Directional Radiative Cooling via Exceptional Epsilon-Based Microcavities

Abstract

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 av ) 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-subwavelengththickness oxide shells. A hexagonal array of SiO 2 /AlO X (100/100 nm) hollow microcavities designed by Bayesian optimization exhibited ε av 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 2 and AlO X , 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.

Directional Thermal Emission Covering Two Atmospheric Windows

“…Besides, directional thermal emission in both polarizations can be realized by harnessing the 3D nature of hollow microcavities. [ 38 ]…”

Functional Radiative Cooling: Basic Concepts, Materials, and Best Practices in Measurements

“…Conventional omnidirectional RC devices suffer from degraded performances in closed systems due to the greenhouse effect, whereas directional RC devices circumvent this issue. Hence, various techniques such as diffraction gratings, epsilon-near-zero materials, , and two-dimensional materials have been utilized to achieve the desired directional thermal emission.…”

Unprecedented Spatial Manipulation and Transformation of Dynamic Thermal Radiation Based on Vanadium Dioxide

“…Advanced infrared (IR) adaptive materials have widespread applications in numerous IR technologies, especially in thermal management and smart IR concealment. − Among these materials, vanadium dioxide (VO 2 ) is particularly attractive, given the remarkable changes in optical properties before and after its phase transition. − Generally, the dynamic thermal radiation properties of VO 2 -based resonators depend largely on the featured structural parameters. − Tang et al developed a temperature-adaptive radiative coating based on VO 2 /BaF 2 /Ag for intelligent regulation of household temperature, which can automatically switch the thermal emittance from 0.20 at 15 °C to 0.90 at 30 °C . Concurrently, Wang et al presented a VO 2 /PMMA/low-E stacking structure, which possessed variable emittance of 0.21 at low and 0.61 at high temperatures to switch radiative cooling automatically .…”