Most reported thermal emitters to date employing photonic nanostructures to achieve narrow bandwidth feature the rainbow effect due to the steep dispersion of the involved high-Q resonances. In this work, we propose to realize thermal emissions with high temporal coherence but free from rainbow effect, by harnessing a novel flat band design within a large range of wavevectors. This feature is achieved by introducing geometric perturbations into a square lattice of high-index disks to double the period along one direction. As a result of the first Brillouin zone halving, the guided modes will be folded to the Γ point and interact with originally existing guided-mode resonances to form a flat band of dispersion with overall high Q. Despite the use of evaporated amorphous materials, we experimentally demonstrate a thermal emission with the linewidth of 23 nm at 5.144 μm within a wide range of output angles (from −17.5° to 17.5°).

迄今为止，大多数报道的热发射器采用光子纳米结构来实现窄带宽，由于所涉及的高 Q 谐振的陡色散而具有彩虹效应。在这项工作中，我们建议通过在大范围的波矢量内利用新颖的平带设计来实现具有高时间相干性但没有彩虹效应的热发射。这一特性是通过将几何扰动引入高折射率圆盘的方格中以使沿一个方向的周期加倍来实现的。由于第一次布里渊区减半，导模将折叠到 Γ 点，并与原来存在的导模谐振相互作用，形成具有整体高 Q 值的平坦色散带。尽管使用了蒸发非晶材料，但我们实验证明，在宽输出角范围（-17.5°至17.5°）内，5.144 μm处线宽为23 nm的热发射。