Two-photon absorption holds potential for many practical applications. We theoretically investigate the onset of this phenomenon in a semiconductor quantum dot–metallic nanoshell composite 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 the nanoshell nearby the quantum dot gives rise to two principal effects: (i) renormalization of the applied field amplitude and (ii) renormalization of the resonance frequencies and radiation relaxation rates of the quantum dot, both depending on the quantum dot level populations. We show that in the perturbation 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 perturbation regime, it is found that the two-photon absorption 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 characteristic features of the two-photon absorption of the composite, emerging from the coupling between both nanoparticles, are bistability and self-oscillations.

中文翻译：

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半导体量子点-金属纳米壳复合物中的等离子体辅助双光子吸收

两光子吸收在许多实际应用中具有潜力。我们在理论上研究了遭受共振连续波激发的半导体量子点-金属纳米壳复合材料中这种现象的发生。该系统中的双光子吸收可能以两种方式发生：由于相干（由于连续的从地面到一个激子到比谢克顿的跃迁），所以是非相干的；而由于相干的两个光子过程（涉及到直接从地面到子）的相干，所以是相干的。量子点中的-biexciton跃迁。靠近量子点的纳米壳的存在引起两个主要影响：（i）重新归一化施加的场振幅和（ii）重新归一化量子点的共振频率和辐射弛豫率，这两者都取决于量子点的水平人口。我们证明在摄动状态下 当激子能级仅略微分布时，这些因素中的每一个都可能引起双光子吸收的抑制或增强。两者之间复杂的相互作用决定了最终的效果。除了扰动机制外，还发现双光子吸收经历了急剧的增强，这种增强与激发的类型无关，而发生在单激子共振或双光子共振中。由两个纳米颗粒之间的耦合产生的复合材料的双光子吸收的其他特征是双稳态和自激振荡。已经发现，双光子吸收经历了急剧的增强，该增强与激发的类型无关地发生，或者进入单激子共振或者进入双光子共振。由两个纳米颗粒之间的耦合产生的复合材料的双光子吸收的其他特征是双稳态和自激振荡。已经发现，双光子吸收经历急剧的增强，该增强与激发的类型无关地发生，或者进入单激子共振或者进入双光子共振。由两个纳米颗粒之间的耦合产生的复合材料的双光子吸收的其他特征是双稳态和自激振荡。