近日，奥地利维也纳理工大学Stefan Rotter团队报道了Floquet散射系统中的特殊点、激光和相干完美吸收。该项研究成果发表在2026年9月9日出版的《光：科学与应用》杂志上。

周期性时变介质，即光子时间晶体（PTCs），为观察非常规波动现象提供了一个有前景的平台。

研究组利用多光谱Floquet散射矩阵分析电磁波从空间有限PTCs中的散射，该矩阵自然地纳入了此类系统固有的频率混合过程。对于无色散、实且时间周期性的介电常数，该矩阵是伪酉的。研究组证明这一性质会导致多个对称性破缺转变：随着驱动强度增加，位于单位圆上的散射矩阵本征值（未破缺对称性区域）在奇异点（EPs）相遇，并在该处分裂为互为倒数的复共轭对（破缺对称性区域）。他们确定了与这些转变相关的对称算符，并表明在时间对称系统中，它对应于时间反演算符。

值得注意的是，在参量共振条件下，一个本征值消失，而其配对者发散，标志着相干完美吸收（CPA）与激光同时发生。由于该方法仅依赖于Floquet散射矩阵，因此不局限于特定几何结构，而是适用于任何周期性时变散射系统。为说明这种普适性，研究组将该方法应用于多种周期性时间调制结构，包括平板、球体和超表面。特别地，研究组表明利用超表面所支持的连续域中准束缚态共振，可以在最小介电常数调制强度下达到CPA和激光条件。该结果为设计具有定制散射特性的时间调制光子系统铺平了道路，为下一代光学器件中光的动态控制开辟了新途径。

附：英文原文

Title: Exceptional points, lasing, and coherent perfect absorption in Floquet scattering systems

Author: Globosits, David, Garg, Puneet, Hpfl, Jakob, Cans Valero, Adri, Weiss, Thomas, Rockstuhl, Carsten, Rotter, Stefan

Issue&Volume: 2026-09-09

Abstract: Periodically time-varying media, known as photonic time crystals (PTCs), provide a promising platform for observing unconventional wave phenomena. We analyze the scattering of electromagnetic waves from spatially finite PTCs using the multispectral Floquet scattering matrix, which naturally incorporates the frequency-mixing processes intrinsic to such systems. For dispersionless, real, and time-periodic permittivities, this matrix is pseudounitary. Here, we demonstrate that this property leads to multiple symmetry-breaking transitions: for increasing driving strength, scattering matrix eigenvalues lying on the unit circle (unbroken symmetry regime) meet at exceptional points (EPs), where they break up into inverse complex conjugate pairs (broken symmetry regime). We identify the symmetry operator associated with these transitions and show that, in time-symmetric systems, it corresponds to the time-reversal operator. Remarkably, at the parametric resonance condition, one eigenvalue vanishes while its partner diverges, signifying simultaneous coherent perfect absorption (CPA) and lasing. Since our approach relies solely on the Floquet scattering matrix, it is not restricted to a specific geometry but instead applies to any periodically time-varying scattering system. To illustrate this universality, we apply our method to a variety of periodically time-modulated structures, including slabs, spheres, and metasurfaces. In particular, we show that using quasi-bound states in the continuum resonances sustained by a metasurface, the CPA and lasing conditions can be attained for a minimal modulation strength of the permittivity. Our results pave the way for engineering time-modulated photonic systems with tailored scattering properties, opening new avenues for dynamic control of light in next-generation optical devices.

DOI: 10.1038/s41377-026-02380-9

Source: https://www.nature.com/articles/s41377-026-02380-9