We study theoretically the nonlinear optical response of twisted AA- and AB-stacked bilayer graphene quantum dots placed in the field of an ultrashort optical pulse. We consider the cases of both linearly and circularly polarized pulses, and we characterize the nonlinear optical response of the system in terms of nonlinear absorption and high harmonic generation. The low-energy states of a bilayer quantum dot system are mainly localized at the quantum dot edges. As a result, such states and correspondingly the nonlinear optical response become very sensitive to the twist angle. In general, the nonlinear optical response of twisted bilayer quantum dots is enhanced when a finite twist angle is introduced. The nonlinear optical absorptance as a function of the twist angle shows a strongly nonmonotonic dependence with the maxima that are realized at a finite angle, with the maximum values that are a few times larger than the absorptance of the untwisted bilayer system. The dependence of the absorptance on the twist angle is partially correlated with the profile of a quantum dot band gap as a function of the angle. The high harmonic generation in twisted bilayer quantum dots shows a very strong sensitivity to the twist angle, especially for large-order harmonics and small quantum dots consisting of 142 atoms. For small-order harmonics, the intensities of the harmonics as a function of the twist angle show nonmonotonic oscillatory behavior with the intensities that change by a few orders of magnitude. For a circularly polarized pulse, some high-order harmonics are strongly suppressed for untwisted quantum dots due to $C_3$ rotational and inversion symmetries, where the inversion symmetry is present only in an AB-stacked untwisted system. These harmonics are strongly enhanced when a finite twist angle is introduced.

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• High-order harmonic generation
• Nonlinear optics
• Ultrafast optics
• Quantum dots
• Twisted bilayer graphene