Department of Applied Physics, Kyung Hee University, Gyeonggi-do 17104, Republic of Korea.
School of Energy and Power Engineering, Huazhong University of Science and Technology, Wuhan 430074, China.
ACS Nano. 2023 Jun 13;17(11):10442-10451. doi: 10.1021/acsnano.3c01184. Epub 2023 May 18.
The advent of nanophotonics enables the regulation of thermal emission in the momentum domain as well as in the frequency domain. However, earlier attempts to steer thermal emission in a certain direction were restricted to a narrow spectrum or specific polarization, and thus their average (8-14 μm) emissivity (ε) and angular selectivity were nominal. Therefore, the practical uses of directional thermal emitters have remained unclarified. Here, we report broadband, polarization-irrelevant, amplified directional thermal emission from hollow microcavities covered with deep-subwavelength-thickness oxide shells. A hexagonal array of SiO/AlO (100/100 nm) hollow microcavities designed by Bayesian optimization exhibited ε values of 0.51-0.62 at 60°-75° and 0.29-0.32 at 5°-20°, yielding a parabolic antenna-shaped distribution. The angular selectivity peaked at 8, 9.1, 10.9, and 12 μm, which were identified as the epsilon-near-zero (via Berreman modes) and maximum-negative-permittivity (via photon-tunneling modes) wavelengths of SiO and AlO, respectively, thus supporting phonon-polariton resonance mediated broadband side emission. As proof-of-concept experiments, we demonstrated that these exceptional epsilon-based microcavities could provide thermal comfort to users and practical cooling performance to optoelectronic devices.
纳米光子学的出现使得在动量域和频率域中对热发射进行调控成为可能。然而，早期在特定方向上引导热发射的尝试仅限于窄谱或特定偏振，因此其平均（8-14μm）发射率（ε）和角选择性是名义上的。因此，定向热发射器的实际用途仍不清楚。在这里，我们报告了覆盖深亚波长厚度氧化物壳的中空微腔的宽带、偏振无关、放大的定向热发射。贝叶斯优化设计的 SiO/AlO（100/100nm）中空微腔的六边形阵列在 60°-75°和 5°-20°处表现出 0.51-0.62 和 0.29-0.32 的 ε 值，产生抛物线形天线分布。角选择性在 8、9.1、10.9 和 12μm 处达到峰值，分别对应于 SiO 和 AlO 的ε-近零（通过 Berreman 模式）和最大负介电常数（通过光子隧道模式）波长，从而支持声子极化激元共振介导的宽带侧发射。作为概念验证实验，我们证明了这些基于ε的特殊微腔可以为用户提供热舒适性，并为光电设备提供实际的冷却性能。
