3D Nanophotonics in Artificially Structured Chalcogenide Materials

在自然界中，蝴蝶的翅膀、甲虫的外壳和墨鱼的交配展示中经常看到的彩虹色，是光的波动特性的结果，它可以对不同的颜色表现出建设性和破坏性的干涉效应。我们可以在实验室里制造重复的结构重复周期接近于光的波长看到类似的效果；一个著名的例子是光盘的反射，但在眼镜上的防反射涂层中也可以看到相反的效果，可以在紫色的高角度反射中看到。事实上，这些衍射反射效应的角度灵敏度可以导致奇妙的彩虹反射色彩显示，而一些三维结构如蝴蝶翅膀则抑制了这种角度变化；就像著名的蓝色形态物种一样，它们在飞行时基本上保持蓝色的翅膀颜色。在这个项目中，我们的目标是建立三维重复结构，可以产生非常强的反射效果，同时减少通常使用二维光栅和镜子看到的角度变化。这些3D周期性材料可以有效地反射从任何角度入射的特定颜色（波长）范围的光，并从本质上阻止光从任何方向穿过材料。这些材料被称为光子带隙材料，因为它们阻挡了一条色带，而且它们具有类似于半导体带隙的特性，可以阻挡电子在某些能带中运动。虽然难以制造，但这些材料可以显示出相当显著和有用的效果。例如，阻断所有方向上的所有传输可以用作对强光（例如激光）的保护，或者可以使带隙对某些分子物种或污染物敏感，从而提供一种传感方式。在我们的团队中，我们一直在研究这些材料的光捕获特性及其对光发射的影响。例如，如果一个发射带完全在带隙内的荧光染料分子在这样的理想材料中被激发，那么它将没有发射光的途径，并保持在其激发态，直到以非辐射途径衰变。然而，如果我们通过去除少量材料来创建一个空腔，则会发生光发射，但会被困在这个空腔中，直到被吸收或通过有限的屏障泄漏到材料的边缘。理论上，这些存储时间可以很长，而空腔体积可以做得非常小，这可以导致荧光团对光的发射和吸收的强烈增强，所谓的“强耦合”是由光-物质相互作用的全量子力学处理所预测的。最后，在这个项目中，我们将开发可靠的技术来制造这些3D光限制材料，并利用它们的新特性将光捕获在微小的“腔”和波导中，从而显示出最强的光-物质相互作用。这些结果将产生全面的影响，从创建包含单个“原子”发射器的新光源到最小的激光器和模仿蝴蝶翅膀反射率的材料。

The iridescent colours often seen in nature in butterfly wings, beetle carapaces and cuttlefish mating displays are a result of the wave nature of light which can show constructive and destructive interference effects for different colours. We can make in the lab repeated structures where the repeat period is close to the wavelength of light and see similar effects; a well-known example being the reflection from a compact disc but also the opposite effect is seen in the anti-reflection coating applied to spectacles which can be seen in the violet coloured high angle reflections. In fact, the angular sensitivity of these diffractive reflection effects can lead to wonderful rainbow reflective colour displays, while some three-dimensional structures such as butterfly wings suppress this angular change; as in the case of the famous blue morphospecies which maintains a largely blue wing colour while flying.In this project, we aim to build three-dimensional repeating structures that can lead to very strong reflection effects while reducing the angular changes normally seen with two-dimensional gratings and mirrors. These 3D periodic materials can effectively reflect light incident from any angle for a particular range of colours (wavelengths) and essentially block light from passing through the material in any direction. These materials are known as photonic bandgap materials because they block a band of colours and because they have properties analogous to the semiconductor bandgaps that block electrons travelling in certain energy bands. Although difficult to fabricate, these materials could exhibit quite striking and useful effects. For instance, blocking all transmission in all directions could be used as protection against bright light (e.g. lasers) or the bandgap could be made sensitive to certain molecular species or pollutants providing a sensing modality. In our team, we have been looking at the light trapping properties of these materials and their effect on light emission. For instance, if a fluorescent dye molecule with emission band entirely within the bandgap is excited inside such an ideal material then it will have no route by which to emit light and remain in its excited state until decaying by a non-radiative route. However, if we create a cavity by removing a small amount of material, the light emission will occur but will be trapped in this cavity until absorbed or leaking through the finite barrier to the edge of the material. Theoretically, these storage times can be very long while the cavity volumes can be made very small which can lead to a strong enhancement of emission and absorption of light by the fluorophore, so-called 'strong coupling' predicted by the full quantum mechanical treatments of the light-matter interaction.Finally, in this project, we will develop reliable techniques to make these 3D light confining materials and exploit their novel properties to trap light in tiny 'cavities' and waveguides thus showing the strongest light-matter interactions possible. These results will have an impact across the board from creating new light sources containing single 'atom' like emitters through to the smallest lasers and materials mimicking the reflectivity of butterfly wings.