Institute for Theoretical Physics, Vienna University of Technology (TU Wien), Vienna, Austria.
Institut de Physique de Nice, Université Côte d'Azur, CNRS, Nice, France.
Nature. 2019 Mar;567(7748):351-355. doi: 10.1038/s41586-019-0971-3. Epub 2019 Mar 4.
Non-Hermitian wave engineering is a recent and fast-moving field that examines both fundamental and application-oriented phenomena. One such phenomenon is coherent perfect absorption-an effect commonly referred to as 'anti-lasing' because it corresponds to the time-reversed process of coherent emission of radiation at the lasing threshold (where all radiation losses are exactly balanced by the optical gain). Coherent perfect absorbers (CPAs) have been experimentally realized in several setups, with the notable exception of a CPA in a disordered medium (a medium without engineered structure). Such a 'random CPA' would be the time-reverse of a 'random laser', in which light is resonantly enhanced by multiple scattering inside a disorder. Because of the complexity of this scattering process, the light field emitted by a random laser is also spatially complex and not focused like a regular laser beam. Realizing a random CPA (or 'random anti-laser') is therefore challenging because it requires the equivalent of time-reversing such a light field in all its degrees of freedom to create coherent radiation that is perfectly absorbed when impinging on a disordered medium. Here we use microwave technology to build a random anti-laser and demonstrate its ability to absorb suitably engineered incoming radiation fields with near-perfect efficiency. Because our approach to determining these field patterns is based solely on far-field measurements of the scattering properties of a disordered medium, it could be suitable for other applications in which waves need to be perfectly focused, routed or absorbed.
非厄米波工程是一个新兴且快速发展的领域，研究的是基础和面向应用的现象。其中一个现象是相干完美吸收——通常被称为“反激光”，因为它对应于激光阈值处辐射相干发射的时间反转过程（所有辐射损耗都被光学增益精确平衡）。已经在几种设置中实验实现了相干完美吸收器（CPA），但有一个显著的例外，即在无序介质（没有工程结构的介质）中没有 CPA。这样的“随机 CPA”将是“随机激光”的时间反转，其中光通过无序内部的多次散射得到共振增强。由于这个散射过程的复杂性，随机激光发出的光场也是空间复杂的，不像常规激光束那样聚焦。因此，实现随机 CPA（或“随机反激光”）具有挑战性，因为它需要在所有自由度上反转这样的光场，以创建当入射到无序介质上时能完美吸收的相干辐射。在这里，我们使用微波技术构建了一个随机反激光，并展示了它在适当设计的入射辐射场中吸收近完美效率的能力。因为我们确定这些场模式的方法仅基于对无序介质散射特性的远场测量，所以它可能适用于其他需要完美聚焦、路由或吸收波的应用。
