Influence of slab interfaces and modulation regime on isolation ratio in graphene space-time crystal slabs: towards high-performance isolation

Abstract

Nonreciprocal devices based on dynamic modulation play a crucial role in on-chip light control such as optical isolation. In this work, we propose new ways for improving isolation performances in space-time crystal slabs by investigating the influence of slab interfaces and modulation regime, taking graphene plasmonic space-time crystals, as examples. First, we show that when slab is truncated by interfaces with the velocity the same as modulation interface, no Bloch-Floquet mode is excited inside the slab by forward incident pulse with central frequency falling in the bandgap, prohibiting forward propagation and leading to the isolation ratio of more than 30 dB. Resultantly, such a comoving slab exhibits an order-of-magnitude larger isolation ratio compared with the cases of fixed interfaces. Next, we show that, in superluminal modulation regime, isolation can be realized based on a completely new mechanism originating from temporal amplification inside the momentum bandgap. Such a modulation opens asymmetric momentum bandgaps, in which Bloch-Floquet modes display exponential amplification. Hence, forward propagating mode experiences significant temporal amplification, while backward one attenuates due to inherent plasmonic loss, resulting in high isolation ratio of around 30 dB. The presented results pave the way towards magnetless optical isolation or circulation with high performances by using space-time modulated systems.