非线性选择性准束缚态在连续谱中的产生：通过对称保护解耦在χ(2)薄 films中
Nonlinearity Selective Quasi Bound States in the Continuum via Symmetry Protected Decoupling in χ(2) Thin Films
AI总结：
本文提出了一种基于对称性解析重叠框架的非线性频率转换方法，通过分离共振场增强与非线性模式投影，揭示了非线性选择性准束缚态在连续谱中的形成机制，统一了泵浦增强、谐波增强和对称控制模式重叠的预测指标。
摘要：
在共振结构中，二次谐波生成通常通过基频和谐波频率的腔内场增强来评估。本文提出了一种非线性频率转换的对称性解析重叠框架，明确分离共振场增强与非线性模式投影。利用简单且可解析的法布里-珀罗薄膜-基底几何结构，证明在ω和2ω均有谱上亮共振的情况下，当泵浦诱导的非线性极化空间奇偶性与辐射2ω驻波模式的奇偶性不兼容时，发射的二次谐波信号可被强烈抑制。该机制导致了非线性选择性的准束缚态在连续谱中出现。除为这些非线性暗态提供紧凑解释外，该框架统一了泵浦增强、谐波增强和对称控制模式重叠于单一预测指标中。更广泛地说，它识别出共振增强伴随构造性非线性耦合的厚度区域，并将其与那些看似有利的共振条件仍因非线性源与辐射谐波模式正交而无转换活性的区域区分开来。
英文摘要：
Second-harmonic generation in resonant structures is commonly evaluated in terms of intracavity field enhancement at the fundamental and harmonic frequencies. Here, we formulate nonlinear frequency conversion within a symmetry-resolved overlap framework that explicitly separates resonant field buildup from nonlinear mode projection. Using a simple and analytically tractable Fabry--Perot thin-film-on-substrate geometry, we show that, even in the presence of spectrally bright resonances at both $ω$ and $2ω$, the emitted second-harmonic signal can be strongly suppressed when the spatial parity of the pump-induced nonlinear polarization is incompatible with that of the radiating $2ω$ standing-wave mode. This mechanism gives rise to nonlinearity-selective quasi-bound states in the continuum. Beyond providing a compact interpretation of these nonlinear dark states, the framework unifies pump enhancement, harmonic enhancement, and symmetry-controlled modal overlap within a single predictive metric. More broadly, it identifies thickness regimes in which resonant buildup is accompanied by constructive nonlinear coupling, and distinguishes them from regimes in which apparently favorable resonance conditions remain conversion-inactive because the nonlinear source is orthogonal to the radiating harmonic mode.
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