derived the analytical bistability condition and the exact steady-state solution of the Bloch equations at the absolute bistability threshold in the similar system [26]. Recently, Knoester et al. [27] explored the switching times from the lower stable branch to the upper stable one. In our previous work, we revealed the bistable properties Abstract We theoretically propose a scheme of a tunable bistable device based on a coupled semiconductor quantum dot-metal nanoparticle nanosystem in the simultaneous presence of a strong pump laser and a weak probe laser with different frequencies. The results show that it is easy to turn on or off the optical bistable effect in such system by switching the polarization direction of the pump field, and the bistability thresholds are highly sensitive to the intensity, frequency, polarization direction of the pump field, and the interparticle distance. In addition, the nonlinear absorption in the two stable states exhibits a ratio as high as 10 4 arising from the three-photon effect, which implies that our nanosystem can also be used as an optical memory cell.
