谱因果性与波的散射
Spectral Causality and the Scattering of Waves
摘要：
因果性--一个系统的输出在时间上不能先于输入的原则--是自然界的普遍属性。在这里，我们表明，类似的输入输出关系也可以实现在频谱域中利用时间调制的非厄米光子系统的特殊属性。具体来说，我们发现了广泛的一类复杂的时间调制超材料的存在，遵守时域等效的良好建立的频域Kramers-Kronig关系（因果关系的直接后果）。我们发现，在这样的时间调制系统的散射响应，输出频率固有地被禁止从频谱前的输入频率，因此，我们称这些系统为“频谱cascade”。我们探讨的后果，这个新引入的概念，几个相关的应用，包括宽带完美吸收，时间掩盖的“事件”，真正的单向传播沿合成尺寸。通过模拟光谱域中因果关系的概念，并提供新的工具，将时间调制超材料（“时间超材料”）的领域扩展到复杂的领域，我们的研究结果可能会打开未探索的机会，并使光子学的各个领域，更广泛地说，波物理和工程的相关技术进步。
英文摘要：
Causality - the principle stating that the output of a system cannot temporally precede the input - is a universal property of nature. Here, we show that analogous input-output relations can also be realized in the spectral domain by leveraging the peculiar properties of time-modulated non-Hermitian photonic systems. Specifically, we uncover the existence of a broad class of complex time-modulated metamaterials which obey the time-domain equivalent of the well-established frequency-domain Kramers-Kronig relations (a direct consequence of causality). We find that, in the scattering response of such time-modulated systems, the output frequencies are inherently prohibited from spectrally preceding the input frequencies, hence we refer to these systems as 'spectrally causal'. We explore the consequences of this newly introduced concept for several relevant applications, including broadband perfect absorption, temporal cloaking of an 'event', and truly unidirectional propagation along a synthetic dimension. By emulating the concept of causality in the spectral domain and providing new tools to extend the field of temporally modulated metamaterials ("chrono-metamaterials") into the complex realm, our findings may open unexplored opportunities and enable relevant technological advances in various areas of photonics and, more broadly, of wave physics and engineering.
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