Wang Neng, Hong Binbin, Wang Guo Ping
Opt Express. 2024 Oct 21;32(22):40092-40107. doi: 10.1364/OE.539505.
We realized fourth-order exceptional points (EP-4s) in the quasienergy bands of anisotropic photonic time-Floquet crystals (APTCs), where the full in-plane permittivity tensors are periodically modulated in time. We developed the Floquet matrix method for APTCs, which provides a comprehensive study of the generation and characteristics of EP-4s. It is shown that an EP-4 is formed by the coalescence of three second-order exceptional points (EP-2s), which are classified into two types based on the band slopes near them. Additionally, the helicities of the four coalescing bands converge rapidly to zero at any given time when approaching the EP-4, following the -dependence. Therefore, a strong polarization sensitivity is observed at the EP-4, which could inspire applications relevant to electromagnetic spins in APTCs.
我们在各向异性光频弗洛凯晶体（APTCs）的准能带中实现了四阶例外点（EP - 4s），其中平面内的完整介电常数张量随时间进行周期性调制。我们开发了用于APTCs的弗洛凯矩阵方法，该方法对EP - 4s的产生和特性进行了全面研究。结果表明，一个EP - 4由三个二阶例外点（EP - 2s）合并而成，根据它们附近的能带斜率可将这三个二阶例外点分为两种类型。此外，当接近EP - 4时，四条合并能带的螺旋度在任何给定时间都迅速收敛到零，遵循 - 依赖性。因此，在EP - 4处观察到强烈的偏振敏感性，这可能会激发与APTCs中的电磁自旋相关的应用。
