Jones Thomas R, Kildishev Alexander V, Segev Mordechai, Peroulis Dimitrios
Elmore Family School of Electrical and Computer Engineering, Purdue University, West Lafayette, IN, USA.
Department of Physics, Technion-Israel Institute of Technology, Haifa, Israel.
Nat Commun. 2024 Aug 8;15(1):6786. doi: 10.1038/s41467-024-51171-6.
When an electromagnetic (EM) wave propagates in a medium whose properties are varied abruptly in time, the wave experiences refractions and reflections known as time-refractions and time-reflections, both manifesting spectral translation as a consequence of the abrupt change of the medium and the conservation of momentum. However, while the time-refracted wave continues to propagate with the same wave-vector, the time-reflected wave propagates backward with a conjugate phase despite the lack of any spatial interface. Importantly, while time-refraction is always significant, observing time-reflection poses a major challenge - because it requires a large change in the medium occurring within a single cycle of the EM wave. For that reason, time-reflection of EM waves was observed only recently. Here, we present the observation of microwave pulses at the highest frequency ever observed (0.59 GHz), and the experimental evidence of the phase-conjugation nature of time-reflected waves. Our experiments are carried out in a periodically-loaded microstrip line with optically-controlled picosecond-switchable photodiodes. Our system paves the way to the experimental realization of Photonic Time-Crystals at GHz frequencies.
当电磁波在一种其特性随时间急剧变化的介质中传播时，该波会经历被称为时间折射和时间反射的折射和反射，这两种现象都因介质的突然变化和动量守恒而表现出频谱平移。然而，虽然时间折射波继续以相同的波矢传播，但时间反射波尽管没有任何空间界面，却以共轭相位向后传播。重要的是，虽然时间折射总是很显著，但观察时间反射却面临重大挑战——因为它需要在电磁波的单个周期内介质发生很大变化。因此，直到最近才观察到电磁波的时间反射。在此，我们展示了在有史以来观测到的最高频率（0.59 GHz）下对微波脉冲的观测，以及时间反射波的相位共轭性质的实验证据。我们的实验是在带有光控皮秒可切换光电二极管的周期性加载微带线中进行的。我们的系统为在吉赫兹频率下实验实现光子时间晶体铺平了道路。
