Directional Radiative Cooling via Exceptional Epsilon-Based Microcavities

• Published in: ACS nano Vol. 17; no. 11; pp. 10442 - 10451
• Main Authors: Cho, Jin-Woo, Lee, Yun-Jo, Kim, Jae-Hyun, Hu, Run, Lee, Eungkyu, Kim, Sun-Kyung
• Format: Journal Article
• Language: English
• Published: United States American Chemical Society 13.06.2023
• Subjects: radiative cooling; Midinfrared photonics; Berreman mode; Bayesian optimization; directional thermal emission
• ISSN: 1936-0851; 1936-086X
• DOI: 10.1021/acsnano.3c01184

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 (εav) 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 SiO2/AlOX (100/100 nm) hollow microcavities designed by Bayesian optimization exhibited εav 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 SiO2 and AlOX, 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.