ICFO-Institut de Ciencies Fotoniques, 08860 Castelldefels, Barcelona, Spain.
Department of Physics, Cornell University, Ithaca, New York 14853, United States.
ACS Nano. 2023 Apr 25;17(8):7377-7383. doi: 10.1021/acsnano.2c11497. Epub 2023 Apr 3.
Photonic crystals and metamaterials are two overarching paradigms for manipulating light. By combining these approaches, hypercrystals can be created, which are hyperbolic dispersion metamaterials that undergo periodic modulation and mix photonic-crystal-like aspects with hyperbolic dispersion physics. Despite several attempts, there has been limited experimental realization of hypercrystals due to technical and design constraints. In this work, hypercrystals with nanoscale lattice constants ranging from 25 to 160 nm were created. The Bloch modes of these crystals were then measured directly using scattering near-field microscopy. The dispersion of the Bloch modes was extracted from the frequency dependence of the Bloch modes, revealing a clear switch from positive to negative group velocity. Furthermore, spectral features specific to hypercrystals were observed in the form of sharp density of states peaks, which are a result of intermodal coupling and should not appear in ordinary polaritonic crystals with an equivalent geometry. These findings are in agreement with theoretical predictions that even simple lattices can exhibit a rich hypercrystal bandstructure. This work is of both fundamental and practical interest, providing insight into nanoscale light-matter interactions and the potential to manipulate the optical density of states.
光子晶体和超材料是两种用于操控光的主要范例。通过结合这两种方法，可以创建超晶体，这是一种具有双曲色散的超材料，经历周期性调制，将类似于光子晶体的方面与双曲色散物理混合在一起。尽管已经进行了多次尝试，但由于技术和设计方面的限制，超晶体的实验实现仍然有限。在这项工作中，创建了具有纳米级晶格常数的超晶体，范围从 25 到 160nm。然后使用散射近场显微镜直接测量这些晶体的 Bloch 模式。从 Bloch 模式的频率依赖性中提取 Bloch 模式的色散，揭示了从正群速度到负群速度的明显转变。此外，还观察到了超晶体特有的谱特征，表现为尖锐的态密度峰，这是由于模间耦合所致，而在具有等效几何形状的普通极化晶体中不应出现这种现象。这些发现与理论预测一致，即使是简单的晶格也可以表现出丰富的超晶体能带结构。这项工作具有基础和实际的意义，为纳米级光物质相互作用提供了深入的了解，并有可能控制光学态密度。
