Abstract

Heating and cooling represent opposing thermal phenomena, yet both heaters and coolers can require optical transparency, depending on their applications. This talk will address two key topics: (i) microwave-transparent heaters for enhancing safety in autonomous driving and (ii) near-infrared reflecting films for cooling windows.

Regarding the first topic, maintaining the optical transparency of protective covers through transparent heaters is essential for ensuring safety in autonomous driving, particularly under extreme weather conditions. However, achieving both high transmittance and low sheet resistance, which are critical performance metrics for transparent heaters, poses significant challenges. Inspired by metamaterial design principles, we report the development of high-performance microwave-transparent heaters for automotive radars. These heaters are composed of ultrathin electrically connected metamaterials (approximately one ten-thousandth of the wavelength) on a millimeter-thick dielectric cover, delivering near-unity transmission at specific frequencies within the W band (75–110 GHz), even with a metal filling ratio exceeding 70%. The fabricated heaters demonstrate exceptionally low sheet resistance (0.41 ohm/sq), enabling the dielectric cover to reach temperatures above 180°C at a nominal bias of 3 V. Defrosting tests confirm their thermal capability to eliminate thin ice layers in sub-zero conditions swiftly.

Regarding the second topic, thermal energy exchange through windows significantly affects building energy efficiency, with windows accounting for up to 40% of total heat losses. To realize zero-energy buildings, various strategies have been explored to enhance window energy efficiency, including the development of cooling windows and the integration of semi-transparent photovoltaics for concurrent energy generation. Passive radiative cooling (PRC) films, which selectively reflect the near-infrared photons of sunlight, offer a promising solution to reduce temperature without energy consumption. Previous research on PRC films has emphasized maximizing visible transmittance while enhancing near-infrared reflectance. However, there is a growing need to address privacy concerns and reduce glare for building residents and automobile drivers. To meet these demands, we developed visible transmittance-modulated PRC films using an active learning algorithm. Glass windows coated with customized PRC films achieved visible transmittance levels of 45%, 62%, and 87% while maintaining high near-infrared reflectance (46–52%). Additionally, we successfully tuned the colors of PRC films while preserving visible transparency and near-infrared protection. 

References

[1] "Microwave-Transparent Metallic Metamaterials for Autonomous Driving Safety," Nat. Commun. 15:4516 (2024).

[2] "Ultrahigh visible-transparency, submicron, and polymer-free radiative cooling meta-glass coating for building energy saving," ACS Photon. 11, 34 (2024).

Biography

Sun-Kyung Kim

Tenured professor, Applied Physics, Kyung Hee University, Gyeonggi-do, Korea.  

Education 

2006, Ph. D. in Physics, KAIST

2002, M. S. in Physics, KAIST 

2000, B. S. in Physics, KAIST 

Career

20010-2013, Postdoctoral Researcher in Chemistry and Chemical Biology, Harvard Univ.

2008-2010 Chief Researcher, LG Innotek

2006-2008, Senior Researcher, LG Electronics 

Research Experience

My current research explores high-index-contrast dielectric or metal/dielectric hybrid photonic materials for manipulating light absorption, emission, and thermal radiation across the ultraviolet, visible, infrared, and microwave spectra. Additionally, it involves the realization of optical materials with exceptional dispersions through the concepts of metamaterials and surface plasmons. My group has expertise in designing functional photonic materials, fabricating complex photonic structures, and characterizing optical performances, which have been successfully incorporated into diverse light absorption, emission, and thermal radiation devices. For example, regarding light absorption devices, we demonstrated a three-dimensional Si grating nanowire photovoltaic system that achieved a record power conversion efficiency at the single nanowire level. In terms of light emission devices, we developed strong-diffraction hollow-cavity growth substrates that enabled high-efficiency InGaN/GaN LED devices, surpassing state-of-the-art commercial LED devices. Finally, for thermal radiation devices, we reported directional radiative coolers that amplified side thermal emissions, thereby providing thermal comfort to users of personal optoelectronic devices such as smartphones.