Non-Hermitian nanophotonics for efficient thermophotovoltaic energy conversion

用清洁和高效的能源系统满足日益增长的全球能源需求是我们这个时代最大的技术挑战之一。有效地将热和光转化为电是克服这一挑战的关键。热光伏是一种很有前途的技术，它可以通过光有效地将热转化为电，而不需要任何移动部件。工业过程、核裂变反应、汽车尾气或吸收阳光产生的热量会从热表面辐射出热光。在太阳能电池中，这种热光的光电转换被称为热光伏转换。热光伏系统的理论效率极限可以高达80%，尽管实验证明远远低于这个数字。效率低的主要原因是从热表面辐射的热光的宽带性质。将热光压缩到窄频带是一项具有挑战性的任务，特别是在高温下工作时。该项目旨在开发新的策略，将热光限制在狭窄的频率范围内，并展示热到光的有效转换。该项目的成功实施将导致发现新工具，以实现对光和热流动的极端控制，教育和培训下一代科学家，并开发用于能源生产和存储应用的高效热电转换技术。考虑到仅并网工业废热就占工业总能耗的近20%，高效的热光伏系统可以在满足地球清洁能源需求方面产生巨大影响。技术描述：热表面的热辐射本质上通常是宽带的，限制了热光伏转换的整体效率。将热辐射限制在一个狭窄的光谱带是解决这个问题的关键。以前的尝试已经研究了各种纳米光子原理来设计窄带热发射体或选择性发射体，但它们的性能仍然不足。所有的纳米结构光学材料在高温下都会降解。它们的光学损耗随温度显著增加，并限制了最大可能的光谱选择性。计算表明，要实现高效的热光伏转换，至少需要20 dB的光谱对比度。如此高的对比度在广泛的红外波长是不可能通过传统的方法，由于高光学损耗在组成材料。在本项目中，采用非厄米物理或量子光学描述共振发射体的非正统方法来设计热发射体。与传统方法不同，非厄米设计利用材料的高光学损耗，同时实现高对比度和高发射率。此外，选择性发射器的量子光学描述允许将其设计为耦合多个纳米级谐振器的系统。这种方法是一种范式的转变，在选择的发射器的设计允许新的多体物理现象被观察到热辐射。因此，谐振器的对称性、拓扑结构和内部相位等新设计工具为极端工程热辐射器提供了前所未有的机会。本项目旨在研究这些新设计工具对热辐射空间和光谱特性的影响，构建具有高对比度、方向性和亮度的选择性发射体，并演示高效的热光伏转换。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

Meeting the rising global energy need with clean and efficient energy systems is one of the greatest technological challenges of our time. Efficient conversion of heat and light to electricity is crucial in overcoming this challenge. Thermophotovoltaics is a promising technique for efficiently converting heat to electricity via light without any moving parts. The heat generated from industrial processes, nuclear fission reaction, automobile exhaust or absorption of sunlight results in thermal light radiating from hot surfaces. Photovoltaic conversion of this thermal light to electricity as in solar cells is called thermophotovoltaic conversion. The theoretical efficiency limit of thermophotovoltaic systems can be as high as 80%, though experimental demonstrations lie far below this number. The primary reason for low efficiency is the broadband nature of thermal light radiating from hot surfaces. Squeezing thermal light into a narrow band of frequencies is a challenging task especially when operating at high temperatures. This project aims to develop novel strategies to confine thermal light to a narrow band of frequencies and demonstrate efficient conversion of heat to light. Successful implementation of the project will result in discovering new tools to achieve extreme control on the flow of light and heat, educate and train next-generation scientists, and develop an efficient heat-to-electricity conversion technology for energy generation and storage applications. Given that the on-grid industrial waste heat alone is nearly 20% of the total industrial energy consumption, efficient thermophotovoltaic systems can make a huge impact in meeting the clean energy needs of the planet.Technical description: Thermal radiation from hot surfaces is typically broadband in nature and limit the overall efficiency of thermophotovoltaic conversion. Confining thermal radiation to a narrow spectral band is the key to this problem. Previous attempts have investigated various nanophotonic principles to design narrowband thermal emitters or selective emitters, though their performance is still inadequate. All nanostructured optical materials degrade at high temperatures. Their optical losses significantly increase with temperature and limit the maximum possible spectral selectivity. Calculations show that a spectral contrast of at least 20 dB is required for efficient thermophotovoltaic conversion. Such high contrast over broad infrared wavelengths is not possible by conventional approach due to high optical losses in the constituent materials. Here in this project, an unorthodox approach using non-Hermitian physics or quantum optical description of resonant emitters is adopted to design thermal emitters. Unlike the conventional approach, non-Hermitian design exploits high optical losses in materials to simultaneously achieve high contrast and high emissivity. Further, a quantum optical description of the selective emitter allows its design as a system of coupled multiple nanoscale resonators. Such an approach is a paradigm shift in the design of selective emitters allowing novel many-body physical phenomena to be observed in thermal emission. As a result, new design tools such as symmetry, topology and internal phase of resonators present an unprecedented opportunity to extreme-engineer thermal emitters. This project aims to investigate the effect of these new design tools on the spatial and spectral properties of thermal radiation, build selective emitters with high contrast, directionality, and brightness, and demonstrate high-efficiency thermophotovoltaic conversion.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.