Plasmon-assisted two-photon absorption in a semiconductor quantum dot -- metallic nanoshell conjugate.

Tho-photon absorption holds potential for many practical applications. We theoretically investigate the onset of this phenomenon in a semiconductor quantum dot -- metallic nanoshell conjugate subjected to a resonant CW excitation. Two-photon absorption in this system may occur in two ways: incoherent -- due to a consecutive ground-to-one-exciton-to-biexciton transition and coherent -- due to a coherent two-photon process, involving the direct ground-to-biexciton transition in the quantum dot. The presence of a nanoshell nearby a quantum dot gives rise to two principal effects: (i) -- renormalization of the applied field amplitude and (ii) -- renormalization of the resonance frequencies and radiative relaxation rates of the quantum dot, both depending on the the quantum dot level populations. We show that in the perturbative regime, when the excitonic levels are only slightly populated, each of these factors may give rise to either suppression or enhancement of the two-photon absorption. The complicated interplay of the two determines the final effect. Beyond the perturbative regime, it is found that the two-photon absorbtion experiences a drastic enhancement, which occurs independently of the type of excitation, either into the one-exciton resonance or into the two-photon resonance. Other features of the two-photon absorption of the conjugate, that emerge due to the nanoparticles coupling, are bistability and self-oscillations, having no analog in the two-photon absorption of an isolated quantum dot.