AI 中文总结
研究时变弗洛凯介质热辐射，推导广义基尔霍夫定律，表明给定频率发射率等于伴随系统谐波分辨吸收率加权和，可由吸收算发射率，还揭示静态介质无法达到的热辐射状态及违反传统定律的时变结构。
AI 中文摘要
基尔霍夫定律从根本上关联了热发射与吸收。对于线性、静态、互易介质，它使每个方向和频率的发射率与吸收率相等，而在非互易系统中，当偏置时间反转时发射与吸收相等。然而，在时变介质中，时间调制打破了时间平移不变性，转换频率并实现与调制驱动的能量交换，因此不再期望吸收率与发射率有同频关系。在此，我们推导了线性时变弗洛凯介质的广义基尔霍夫定律。我们表明给定频率下的发射率等于伴随系统谐波分辨吸收率的加权和，权重考虑了热占据和光子通量转换。这一关系具有实际和基本的意义。在实际方面，它允许从吸收计算发射率，简化时变热发射器的设计。更根本的是，它揭示了静态介质中无法达到的热辐射状态。特别是，我们识别出时变结构，其在所有方向上同一频率下表现出强发射且吸收可忽略不计，几乎最大程度地违反了传统形式的基尔霍夫定律。
英文摘要
Kirchhoff's law fundamentally relates thermal emission to absorption. For linear, static, reciprocal media, it equates the emissivity and absorptivity for each direction and frequency, while in nonreciprocal systems emission and absorption are equal when the bias is time-reversed. In time-varying media, however, temporal modulation breaks time-translation invariance, converts frequencies, and enables energy exchange with the modulation drive. As a result, a same-frequency relation between absorptivity and emissivity can no longer be expected. Here, we derive a generalized Kirchhoff's law for linear time-varying Floquet media. We show that the emissivity at a given frequency equals a weighted sum of harmonic-resolved absorptivities of the adjoint system, with weights accounting for thermal occupation and photon-flux conversion. This relation has both practical and fundamental consequences. In practical terms, it allows emissivity to be calculated from absorption, simplifying the design of time-varying thermal emitters. More fundamentally, it reveals thermal radiation regimes inaccessible in static media. In particular, we identify time-varying structures that exhibit strong emission with negligible absorption at the same frequency for all directions, yielding a near-maximal violation of the conventional form of Kirchhoff's law.
Comments22 pages and 7 figures including supplementary materials
