Abstract We put forward the multipolar model which captures the physics behind linear and nonlinear response driven by high-quality (high-Q) supercavity modes in subwavelength particles. We show that the formation of such trapped states associated with bound states in the continuum (quasi-BIC) can be understood through multipolar transformations of coupled leaky modes. The quasi-BIC state appears with increasing the order of the dominating multipole, where dipolar losses are completely suppressed. The efficient optical coupling to this state in the AlGaAs nanodisk is implemented via azimuthally polarized beam illumination matching its multipolar origin. We establish a one-to-one correspondence between the standard phenomenological non-Hermitian coupled-mode theory and multipolar models. The derived multipolar composition of the generated second-harmonic radiation from the AlGaAs nanodisk is then validated with full-wave numerical simulations. Back-action of the second-harmonic radiation onto the fundamental frequency is taken into account in the coupled nonlinear model with pump depletion. A hybrid metal-dielectric nanoantenna is proposed to augment the conversion efficiency up to tens of per cent due to increasing quality factors of the involved resonant states. Our findings delineate novel promising strategies in the design of functional elements for nonlinear nanophotonics applications.

中文翻译：

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从亚波长谐振器中的高 Q 准 BIC 状态产生多极二次谐波

摘要 我们提出了多极模型，该模型捕捉了由亚波长粒子中的高质量（高 Q）超腔模式驱动的线性和非线性响应背后的物理原理。我们表明，可以通过耦合泄漏模式的多极变换来理解与连续体（准 BIC）中的束缚态相关的这种俘获态的形成。准 BIC 状态随着主导多极子的阶数的增加而出现，其中偶极损耗被完全抑制。在 AlGaAs 纳米盘中与这种状态的有效光学耦合是通过与其多极原点匹配的方位偏振光束照明来实现的。我们建立了标准现象学非厄米耦合模式理论和多极模型之间的一一对应关系。然后通过全波数值模拟验证从 AlGaAs 纳米盘产生的二次谐波辐射的衍生多极成分。在具有泵损耗的耦合非线性模型中考虑了二次谐波辐射对基频的反作用。由于所涉及的共振态的质量因素增加，因此提出了一种混合金属电介质纳米天线，以将转换效率提高到百分之几十。我们的研究结果描绘了非线性纳米光子学应用功能元件设计中新的有前途的策略。在具有泵损耗的耦合非线性模型中考虑了二次谐波辐射对基频的反作用。由于所涉及的共振态的质量因素增加，因此提出了一种混合金属电介质纳米天线，以将转换效率提高到百分之几十。我们的研究结果描绘了非线性纳米光子学应用功能元件设计中新的有前途的策略。在具有泵损耗的耦合非线性模型中考虑了二次谐波辐射对基频的反作用。由于所涉及的共振态的质量因素增加，因此提出了一种混合金属电介质纳米天线，以将转换效率提高到百分之几十。我们的研究结果描绘了非线性纳米光子学应用功能元件设计中新的有前途的策略。