Tailoring optical properties of randomly nanotextured layers via Anderson localization

该项目结合了实验、理论和纳米工程方法，以确定一种通用的设计概念，使光在无序的薄光学层中有效定位。材料的吸收率不容易调节。然而，局域态的光捕获增强了薄层中的光-物质相互作用，从而增强了吸收。因此，调整局域模式与外场的耦合效率是一种增强吸收的有趣机制。随机纳米结构的吸收剂可以增强薄吸收层的吸收。然而，关于增强机理的争论仍然存在，高效吸收剂的明确设计策略仍然缺乏。我们建议研究光定位在无序但合适的定制二维层中的应用，以实现增强甚至完全吸收。在一项初步研究中，我们证明了光在薄纳米结构的a- si:H吸收层中的局部化在吸收率的长波长尾部占主导地位。在此基础上，我们将系统地调整这些层中的无序度，以确定支持有效光定位的关键纳米纹理参数。最初，a- si:H将作为模型吸收材料，所使用的实验方法的适用性已经得到证明。在项目的过程中，其他材料，如金属卤化物钙钛矿和透明导电氧化物（TCOs）作为宽带间隙介电材料也将进行研究。采用自上而下（聚焦离子束（FIB））和自下而上（生长控制，蚀刻）纳米织构方法结合界面表征（TEM， SEM， AFM）制备和表征纳米织构吸收层，然后通过热离子发射显微镜结合相干二维纳米显微镜和散射光光谱显微镜对纳米织构吸收层进行研究。该项目的第一个里程碑是在采用自上而下的纳米制造方法定制的纳米纹理层中展示有效的光捕获。自上而下的方法允许定制无序层的系统变化，从而通过定位建立有效的光捕获的通用设计概念。纳米结构层响应的理论建模（FDTD求解器）有助于识别关键的纳米纹理特性。在此基础上，我们将局域模式与外辐射场之间的耦合效率调整到临界耦合状态，即等耦合和内损耗。对于这种情况，期望局域模态的完全吸收。此外，通过将该方案扩展到吸收率低于a- si:H的材料，研究了在无序介电层中通过局域化形成高质量（高q）随机谐振腔的前景。

This project combines experimental, theoretical, and nanoengineering approaches to identify a universal design concept enabling efficient localization of light in disordered thin optical layers. The absorptivity of a material cannot be easily tuned. However, trapping of light in localized states enhances the light-matter interaction in thin layers and thus enhances absorption. Tailoring the coupling efficiency of localized modes to the external field therefore is an interesting mechanism to enhance absorption. Randomly nanotextured absorbers are known to enhance absorption in thin absorber layers. However, controversies about the enhancement mechanism persist and clear design strategies for efficient absorbers are still missing. We propose investigating theapplication of light localization in disordered but suitable tailored twodimensional layers to achieve enhanced or even perfect absorption. In a preliminary study we demonstrated that light localization in a thin nanotextured a-Si:H absorber layer dominates absorption in the long-wavelength tail of the absorptivity. Based on this we will systematically tailor the disorder in such layers to identify the critical nanotexture parameters that support efficient light localization. Initially a-Si:H will serve as a model absorber material for which the applicability of the used experimental methods is already demonstrated. In the course of the project also other materials such as metal-halide perovskites and transparent conductive oxides (TCOs) as wide-band gap dielectrics will be investigated. Top-down (focused ion beam (FIB)) and bottom-up (growth control, etching) nanotexturing methods in combination with interface characterization (TEM, SEM, AFM) are applied to prepare and characterize nanotextured absorber layers that are then investigated by thermionic emission microscopy in combination with coherent 2D nanoscopy and spectromicroscopy of scattered light. The first milestone of the project is the demonstration of efficient light trapping in nanotextured layers custom made by employing top-down nanofabrication methods. The top-down approach allows a systematic variation of tailored disordered layers and thus the establishment of universal design concepts for efficient light trapping via localization. The identification of crucial nanotexture properties is assisted by theoretical modeling (FDTD solver) of the response of nanostructured layers. Based on this we will tune the coupling efficiency between localized modes and external radiation fields towards the regime of critical coupling, i.e. equal coupling and internal loss. For this case perfect absorption of the localized modes is expected. In addition, the prospect of forming high-quality (high-Q) random resonators via localization in disordered dielectric layers is investigated by extending the scheme to materials with a lower absorptivity than a-Si:H.