Galiffi Emanuele, Yin Shixiong, Alú Andrea
Photonics Initiative, Advanced Science Research Center, City University of New York, NY, NY 10031, USA.
Physics Program, Graduate Center, City University of New York, NY, NY 10016, USA.
Nanophotonics. 2022 Jul 20;11(16):3575-3581. doi: 10.1515/nanoph-2022-0200. eCollection 2022 Sep.
The advent of novel nonlinear materials has stirred unprecedented interest in exploring the use of temporal inhomogeneities to achieve novel forms of wave control, amidst the greater vision of engineering metamaterials across both space and time. When the properties of an unbounded medium are abruptly switched in time, propagating waves are efficiently converted to different frequencies, and partially coupled to their back-propagating phase-conjugate partners, through a process called time-reversal. However, in realistic materials the switching time is necessarily finite, playing a central role in the resulting temporal scattering features. By identifying and leveraging the crucial role of electromagnetic momentum conservation in time-reversal processes, here we develop a general analytical formalism to quantify time-reversal due to temporal inhomogeneities of arbitrary profile. We deploy our theory to develop a formalism, dual to spatial tapering, that enables the tailoring of a desired time-reversal spectral response, demonstrating its use for the realization of broadband frequency converters and filters.
新型非线性材料的出现，在跨时空工程超材料这一更为宏大的愿景中，引发了人们对利用时间不均匀性来实现新型波控制形式的前所未有的兴趣。当无界介质的特性在时间上突然切换时，传播的波会通过一种称为时间反转的过程有效地转换为不同频率，并部分耦合到它们的反向传播相位共轭波。然而，在实际材料中，切换时间必然是有限的，这在产生的时间散射特性中起着核心作用。通过识别并利用电磁动量守恒在时间反转过程中的关键作用，我们在此开发了一种通用的解析形式体系，以量化由于任意轮廓的时间不均匀性导致的时间反转。我们运用我们的理论来开发一种与空间渐变对偶的形式体系，该体系能够定制所需的时间反转光谱响应，展示了其在实现宽带频率转换器和滤波器方面的应用。
