纳米谐振腔中高$Q$准BIC态的多极二次谐波产生
Multipolar second-harmonic generation from high-$Q$ quasi-BIC states in nanoresonators
摘要：
多模干涉和多极相互作用控制光学纳米谐振器和非线性纳米天线的功能。最近，高品质的超腔模式（准BIC状态）在个别的亚波长介质粒子的激发已被预测，以提高在纳米尺度上的非线性频率转换。在这里，我们提出了多极模型，它捕捉背后的物理驱动的高$Q$模式的纳米谐振器的线性和非线性响应。我们表明，准BIC状态的AlGaAs纳米盘的形成可以理解通过耦合漏模的多极变换。特别地，杂化轴对称TE偏振模可以被看作是多极子的叠加，其母多极子的基主要由磁偶极子和八极子构成。参数空间中的准BIC点出现在偶极损耗被完全抑制的地方。通过与其多极原点匹配的方位角偏振光束照明来实现与该状态的有效光学耦合。我们建立了一个标准的非厄米耦合模理论和多极模型，使透明的散射特性的解释之间的一一对应。使用我们的方法，我们得到的多极组成所产生的二次谐波辐射的AlGaAs纳米盘，并验证它与全波数值模拟。在考虑抽运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。提出了一种混合金属-介质纳米天线，将转换效率提高了几十个百分点，实现了非线性参量下转换。我们的研究结果描绘了深入的概念框架和新的有前途的策略，在非线性纳米光子学应用的功能元件的设计。
英文摘要：
Multimode interference and multipolar interplay govern functionalities of optical nanoresonators and nonlinear nanoantennas. Recently, excitation of the high-quality supercavity modes (quasi-BIC states) in individual subwavelength dielectric particles has been predicted to boost the nonlinear frequency conversion at the nanoscale. Here, we put forward the multipolar model which captures the physics behind linear and nonlinear response driven by such high-$Q$ modes in nanoresonators. We show that formation of the quasi-BIC state in the AlGaAs nanodisk can be understood through multipolar transformations of coupled leaky modes. In particular, the hybridized axially symmetric TE-polarized modes can be viewed as superpositions of multipoles, with a basis of parent multipoles constituted mainly by magnetic dipoles and octupole. The quasi-BIC point in the parameter space appears where dipolar losses are totally suppressed. The efficient optical coupling to this state is implemented via azimuthally polarized beam illumination matching its multipolar origin. We establish a one-to-one correspondence between the standard non Hermitian coupled-mode theory and multipolar models that enables transparent interpretation of scattering characteristics. Using our approach, we derive the multipolar composition of the generated second-harmonic radiation from the AlGaAs nanodisk and validate it 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 and actualize the nonlinear parametric downconversion. Our findings delineate the in-depth conceptual framework and novel promising strategies in the design of functional elements for nonlinear nanophotonics applications.
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