arXiv subjects

Yeonghoon Jin

Publications and source records attributed to Yeonghoon Jin.

5 recordsLinked to original sources

arXiv2026

Cool windows: simultaneously engineering high visible transparency and strong solar rejection

For window applications in hot climates, it is desirable to have windows with high visible transparency, while maintaining strong reflectance in both the ultraviolet and near infrared, to minimize unwanted heat gain. Given that more than 70% of incident solar energy is at wavelengths shorter than 1000 nm, achieving spectrally abrupt transitions from transparent to reflecting at the boundaries of the visible is essential. Such abrupt transitions at multiple wavelengths typically would require tens of dielectric layers, which is impractical for most window applications. Here, we propose and realize a structure comprising only eight planar layers that achieves sharp reflectance changes at ~390 and ~680 nm, resulting in high visible transmittance (>70%), and high near-infrared (>80%) and UV (>60%) reflectance, as well as high mid-infrared emissivity (>90%) for additional radiative cooling. We demonstrate that our engineered window reduces air temperature by up to 3.8 {\deg}C inside an enclosed box simulating a vehicle, compared to a box with a reference window.

physics.optics↗

arXiv2025

Passive radiative cooling using temperature-dependent emissivity can sometimes outperform static emitters

In passive sky-facing radiative cooling, wavelength-selective thermal emitters in the atmospheric transparency window of 8-13 $\mu$m can reach lower temperatures compared to broadband emitters, but broadband emitters always have higher cooling power when the emitter is warmer than the ambient. Here, we propose a temperature-tunable thermal emitter that switches between a wavelength-selective state -- with high emissivity only in the atmospheric transparency window of 8-13 $\mu$m -- and a broadband-emissive state with high emissivity in the 3-25 $\mu$m range, thus maintaining high cooling potential across all temperatures. We also propose a realization of such a temperature-tunable emitter using the phase transition of vanadium dioxide (VO$_2$), which can be tuned to the ambient temperature using a combination of doping and defect engineering.

physics.optics↗

arXiv2024

Large tuning of the optical properties of nanoscale NdNiO3 via electron doping

We synthesized crystalline films of neodymium nickel oxide (NdNiO3), a perovskite quantum material, switched the films from a metal phase (intrinsic) into an insulator phase (electron-doped) by field-driven lithium-ion intercalation, and characterized their structural and optical properties. Time-of-flight secondary-ion mass spectrometry (ToF-SIMS) showed that the intercalation process resulted in a gradient of the dopant concentration along the thickness direction of the films, turning the films into insulator-metal bilayers. We used variable-angle spectroscopic ellipsometry to measure the complex refractive indices of the metallic and insulating phases of NdNiO3. The insulator phase has a refractive index of n ~ 2 and low absorption in the visible and near infrared, and analysis of the complex refractive indices indicated that the band gap of the insulating phase is roughly 3-4 eV. Electrical control of the optical band gap, with corresponding large changes to the optical refractive indices, creates new opportunities for tunable optics.

cond-mat.mtrl-sci↗

arXiv2024

Preventing overfitting in infrared ellipsometry using temperature dependence: fused silica as a case study

Fitting oscillator models to variable-angle spectroscopic ellipsometry (VASE) data can lead to non-unique, unphysical results. We demonstrate using temperature-dependent trends to prevent overfitting and ensure model physicality. As a case study, we performed mid-infrared VASE measurements on fused silica (SiO2) of various grades, from room temperature to 600 {\deg}C. We fitted oscillator models independently at each temperature, and confirmed the model's physical validity by observing the expected monotonic trends in vibrational oscillator parameters. Using this technique, we generated a highly accurate dataset for the temperature-dependent complex refractive index of fused silica for modeling mid-infrared optical components such as thermal emitters.

physics.optics↗

arXiv2024

A gradient atmospheric model reveals enhanced radiative cooling potential and demonstrates the advantages of broadband emitters

Passive radiative cooling toward the sky is a developing technology for adaptation in hot climates. Previous calculations of cooling performance have generally used uniform atmospheric models that assume a single sky temperature and atmospheric transmittance spectrum. Here, we introduce a gradient atmospheric model that accounts for altitude-dependent temperature and gas composition, revealing that uniform models underestimate cooling power by 10 - 40%. Using our improved model, we systematically compared broadband emitters (BEs) and wavelength-selective emitters (SEs) for sky-facing radiative cooling at various locations on Earth. We find that the differences in cooling power between the two types of emitters in the sub-ambient temperature range are generally small, even under ideal conditions. Furthermore, in practice, BEs actually have superior performance than realistic SEs, because they have fewer design degrees of freedom and thus can be engineered to have lower solar absorption. Our analysis suggests that large-scale deployment of sky-facing passive radiative cooling technologies should prioritize the development of scalable, low-cost surfaces with minimal solar absorption, rather than focusing on achieving selective thermal emission.

physics.optics↗